H`CBCB @@@ @@@@I:CBpCB EN DB eCBV >n Edwards1994Kg Lauenroth1997O Rice19971lW2X Rich1997Richards19858Richards19871Richards1994Richards2001 Richardson2002 Riddle19999 Ridolfi1999 Ridolfi2000 Ridolfi2000 Ridolfi2000 Ridolfi2001 Ridolfi2001 Ridolfi2001 Robinson1983Robinson1999cRobinson1999 Robles19988; Rodriguez1998Rodriguez-Iturbe1999Rodriguez-Iturbe2000Rodriguez-Iturbe2000Rodriguez-Iturbe2000Rodriguez-Iturbe2001Rodriguez-Iturbe2001Rodriguez-Iturbe2001Rodriguez-Iturbe20010? Rood19989 Rorison1990 Rose20000k Rose20010n Rothstein1996B Roundy1989= Roundy1997F Roy1986 Rundel19900 Rundel19921 Rundel19981 Rundel1999 Running1996h Rusek2000 Ruxton1999 Ryan20012 Ryel1996 Ryel2001 Ryel2002Rylander20000 Saether Bernt1997 Sain20000 Sala1982 j Sala1985lv Sala1987w Sala1988x Sala1989t Sala1992 Sala19949l Sala1994\ Sala19959 Sala19969D Sala19969{ Sala19969 Sala19969c Sala19971d Sala1997^ Sala19989r Sala1998q Sala19989_ Sala19999e Sala19999 Sala19999[ Sala20000h Sala2000v Sala20000i Sala2001k Sala200100 Saliendra19942 Saliendra1995@ Salih1999D Salleo1996 Sandgren1983 Sandquist1991 Sandquist1992 Sandquist1992 Sandquist1993 Sandquist1994 Sandquist1995 Sandquist1995FSaracino19989 Sarig1993 Schfer2000 Schenk20000N Schimel David1995 Schlesinger1987k Schlesinger2001 Schmidt1999 Schmidt1999 Schmitt1986D Schulze1996{ Schulze1996 Schulze1996Schuster19919Schuster1992Schuster19929Schuster19933Schuster19949 Schwinning2000l Schwinning2001 Schwinning2002 Scrimgeour1999XSeligman1998i Semmartin2001 Sharifi1990 Shein1995!Sherrill1990 Sherwin1999w Shmida1988 Shmida1990( Shmida1990hSilver Whendee2000 Simunek2001} Singh1996 Singh1998I Skerbek1988! Smedley1990 Smith1995c Smith1997 Smith2001 Snyder2000> Sobrado1992x Soriano1989q Sosebee1993p Sosebee1993 Sosebee1993r Sosebee1994 Sosebee1995s Sosebee1996t Sosebee1998 Sosebee2000 Sparks19972 Sperry1988( Sperry1988 Sperry1989t Sperry1990t( Sperry1993) Sperry19930 Sperry1994+ Sperry19944 Sperry199552 Sperry199593 Sperry19965 Sperry199776 Sperry19988 Sperry19997 Sperry1999J Sperry2000 Sperry2000 Sperry20000 Sperry200009 Squeo1988; Squeo1998< Squeo1999x Stadelmann1974 Stark1994 Steinberger1993 Steiner2000 Stephan2001Stephens1993 Stewart1999c Stewart1999 Stirzaker1996E Strauss1988@Sugimoto1999+Sullivan1994Svensson1999 Sykes2000k Teeri1995 Tegeder1994Tenhunen2000h Terri1993Theodose1997BThompson1995%Thompson2000Thorburn1995 Thulke20000 Thurow1995h Tiedje2000d Tielborger1997  Tielbrger2000 Ting20000 Todd19977 Todd19988 Tremmel1999 Tremmel19995 Turkington20014 Turner19868 Turner1990( Turner19900  Tyree1988 Tyree1988 Tyree1989 Tyree1990Unsworth1999f Valentini1992B Valone19959 Valone1997hvan der Putten Wim2000 Van Devender1990hvan Veen Johannes2000van Wijk2001G Varnamkhasti1995 Vasquez1996 Vavrek1996~ Veenendaal19969 Villagran1988 Vinton19989(Virginia1996Virginia19999Virginia1999Vitousek1998k Vogel1995 Vrugt2001J Walker1985 Walker1996 Walker2001k Wall2001eh Wall Diana2000Wx Walter1974p Wan1993q Wan1993 Wan1993r Wan1994 Wan1995s Wan1996t Wan1998 Wan2000 Wand1999 Wang1995 Ward20000h Wardle David2000m Watson1997V Watts1998k Wear2001e Webb19922 Weber Gerhard2001 Welker19989 Weltzin1997 Weltzin1999 Weltzin2000 West1994 Wester20000 Westoby1997 White1977Whitford1989Whitford1993(Whitford1996<Whitford1996Whitford1997Whitford20000 Wiegand1998 Wiegand2000 Wiegand2000 Wiegand2000 Wiegand2001 Wiegand2001 Wight1994 Wilcox19989 Wilcox19989 Wilcox1999\Wilcox Bradford1996 Willgoose2001Williams19959Williams19969Williams2000Williams20009Williams2000Williams2001h Wolters2000Wondzell1995Wondzell1996; Wraith1995 Wu1999 Xiao1995 Xiao1996a Xiao1996 Xiao2001 Xu20000 Yan2000u Yanai1997 Yang19969 Yang Anthony1993[ Yeakley1994 Yeakley1994 Yifeng1996 Yoder1995 Yoder1999 Yoder2000 Yoder2001 Yoder2002h Zak1993k Zak1995n Zak1996o Zak1998M Zakir1987 Zartman2000 Zhang1999D Zippo199699D Zippo199699D Zippo1996D Zippo1996D Zippo199699D Zippo199699D Zippo199699D Zippo1996o1996on1999f Sykes2000 Tegeder1994*Tenhunen2000h Terri James1993Theodose1997BThompson1995%Thompson2000 Thulke20000 Thurow1995h Tiedje2000d Tielborger1997  Tielbrger2000 Tremmel1999) Tremmel19995 Turkingtonb4 Turner19868 Turner1990( Turner19900  Tyree1988f Valentini1992B Valone19959 Valone1997hvan der Putten Wim2000 Van Devender1990hvan Veen Johannes2000G Varnamkhasti1995 Vasquez1996 Vavrek1996~ Veenendaal19969 Villagran1988 Vinton19989(Virginia1996 Virginia19999)Virginia19994W1995J Walker1985k Wall2001eh Wall Diana2000W Wan2000 Wand1999 Wang1995 Ward20000h Wardle David2000mV Watts1998k Wear2001e Webb19922 Weber Gerhard2001 Welker19989 Weltzin Jake1997 Weltzin Jake1999 Weltzin Jake2000 West1994 Wester20000Whitford1989Whitford1993(Whitford1996<Whitford1996Whitford1997 Wiegand1998 Wiegand2000 Wiegand2000 Wiegand2000 Wiegand2001 Wiegand2001 Wight1994\Wilcox Bradford1996`Wilcox Bradford1998Williams2000Williams2000h Wolters2000; Wraith1995 Xiao1995 Xiao1996a Xiao1996 Xiao2001 Yan2000 Yang19969 Yang Anthony1993[ Yeakley1994 Yifeng1996 Yoder1995 Yoder1999h Zak Donald1993M Zakir1987 Zartman2000D Zippo1996 {p '(x8Ud+Cr|go/?lLivhX#,>I7tc$@[~ !JADks "jN<M0WbP\%4O]&z*3G-qY1`a.9Z_umE^n2B:6;5FHQRTS )=== AuthorsJournals#Keywords   '9 (x.>gC? &""I!$o/ `&AJ:0j0Ypd84~-*3Gl1{ 1MGI1<11]8'h6 )$2B;9O5FH))=WQ<@'E,H,$'P33+CNLG<0bRTSZ]DA?P*AC'4%##4D,\nks7tcsn7U^6N~.'Vavrek, M. C. McGraw, J. B. Yang, H. S.r 1996jcWithin-population variation in demography of Taraxacum officinale: Maintenance of genetic diversityEcology777c 2098-21072Differential response of genotypes to temporal environmental heterogeneity may contribute to the long-term persistence of those genotypes within a population. We tested whether season-dependent fitness components of genotypes could be responsible for the maintenance of genetic diversity within a population. Clonal replicates of five genotypes of Taraxacum officinale were planted in each of four seasons (spring-April to July, summer-July to October, autumn-October to January, and winter-January to April). Individuals were planted directly into natural, field vegetation. Fitness components (establishment, survival, growth, seed production, and leaf area dominance) and an integrated measure of fitness (finite rate of increase) were measured for each genotype in each season. Differential genotypic responses to seasons were observed for all fitness components. Despite seasonal differences, genotypic performances summed across seasons were equivalent. This result indicates that temporal heterogeneity may have a substantial effect on within-population genetic structure. The persistence of genotypes of contrasting seasonal performance through time is expected if long-term fitness values remain similar.e6/Veenendaal, E. M. Ernst, W. H. O. Modise, G. S. 1996}Effect of seasonal rainfall pattern on seedling emergence and establishment of grasses in a savanna in south-eastern BotswanaE"Journal of Arid Environments323305-317,The emergence and spatial distribution of grass seedlings were studied in a degraded savanna in south-eastern Botswana. When the rainy season started early, several separate establishment opportunities occurred, but with late rains only one germination wave was observed. Small rainfall events of 10-12 mm were unable to trigger germination. The phenology of emergence confirms the presence of a drought avoidance syndrome in this semi-arid climate. Seedling densities varied from a few in perennial Eragrostis rigidior grassland to 3000 m-2 in annual Tragus berteronianus grassland influenced by runoff processes. Seedlings emerged mainly from the top 1 cm of the soil. Vertical distribution of seeds showed a concentration in the top 1 cm of soil and the litter layer. Grass seedling survival was positively correlated with seed size. The importance of seedlings for the regeneration of grasslands is discussed.eD)|Zmicroarthropods microclimatemicroorganismsmineral-nutritionmodel modeling modelling models moisturemoisture transport moisture use mojave desert monosperma multicomponent decompositionmycorrhizal fungin-15 nasa/dao new-mexiconino southern oscillation nitrate nitrogennitrogen availabilitynitrogen mineralizationnitrogen uptake north-america north-atlantic oscillationnorthern new-mexico oak hybrids oak standosmotic adjustmentosmotic pressure overstory pacific pallidumparameterizationpatagonian steppe pattern patternspercolation theory perspectivephosphate uptakephotosynthesis picea-abiespinonpinon-juniper woodlandPinus Pinus taeda pinus-edulis pinyonpinyon-juniper woodlandsplant plant community compositionplant functionalplant mortalityplant responses plant sizeplant water uptakeplant-communities plant-roots plantsponderosa pine hillslope poplarpopulus-balsamiferapotential evaporationpotential gradients precipitationprecipitation regime productivityProsopis glandulosaPseudoroegneria spicata quercus 1201-1215$://A1997YD79000015>7Breshears, D. D. Rich, P. M. Barnes, F. J. Campbell, K.b[Overstory-imposed heterogeneity in solar radiation and soil moisture in a semiarid woodlandoEcological Applications  Ecol. Appl.n 1997 Novn74YD790 ECOL APPLISI:A1997YD79000015A289-299$://A1997XK15200003PNHBreshears, D. D. Myers, O. B. Johnson, S. R. Meyer, C. W. Martens, S. N.~Differential use of spatially heterogeneous soil moisture by two semiarid woody species: Pinus edulis and Juniperus monospermaJournal of EcologyJ. Ecol. 1997 Jun853Y XK152 J ECOLISI:A1997XK15200003A 4244-4255e$://000165989100002 2,Rajagopalan, B. Cook, E. Lall, U. Ray, B. K.Spatiotemporal variability of ENSO and SST teleconnections to summer drought over the United States during the twentieth centuryJournal of Climatenino southern oscillation; intraseasonal extreme rainfall; north-atlantic oscillation; sea-level pressure; el-nino; surface-temperature; interdecadal variability; interannual variability; precipitation regime; decadal variabilityPresented are investigations into the spatial structure of teleconnections between both the winter El Nino-Southern Oscillation (ENSO) and global sea surface temperatures (SSTs), and a measure of continental U.S. summer drought during the twentieth century. Potential nonlinearities and nonstationarities in the relationships are noted. During the first three decades of this century, summer drought teleconnections in response to SST patterns linked to ENSO are found to be strongest in the southern regions of Texas, with extensions into regions of the Midwest. From the 1930s through the 1950s, the drought teleconnection pattern is found to extend into southern Arizona. The most recent three decades show weak teleconnections between summer drought over southern Texas and Arizona, and winter SSTs, which is consistent with previous findings. Instead, the response to Pacific SSTs shows a clear shift to the western United States and southern regions of California. These epochal variations are consistent with epochal variations observed in ENSO and other low-frequency climate indicators. This changing teleconnection response complicates statistical forecasting of drought.J. Clim. 20001324385EF J CLIMATEISI:000165989100002\Water sources used by riparian trees varies among stream types on the San Pedro River, Arizona,%Snyder Keirith, A. Williams David, G.cF?AgriculturalJDWiegand, Thorsten Milton Suzanne, J. Esler Karen, J. Midgley Guy, F. 2000Live fast, die young: Estimating size-age relations and mortality pattern of shrubs species in the semi-arid Karoo, South Africa Plant Ecology 150 1-2i115-131.JDWe present a technique for estimating size-age relations and size-dependent mortality patterns of long-lived plants. The technique requires two sets of size data of individual (non-marked) plants that should be collected with a time-lag of several years in the same area of a study site. The basic idea of our technique is to assume general (three parameter) families of size-dependent functions which describe growth and mortality that occurred between the two data gathering events. We apply these growth and mortality functions to the size data of the early data set and construct predicted size-class distributions to compare it, in a systematic way, to the size-class distribution of the later data set. In a next step we calculate the size-age relations from the resulting growth functions, which yield the smallest difference between observed and predicted size-class distribution. Applying this technique to size data of five dominant shrub species at the Tierberg study site in the semiarid Karoo, South Africa produced new insight into the biology of these species which otherwise cannot be obtained without frequent measurements of marked plants. We could relate characteristics of growth behavior and mortality, for certain subgroups of the five species, to the life-history attributes evergreen vs. deciduous, succulent vs. woody, and early reproductive vs. late reproductive. The results of our pilot-study suggest a broad applicability of our technique to other shrublands of the world. This requires at least one older record of (individual) shrub-size data and performance of resampling.^WPatterns and causes of spatial variation of the reproductive success of a desert annualAKadmon, R. Shmida, A.e Oecologian831139-144S 1990d]This study examines patterns and causes of variation in the reproductive success of the desert annual Stipa capensis. Three nested scales of variation were analyzed: variation between individuals of the same plot, variation between different plots of the same habitat, and variation between different habitats in the same region. Perturbation experiments (irrigation and neighbors removal) were performed to test the effects of heterogeneity in soil water and neighborhood competition on the magnitude of variation in each scale. The results demonstrate that variation of reproductive success was highest within plots, lowest between plots, and moderate between habitats. Soil water heterogenity contributed to spatial variation in all scales but was mot important for differences between habitats. Neighborhood competition increased the variation within plots, but decreased the variation between habitats. The results further demonstrate that water limitation was negatively correlated with the position of the habitat along the run-off/run-on gradient. An opposite trend was obtained for the effect of competition. Kadmon, Ronen 1993VOPopulation dynamic consequences of habitat heterogeneity: An experimental studyEcology743816-825c4.Population dynamic consequences of habitat heterogeneity were investigated in a population of the desert annual Stipa capensis by measuring demographic responses of subpopulations inhabiting different habitats (slopes, depressions, and wadis) to natural and experimental changes in the amount of yearly rainfall. The results indicate that rainfall fluctuations affect the dynamics of the studied population by influencing both the percentage of germination and the number of seeds produced per germinated plant. However, the effect of changes in rainfall on both demographic parameters depends on habitat conditions, with slope subpopulations exhibiting the largest, and wadi subpopulations the smallest, effects. The fact that demographic responses to rainfall fluctuations are habitat dependent has two major implications. First, subpopulations inhabiting different habitats show considerable differences in their year-to-year fluctuations in density. Secondly, since seed production per seedling is habitat dependent, the distribution of the seedling population among the various habitats is a major determinant of the total number of seeds produced by the population in a given year. The results further indicate that most of the seeds (75-99.9%, depending on rainfall conditions) are produced in the depressions and the wadis, which taken together account for only 10% of the total area. This finding indicates that the ecological conditions in these spatially restricted habitats are critical for the dynamics of the whole population. The overall results suggest that taking into account factors such as the number and types of habitats available, the relative area occupied by each habitat and the distribution of the individuals among the available habitats may be important in explaining observed patterns of population dynamics.t that was reflected in only little variation in the RUE during the period of study. Thus, in much of the region, NPP seems to be in step with rainfall, recovering rapidly following drought and not supporting the fears of widespread, subcontinental scale desertification taking place in the 9-year period that is studied. In fact the results show a small but systematic increase in RUE for the Sahel as a whole from 1982 to 1990, although some areas contained within the region did have persistently low values.tJs derived between 26 and 33% of its transpiration water from upper soil layers. Similarly, at the ephemeral stream site during the summer rainy period, Prosopis velutina derived a greater fraction of its transpiration water from upper soil layers, than at a perennial stream site where groundwater depth was less than 2 m. Measurements of transpiration flux combined with stable isotope data revealed that Populus fremontii transpired a greater quantity of water from upper soil layers at the ephemeral stream site than at the perennial stream site. These results imply that transpiration from groundwater and unsaturated soil layers by riparian vegetation may depend on the interaction between site conditions and species assemblage.hange and plant water potential corresponded to the seasonality of rainfall at different sites. However, no correlation between a species' ability to use summer rainfall and its tolerance to water deficits at4-Wiegand, T. Jeltsch, F. Bauer, S. Kellner, K.9 1998D=Simulation models for semi-arid rangelands of southern Africa2+African Journal of Range and Forage Science15 1-2N 48-603vpIn semi-arid regions, the effects of grazing or sparing management on natural communities of long-lived plants generally take decades to become evident. Event-driven dynamic behaviour, unpredictable and low rainfall, and complicated interactions between species make it difficult to gather sufficient understanding of vegetation dynamics to be able to develop guidelines for sustainable management. Simulation models that consider the essential processes that determine vegetation dynamics offer scope for quantitatively exploring long-term vegetation dynamics of arid and semi-arid rangelands. In this paper we review three models that were aimed to provide an understanding of the vegetation dynamics and management of different typical vegetation types in South Africa, including the Karoo shrubland, the shrub-grassland of the southern Kalahari, and pure semi-arid grasslands.HO Archer, Steve 1995vpTree-grass dynamics in a Prosopis-thornscrub savanna parkland: Reconstructing the past and predicting the future Ecoscience21 83-99aAlthough trends toward increased woody plant abundance in grasslands and savannas in recent history have been reported worldwide, our understanding of the processes involved is limited. Here I review and integrate a series of studies which quantify the rates, dynamics, spatial patterns and successional processes involved in tree patch and woody plant community development at a savanna parkland site in southern Texas, U.S.A. Stable carbon isotope ratios of soil organic carbon indicate C-3 woody plants currently occupy sites once dominated by C-4 grasses. Historical aerial photographs (1941-1990), tree ring analysis and plant growth models all indicate this displacement has occurred over the past 100 to 200 years. Succession from grass-to woody plant-domination occurs when the N-2-fixing arborescent, honey mesquite (Prosopis glandulosa (Torr.) var. glandulosa), invades and establishes in herbaceous patches. Over time, this plant modifies soils and microclimate to facilitate the ingress and establishment of additional woody species. The result is a landscape comprised of shrub clusters of varying ages organized around a Prosopis nucleus. As new clusters form and existing clusters enlarge, coalescence occurs. This process appears to be in progress on upland portions of the landscape and has progressed to completion on lowlands. Rates of cluster development and patterns of distribution appear regulated by subsurface variations in clay content and by variations in annual rainfall. Simulation models based on reconstructions and forward projections indicate succession from grassland to woodland steady states would require 400-500 years, with the most dramatic changes occurring over a 200-year period. The shrubs initially facilitated by Prosopis appear to contribute to its demise and prevent its re-establishment. Structure and function of future communities may therefore depend on how remaining woody plants react to changes in microclimate and nitrogen cycling that occur after Prosopis is gone.$Austin, A. T. Vitousek, P. M. 1998@9Nutrient dynamics on a precipitation gradient in Hawai'i. Oecologia. 113519-529We evaluated soil and foliar nutrients in five native forests in Hawai'i with annual rainfall ranging from 500 mm to 5500 mm. All of the sites were at the same elevation and of the same substrate age-, all were native-dominated forests containing Metrosideros polymorpha Gaud. Soil concentrations of extractable NO3-N and PO4-P, as well as major cations (Ca, Mg, and K), decreased with increasing annual precipitation, and delta15N values became more depleted in both soils and vegetation. For M. polymorpha leaves, leaf mass per area (LMA) and lignin concentrations increased significantly, while delta13C values became more depleted with increasing precipitation. Foliar phosphorus, and major cation (Ca, M-, and K) concentrations for M. polymorpha all decreased significantly with increasing precipitation. For other native forest species, patterns of LMA, delta13C, and delta15N generally mirrored the pattern observed for M. polymorpha. Decreasing concentrations of available rock-derived nutrients in soil suggest that the effect of increased rainfall on leaching outweighs the effect of increasing precipitation on weathering. The pattern of decreased foliar nutrient concentrations per unit leaf area and of increased lignin indicates a shift from relatively high nutrient availability to relatively high carbon gain by producers as annual precipitation increases. For nitrogen cycling, the pattern of higher inorganic soil nitrogen concentrations in the drier sites, together with the progressively depleted delta15N signature in both soils and vegetation, suggests that nitrogen cycling is more open at the drier sites, with smaller losses relative to turnover as annual precipitation increases. Austin, A. T. Sala, O. E.. 1999b[Foliar delta15N is negatively correlated with rainfall along the IGBP transect in Australia,&Australian Journal of Plant Physiology26293-295o? >(!Mahoney, John M. Rood, Stewart B.t 1998XQStreamflow requirements for cottonwood seedling recruitment: An integrative modelWetlands . Dec.184634-645~xThis paper describes the 'recruitment box,' an integrative model that defines the stream stage patterns that enable successful establishment of riparian cottonwood seedlings. In western North America, cottonwood seed dispersal generally occurs after annual peak river flows. The receding stream exposes moist sites upon which seeds land after transport by wind and water. Germination is rapid, and initial seedling establishment is often prolific. However, the vast majority of seedlings die, primarily due to drought stress, as root growth is insufficient to maintain contact with the receding zone of moisture. Cottonwood roots grow about 0.5 to 1 cm per day or 60 to 100 cm in the first year. Along the 'losing' streams in semi-arid regions, the riparian water table is an almost horizontal extension from the stream stage. A capillary fringe exists above the water table and is often 30 to 40 cm in elevation, but can range from about 5 to 130 cm depending on substrate texture. The combinati on of root growth and capillary fringe define the successful recruitment band, which is usually from about 0.6 to 2 m in elevation above the late summer stream stage. Within this range, higher elevation establishment occurs (i) for the Aigeiros cottonwoods, Populus deltoides, and P. fremontii, which grow more rapidly than Tacamahaca species and occur in warmer areas with longer growing seasons; (ii) along larger rivers that are characterized by more gradual stage fluctuations; and (iii) along streams with finer substrate. The rate of stream stage decline is also critical for seedling survival and should not exceed 2.5 cm per day. The recruitment box model is consistent with dendrochronological interpretations that moderate flood events are naturally required for cottonwood recruitment. Flood events with recurrences of about 1 in 5 to 1 in 10 years often satisfy the model and provide stream stage patterns with a gradual decline through the recruitment box. The model will facilitate analyses of the reproductive ecology of riparian cottonwoods and also permit the prescription of stream stage patterns for cottonwood seedling recruitment along dammed rivers.the key factor controlling invasibility, the susceptibility of an environment to invasion by non-resident species. The theory is mechanistic and quantitative in nature leading to a variety of testable predictions. 3 We conclude that the elusive nature of the invasion process arises from the fact that it depends upon conditions of resource enrichment or release that have a variety of causes but which occur only intermittently and, to result in invasion, must coincide with availability of invading propagules.Ecosystem responses to changes in plant functional type composition: An example from the Patagonian Bowers, Janice E. 1997f_Demographic patterns of Ferocactus cylindraceus in relation to substate age and grazing history Plant Ecology 1331 37-48Three subpopulations of Ferocactus cylindraceus, a short-columnar cactus of the Sonoran and Mojave deserts, were sampled in Grand Canyon, Arizona, USA, at sites representing a range of substrate ages and different grazing histories. Age-height relations were determined from annual growth, then used to estimate probable year of establishment for each cohort. Eight years between 1944 and 1992 were especially favorable for establishment. Six of these 8 years coincided with El Nino-Southern Oscillation conditions, indicating that as for many woody plants in arid regions, somewhat unusual climatic conditions are necessary if populations are to replace themselves. Comparison of age structures showed that established and developing populations have somewhat different dynamics in that the rate of population increase was slowest on the youngest terrace. On the ancient terraces, about half the plants were less than 25 years old. Plants older than 40 years were few; however the oldest plants in the study (about 49 years) grew on the ancient terraces. On the recent terrace, 76% of the subpopulation was 25 years or younger, and the oldest living plant was about 36 years of age. The age structures of subpopulations on grazed and ungrazed sites also differed markedly. On ungrazed sites, subpopulations were more or less at equilibrium, with enough young plants to replace old ones as they died. In contrast, the subpopulation on the grazed site was in a state of marked disequilibrium. Grazing before 1981 largely extirpated a palatable subshrub that was probably an important nurse plant. Until the shrub population at Indian Canyon recovers from decades of burro grazing, a rebound in F. cylindraceus establishment is not to be expected.a X189-204$://000170837100013o,%Li, K. Y. De Jong, R. Boisvert, J. B.nLEAn exponential root-water-uptake model with water stress compensation7Journal of Hydrologyroot-water-uptake model; soil water simulation; root distribution; SWAP soil-water; moisture use; crop; extraction; fertilizer; simulation; patterns; balance; field; wheat Modelling soil water flow under cropped conditions requires a description of water uptake by plant roots. Most macroscopic root-water-uptake models distribute potential transpiration across the root zone based on the root distribution pattern, without accounting for the distribution of water stress in the soil profile. An exponential root-water-uptake model was modified by incorporating a weighted stress index which accounts for both root distribution and soil water stress. This new model. called water stress compensating exponential root- water-uptake model, is represented as a function of potential transpiration, soil water availability and root-length density. The root length fraction in the various soil layers can be estimated from its value in the surface (0-10 cm) layer. Both the exponential- and the water stress compensating- root water uptake models were incorporated in the Soil-Water-Atmosphere- Plant (SWAP) simulation model, and tested against soil water content data from a long-term crop rotation experiment in the semi-arid region of western Canada. The results showed that the new water stress compensating model simulated soil water contents significantly better than the exponential model. especially during the second half of the growing season at the lower depths (60-120 cm). The model is user-friendly and application oriented, and can be used for various cereal crops. (C) 2001 Elsevier Science B.V. All rights reserved.r J. Hydrol. 2001 Oct 31 252e 1-4v469VQ J HYDROLISI:000170837100013n<5Lin, Guanghui Phillips, Susan L. Ehleringer, James R. 1996d^Monsoonal precipitation responses of shrubs in a cold desert community on the Colorado Plateau Oecologia 106e1 8-17 zSouth-eastern Utah forms a northern border for the region currently influenced by the Arizona monsoonal system, which feeds moisture and summer precipitation into western North America. One major consequence predicted by global climate change scenarios is an intensification of monsoonal (summer) precipitation in the arid land areas of the western United States. We examined the capacity of dominant perennial shrubs in a Colorado Plateau cold desert ecosystem of southern Utah, United States, to use summer moisture inputs. We simulated increases of 25 and 50 mm summer rain events on Atriplex canescens, Artemisia filifolia, Chrysothamnus nauseosus, Coleogyne ramosissima, and Vanclevea stylosa, in July and September with an isotopically enriched water (enriched in deuterium but not 180). The uptake of this artificial water source was estimated by analyzing hydrogen and oxygen isotope ratios of stem water. The predawn and midday xylem water potentials and foliar carbon isotope discrimination were measured to estimate changes in water status and water-use efficiency. At. canescens and Ch. nauseosus showed little if any uptake of summer rains in either July or September. The predawn and midday xylem water potentials for control and treatment plants of these two species were not significantly different from each other. For A. filifolia and V styles, up to 50% of xylem water was from the simulated summer rain, but the predawn and midday xylem water potentials were not significantly affected by the additional summer moisture input. In contrast, C. ramosissima showed significant uptake of the simulated summer rain ( gt 50% of xylem water was from the artificial summer rain) and an increase in both predawn and midday water potentials. The percent uptake of simulated summer rain was greater when those rains were applied in September than in July, implying that high soil temperature in midsummer may in some way inhibit water uptake. Foliar carbon isotope discrimination increased significantly in the three shrubs taking up simulated summer rain, but pre-treatment differences in the absolute discrimination values were maintained among species. The ecological implications of our results are discussed in terms of the dynamics of this desert community in response to changes in the frequency and dependability of summer rains that might be associated with a northward shift in the Arizona monsoon boundary.y activities of the kangaroo rats or indirectly as a consequence of the increase in grass cover. No experimental effect on species diversity of winter annual dicots was detected. Our study site was located in a natural transition between desert scrub and grassland, where abiotic conditions and the effects of organisms may be particularly influential in determining the structure and composition of vegetation. Under these conditions kangaroo rats have a dramatic effect on plant cover and species composition.uzada del Muerto basin of the Chihuahuan Desert of southern New Mexico, USA, has undergone a marked transition of plant communities. Shrubs such as mesquite (Prosopis glandulosa) have greatly increased or now dominate in areas that were previously dominated by perennial grasses. The replacement of grasses by shrubs requires an establishment phase where small shrubs must compete dirCui, M. Caldwell, M. M. 1997Growth and nitrogen uptake by Agropyron desertorum and Pseudoroegneria spicata when exposed to nitrate pulses of different durationu,&Australian Journal of Plant Physiology2458637-642|vPlant growth and nitrate uptake were measured for two Great Basin perennial grasses, Agropyron desertorum and Pseudoroegneria spicata, in sand-filled pots in either monoculture or mixed culture (2 plants/pot). All plants were supplied with the same initial amount of nitrate but delivered in five different pulse durations ranging from 0.5 to 72 h. The pulse duration was controlled by flushing the pots with water at different times after applying the nitrate pulse. The same concentration of nitrate was used in all pulse treatments. Increasing the pulse duration led to significantly increased plant biomass production for both species in both mono- and mixed cultures, and to reduced root/shoot biomass ratio. Biomass and root/shoot ratio were greater for Agropyron in mixed culture than in monoculture. To assess root nitrate uptake capacity, a 30-min tracer pulse was applied to all plants. Plants that had been exposed to longer pulses acquired significantly more nitrate than those that had been exposed to shorter pulses for both total plant acquisition and acquisition per unit root length. With greater root/shoot ratio than Pseudoroegneria, total nitrate acquisition by Agropyron was significantly greater at most pulse durations. Root nitrate uptake per unit mass was also greater for Agropyron than for Pseudoroegneria, indicating that Agropyron is more responsive to nitrate pulsing. 173-184$://000173912100002yD>Ryel, R. J. Caldwell, M. M. Yoder, C. K. Or, D. Leffler, A. J.Hydraulic redistribution in a stand of Artemisia tridentata: evaluation of benefits to transpiration assessed with a simulation modelr Oecologianhydraulic lift/redistribution; soil-water model; root distribution; water use; Artemisia tridentata water-use; soil-water; lift; roots; transport; plants; acquisition; rhizosphere; balance; efflux\VThe significance of soil water redistribution facilitated by roots (an extension of "hydraulic lift", here termed hydraulic redistribution) was assessed for a stand of Artemisia tridentata using measurements and a simulation model. The model incorporated water movement within the soil via unsaturated flow and hydraulic redistribution and soil water loss from transpiration. The model used Buckingham-Darcy's law for unsaturated flow while hydraulic redistribution was developed as a function of the distribution of active roots, root conductance for water, and relative soil-root (rhizosphere) conductance for water. Simulations were conducted to compare model predictions with time courses of soil water potential at several depths, and to evaluate the importance of root distribution, soil hydraulic conductance and root xylem conductance on transpiration rates and the dynamics of soil water. The model was able to effectively predict soil water potential during a summer drying cycle, and the rapid redistribution of water down to 1.5 m into the soil column after rainfall events. Results of simulations indicated that hydraulic redistribution could increase whole canopy transpiration over a 100-day drying cycle. While the increase was only 3.5% over the entire 100-day period, hydraulic redistribution increased transpiration up to 20.5% for some days. The presence of high soil water content within the lower rooting zone appears to be necessary for sizeable increases in transpiration due to hydraulic redistribution. Simulation results also indicated that root distributions with roots concentrated in shallow soil layers experienced the greatest increase in transpiration due to hydraulic redistribution. This redistribution had much less effect on transpiration with more uniform root distributions, higher soil hydraulic conductivity and lower root conductivity. Simulation results indicated that redistribution of water by roots can be an important component in soil water dynamics, and the model presented here provides a useful approach to incorporating hydraulic redistribution into larger models of soil processes. Oecologia 2002 Jan 1302522TB OECOLOGIAISI:000173912100002LZTSchuster, William S. F. Sandquist, Darren R. Phillips, Susan L. Ehleringer, James R. 1992rkComparisons of carbon isotope discrimination in populations of aridland plant species differing in lifespan Oecologia913332-337F@Carbon isotope discrimination (DELTA) was compared between populations of dominant perennial plant species, differing in life expectancy, in two deserts with contrasting vegetation types. In both deserts, plants of the shorter-lived species showed significantly higher DELTA and greater intrapopulation variance in this character compared to the long-lived species. These results indicate underlying differences in gas-exchange physiology, and suggest a positive correlation between water-use efficiency and lifespan in desert plants. Differences in variance for this character may reflect greater microenvironmental variation experienced by shorter-lived plants and/or different forms of selection acting on water-use traits. Spatial distributions were significantly clustered for the shorter-lived species and significantly uniform for the long-lived species, indicating that competition has been important the development of the long-lived populations. The long-lived Larrea tridentata showed a significant, negative correlation between DELTA and Thiessen polygon area, suggesting a positive relationship between water use efficiency and longevity within this species. This relationship was weakly supported in the other warm desert species, Encelia farinosa, but was not observed within populations of the cold desert species, Gutierrezia microcephala and Coleogyne ramosissima. These results suggest that DELTA reflects key aspects of plant metabolism related to lifespan; these differences may ultimately influence interactions among desert plants and the structure of desert plant communities.(!`0w"A. Shmida T. L. Burgess 1988JDPlant growth form strategies and vegetation types in arid ecosystems HBM. J. A. Werger P. J. M. v.d. Aart H. J. During J. T. A. Verhoeven*#Plant Form and Vegetation Structure  The Haguel SPB Academic Publishing 211-241.4.Singh, J. S. Milchunas, D. G. Lauenroth, W. K. 1998JCSoil water dynamics and vegetation patterns in a semiarid grasslands Plant Ecologyh 134 1n 77-89/tmLong-term (1985-1992) dynamics and spatial variations in soil water below the evaporative zone were evaluated for a shortgrass steppe with a low and variable precipitation regime. Each of a sandy loam, clay loam, and two sandy clay loam sites comprised a toposequence with upland, midslope and lowland positions. Soil water was monitored at 15 cm intervals providing estimates covering 22.5 to 97.5 cm depths. Soil water throughout the profile was highest in the clay loam site and lowest in the sandy loam site. However, stored soil water did not vary systematically among slope positions. Total vegetation cover was highest on the lowland in two sites, but was greatest on the midslope position in the other two. Total vegetation cover was greatest on the CL site, which was the wettest in terms of soil water. Soil water depletion was related to the depth-distribution of roots. There was an inverse relationship between aboveground production and soil water content of the 30, 45 and 60 cm layers during the growth period. Root distributions through the profile did not, however, vary with soil texture or with different soil water profiles controlled by texture. The less variable water content of deeper soil layers is a resource which potentially buffers the impact of pronounced variability in precipitation and thus contributes to vegetation stability of the shortgrass community.F?Seasonal carbon isotope discrimination in a grassland community?piSmedley, M. P. Dawson, T. E. Comstock, J. P. Donovan, L. A. Sherrill, D. E. Cook, C. S. Ehleringer, J. R. Oecologiai853r314-320s 1990pjGrassland communities of arid western North America are often characterized by a seasonal increase in ambient temperature and evaporative demand and a corresponding decline in soil moisture availability. As the environment changes, particular species could respond differently, which should be reflected in a number of physiological processes. Carbon isotope discrimination varies during photosynthetic activity as a function of both stomatal aperture and the biochemistry of the fixation process, and provides an integrated measure of plant response to seasonal changes in the environment. We measured the seasonal course of carbon isotope discrimination in 42 grassland species to evaluate changes in gas exchange processes in response to these varying environmental factors. The seasonal courses were then used to identify community-wide patterns associated with life form, with phenology and with differences between grasses and forbs. Significant differences were detected in the following comparisons: (1) Carbon isotope discrimination decreased throughout the growing season; (2) perennial species discriminated less than annual species; (3) grasses discriminated less than forbs; and (4) early flowering species discriminated more than the later flowering ones. These comparisons suggested that (1) species active only during the initial, less stressful months of the growing season used water less efficiently, and (2) that physiological responses increasing the ratio of carbon fixed to water lost were common in these grassland species, and were correlated with the increase in evaporative demand and the decrease in soil moisture.>7Smith, S. D. Herr, C. A. Leary, K. L. Piorkowski, J. M.E 1995tnSoil-plant water relations in a Mojave Desert mixed shrub community: A comparison of three geomorphic surfaces"Journal of Arid Environments293t339-351p*#Comparative plant water relations and soil moisture content of three geomorphic surfaces were assessed in a northern Mojave Desert mixed shrub community. The adjacent geomorphic surfaces studied were an ephemeral wash (Wash), a dissected alluvial fan remnant (Bench), and a montane slope (Slope). Perennial vegetation transpired for 2-6 months during a typical precipitation year. Plant water relations differed between species (on the same geomorphic surface) and between surfaces (for the same species). Plant water stress was greatest on the Bench, which had the finest textured soils and was underlaid by an indurated petrocalcic layer. Plants from the Wash and Slope sites had higher water potentials and stomatal conductances, presumably due to coarser textured, deeper soils in the Wash and water storage in fractured bedrock on the Slope. Soil water uptake patterns closely approximated relative transpiration on each surface. No evidence of deep percolation below the rooting zone was found on any of the three surfaces during a normal rainfall year.s& Smith, Melinda D. Knapp, Alan K. 2001vpPhysiological and morphological traits of exotic, invasive exotic, and native plant species in tallgrass prairie.'International Journal of Plant Sciencese 162e4n785-792. We compared 13 traits of invasive exotic, noninvasive exotic, and ecologically similar native species to determine if there are generalizable differences among these groups that relate to persistence and spread of exotic species in tallgrass prairie plant communities. When species were grouped as invasive (two species), noninvasive (five species), and native (six species), no differences were found for the suite of traits examined, likely because of the high variability within and between groups. However, when exotic species, regardless of invasiveness, were compared with the native species, specific leaf area was ca. 40% higher for the exotic species, a result that is consistent with that of other studies. This pattern was also observed for five of seven pairwise comparisons of exotic and native species with similar life history traits. In contrast, total end-of-season biomass was as much as three times higher for the native species in five of seven of the native-exotic species pairs. For other traits, differences between exotic and native species were species-specific and were generally more numerous for noninvasive than for invasive exotic species pair-wise comparisons. Thus, contrary to predictions, exotic species capable of successfully invading tallgrass prairie did not differ considerably from native species in most traits related to resource utilization and carbon gain. Moreover, invasive exotic species, those capable of displacing native species and dominating a community, were not distinct for the observed traits from their native counterparts. These results indicate that other traits, such as the ability to respond to resource pulses or herbivory, may explain more effectively why certain invasive species are able to invade these communities aggressively.pse comparisons of exotic and native species with similar life history traits. In contrast, total end-of-season biomass was as much as three times higher for the native species in five of seven of the native-exotic species pairs. For other traits, differences between exotic and native species were species-specific and were generally more numerous for noninvasive than for invasive exotic species pair-wise comparisons. Thus, contrary to predictions, exotic species capable of successfully invading tallgrass prairie did not differ considerably from native species in most traits related to resource utilization and carbon gain. Moreover, invasive exotic species, those capable of displacing native species and dominating a community, were not distinct for the observed traits from their native counterparts. These results indicate that other traits, such as the ability to respond to resource pulses or herbivory, may explain more effectively why certain invasive species are able to invade these communities aggressively.p FLEReed, Daniel C. Ebeling, Alfred W. Anderson, Todd W. Anghera, Michele 1996zsDifferential reproductive responses to fluctuating resources in two seaweeds with different reproductive strategies2Ecologyn771300-316  H AReproduction is closely tied to environmental conditions and the availability of resources, and thus typically varies with season. Consequently, perennial organisms that reproduce continuously are generally restricted to tropical regions with relatively aseasonal climates. The temperate marine alga Macrocystis pyrifera is a rare exception in this regard, as most individuals reproduce throughout the year in a seasonally variable habitat. Here we measure reproductive responses of the giant kelp Macrocystis during a period in which resources and environmental conditions fluctuated greatly and contrast these responses with those of the palm kelp, Pterygophora californica, a sympatric species that exhibits strictly seasonal reproduction. The quantity and quality of spore production tracked resource availability within and among years for Macrocystis, but not for Pterygophora. Reproductive allocation and spore standing stock in Macrocystis were negatively correlated with seawater temperature and positively correlated with the nitrogen content of adult plants. Macrocystis generally displayed two seasonally distinct peaks in spore production per year (winter and spring). The only disruption of this pattern coincided with a warmwater El Nino event. Although seawater temperature and the nitrogen content of adults were inversely related in Pterygophora, neither variable was significantly correlated with the quantity or quality of spore production in this species. Unlike Macrocystis, Pterygophora exhibited a well-defined reproductive season in which plants displayed a single broad peak in spore production that varied little in timing and magnitude among years, even during El Nino conditions. Spore C/N ratios remained relatively constant over time in both species, despite large seasonal fluctuations in C/N ratios of vegetative tissue of adults plants. Nonetheless, spore C/N ratios were positively correlated with seawater temperature in Macrocystis, but not in Pterygophora. Spore viability (swimming and germination) varied considerably, and often unpredictably, over time for both species. Our results support the general idea that environmental conditions and resources exert a much greater influence on the quantity and quality of reproduction in species that reproduce continuously than on the majority of species that are strictly seasonal in onset of reproduction. The differential responses of Macrocystis and Pterygophora may reflect their different morphologies and life-spans. Macrocystis is relatively short lived and may "hedge its bets" by reproducing continuously rather than risk delaying reproduction. Conversely, since Pterygophora lives much longer, plants can afford to release spores only during times when the chances for reproductive success are predictably greatest because these plants are likely to reproduce again in subsequent years.lXDNHSalleo, Sebastiano Lo Gullo, M. Assunta De Paoli, Dorotea Zippo, Manuela 1996ngXylem recovery from cavitation-induced embolism in young plants of Laurus nobilis: A possible mechanismNew Phytologist5 132-16 47-56 Xylem recovery from cavitation-induced embolism was studied in 1-yr-old twigs of Laurus nobilis L. Cavitation was induced by applying-pre-established pressure differentials (DELTA-P-o-i) across the pit membranes of xylem conduits. DELTA-P-o-o were 1.13, 1.75 and 2.26 MPa, corresponding to about 50, 77 and 100 of the measured leaf water potential at the turgor loss point. DELTA-P-o-i, were obtained either by increasing xylem tensions or by applying positive pressures from outside, or by a combination of the two. The percentage loss of hydraulic conductivity (PLC) did not change, regardless of how the DELTA-P-o-i were obtained. This confirmed that xylem cavitation was nucleated by microbubbles coming from outside the vessels. Positive pressures, however, amplified (up to 75%) and sped up the xylem refilling (20 min) in comparison with that measured in unpressurized twigs (c.50% in 15 h). Twigs girdled proximally to their pressurized segment 1 min after the desired pressure value had been reached, did not recover from embolism. The later the twigs were girdled with respect to when they were tested for PLC, the higher was their recovery from embolism, suggesting that some messenger was transported in the phloem which stimulated xylem refilling. Indol-3-acetic acid (IAA) applied to the exposed cortex of both pressurized and unpressurized twigs, induced an almost complete recovery from PLC. We hypothesize that the refilling of cavitated xylem might be a result of an auxin-induced increase in the phloem loading with solutes. This would cause radial transport of solutes to cavitated xylem conduits via the rays, thus decreasing their osmotic potential and making them refill. No positive xylem pressure potentials were measured during xylem recovery from PLC.leSandquist, Darren R. Schuster, William S. F. Donovan, Lisa A. Phillips, Susan L. Ehleringer, James R. 1993xqDifferences in carbon isotope discrimination between seedlings and adults of southwestern desert perennial plants1Southwestern NaturalistG383s212-217l,%Carbon isotope discrimination (DELTA) is associated with long-term water-use efficiency (ratio of photosynthesis to transpiration), and varies within and among desert plant species. The variation of DELTA within aridland perennial plants may be related to life-history class attributes, while differences in DELTA between long- and shorter-lived species may be based on life-form dependent tradeoffs of growth and survival. We examined these patterns in two desert environments and asked if life-history class and life-form related differences in DELTA occurred in Coleogyne ramosissima, Larrea tridentata, Encelia farinosa, ambrosia dumosa and Gutierrezia microcephala. The mean DELTA values of seedlings were greater than those of adults for all species except G. microcephala. The difference between life-history classes may represent a shift from low water-use efficiency (high DELTA) during establishment to high water-use efficiency (low DELTA) as an adult. Our results suggest that this shift is based on genetically- and/or environmentally-induced ontogenetic changes in plant metabolism. Such changes may be beneficial if a high DELTA and low DELTA are favored at different developmental stages. Interspecific comparisons showed that adult long-lived plants had lower DELTA values than adults of shorter-lived species in the same community, but for seedlings the same pattern was found for only one pair of species. Thus the previously shown trend for long-lived species to have lower DELTA values than shorter-lived species may depend on the stage of development.e *onsediment transportJackson, R. B. Schenk, H. J. Jobbagy, E. G. Canadell, J. Colello, G. D. Dickinson, R. E. Field, C. B. Friedlingstein, P. Heimann, M. Hibbard, K. Kicklighter, D. W. Kleidon, A. Neilson, R. P. Parton, W. J. Sala, O. E. Sykes, M. T.l 2000RKBelowground consequences of vegetation change and their treatment in modelsiEcological Applications 102470-483p Apr Ecol. Appl.ISI:000086008300013belowground processes and global change, biogeochemistry, ecosystem models, global change, plant life forms, roots, shrub encroachment, soil carbon and nutrients, water balance~wThe extent and consequences of global land-cover and land-use change are increasingly apparent. One consequence not so apparent is the altered structure of plants belowground. This paper examines such belowground changes, emphasizing the interaction of altered root distributions with other factors and their treatment in models. Shifts of woody and herbaceous vegetation with deforestation, afforestation, and woody plant encroachment typically alter the depth and distribution of plant rests, influencing soil nutrients, the water balance, and net primary productivity (NPP). For example, our analysis of global soil data sets shows that the major plant nutrients C, N, P, and K are more shallowly distributed than are Ca, Mg, and Na, but patterns for each element vary with the dominant vegetation type. After controlling for climate, soil C and N are distributed more deeply in arid shrublands than in arid grasslands, and subhumid forests have shallower nutrient distributions than do subhumid grasslands. Consequently, changes in vegetation may influence the distribution of soil carbon and nutrients over time (perhaps decades to centuries). Shifts in the water balance are typically much more rapid. Catchment studies indicate that the water yield decreases 25-40 mm for each 10% increase in tree cover, and increases in transpiration of water taken up by deep roots may account for as much as 50% of observed responses. Because models are increasingly important for predicting the consequences of vegetation change, we discuss the treatment of belowground processes and how different treatments affect model outputs. Whether models are parameterized by biome or plant life form (or neither), use single or multiple soil layers, or include N and water limitation will all affect predicted outcomes. Acknowledging and understanding such differences should help constrain predictions of vegetation change.296DT ECOL APPL$://000086008300013_ NHBreshears, David D. Rich, Paul M. Barnes, Fairley J. Campbell, Katherine 1997b[Overstory-imposed heterogeneity in solar radiation and soil moisture in a semiarid woodlandEcological Applicationsn7e4o 1201-1215o Degradation of semiarid ecosystems is a major environmental problem worldwide, characterized by a reduction in the ratio of herbaceous to woody plant biomass. These ecosystems can be described as a set of canopy patches comprising woody plants and the intercanopy patches that separate them, yielding an overstory with intermediate closure. Field measurements of microclimate at the scale of canopy patches, particularly for near-ground solar radiation and soil moisture, are largely lacking from both nondegraded and degraded ecosystems. We tested for relationships among spatial patterns of the overstory, near-ground solar radiation, and soil moisture in a semiarid pinon-juniper woodland in northern New Mexico that had a highly heterogeneous overstory (apprxeq50% canopy cover) and was not degraded with respect to ground cover and erosion rates. We used measurements taken every 1 m along a 102-m transect-solar radiation indices were estimated monthly and annually using hemispherical photographs, and soil moisture was measured over 4 yr using time-domain reflectometry (TDR)-and analyzed the data using general least squares linear models that accounted for spatial autocorrelation and temporal heteroscedasticity. Time-averages of solar radiation and of soil moisture both were spatially autocorrelated at scales of up to 4 m (P < 0.05), corresponding approximately to the average lengths of both canopy and intercanopy patches and to the scale of spatial autocorrelation in the canopy/intercanopy pattern of the overstory (3 m; P < 0.05). For near-ground solar radiation, we found expected spatial variation between patches (canopy < intercanopy; P < 0.0001) and within patches for centers vs. edges (canopy center < canopy edge and intercanopy center > intercanopy edge; P < 0.0001) and for north vs. south edges (canopy north edge < canopy south edge and intercanopy south edge < intercanopy north edge; P < 0.0001). For soil moisture, canopy locations were significantly drier than intercanopy locations (P < 0.0001), and edge locations were significantly wetter than center locations both overall and within both patch types (P < 0.0001). Spatial heterogeneity in soil moisture was attributed primarily to canopy interception and drip on the basis of large differences in snow cover between canopy and intercanopy locations. Spatial autocorrelation in the residuals for soil moisture of up to 7 m was attributed to transpiration by woody plants at scales corresponding to belowground root distributions. The spatial heterogeneities in near-ground solar radiation and soil moisture are of sufficient magnitude to affect biotic processes of woody and herbaceous plants, such as growth and seedling establishment. Because land degradation problems in semiarid shrublands and woodlands appear to result from differential impacts to intercanopy vs. canopy patches, our results can be used to help design effective mitigation and remediation strategies. More generally, our results demonstrate how the physical presence of woody canopies reinforces spatial heterogeneity in microclimate and, because our site has intermediate closure of the overstory, bridge the gap along a grassland-forest continuum between related studies in relatively open savannas and in forests with nearly closed canopies.dFtpxtwj"Sala, O. E. Lauenroth, W. K. 1982D=Small rainfall events: an ecological role in semiarid regions Ecology}53301-304"Sala, O. E. Lauenroth, W. K. 1985`YRoot Profiles and the Ecological Effect of Light Rainshowers in Arid and Semiarid Regions"American Midland Naturalist 1142o406-408aA previous study reported results from an evaluation of root biomass under nine vegetation types in Montana [USA] and concluded that since root biomass in the top 1-cm soil layer was low, this should be taken as evidence that light rainshowers are of little value to plants. The objective of this note is to present an analysis which will test the generality of this conclusion. Our calculations for a range of sites showed that even under very dry conditions, a 5-mm rainshower should wet the root zone and become a potential soil water resource for plants. Furthermore, basing our assertion on root morphology and the distribution of roots in the soil, we suggest that the value of light rainshowers to plants may vary with their life form.>7Sala, O. E. Parton, W. J. Joyce, L. A. Lauenroth, W. K. 1988D=Primary Production of the Central Grassland Region of the USAEcology Washington691 40-45Aboveground net primary production of grasslands is strongly influenced by the amount and distribution of annual precipitation. Analysis of data collected at 9500 sites throughout the central United States confirmed the overwhelming importance of water availability as a control on production. The regional spatial pattern of production reflected the east-west gradient in annual precipitation. Lowest values of aboveground net primary production were observed in the west and highest values in the east. This spatial pattern was shifted eastward during unfavorable years and westward during favorable years. Variability in production among years was maximum in northern New Mexico and southwestern Kansas and decreased towards the north and south. The regional pattern of production was largely accounted for by annual precipitation. Production at the site level was explained by annual precipitation, soil water-holding capacity, and an interaction term. Our results support the inverse texture hypothesis. When precipitation is <370 mm/yr, sandy soils with low water-holding capacity are more productive than loamy soils with high water-holding capacity, while the opposite pattern occurs when precipitation is >370 mm/yr. PIResource partitioning between shrubs and grasses in the Patagonian stepper@9Sala, O. E. Golluscio, R. A. Lauenroth, W. K. Soriano, A. Oecologia814!501-505 1989Experiments were conducted in the Patagonian steppe in southern South America to test the following hypotheses; (a) grasses take up most of the water fro m the upper layers of the soil and utilize frequent and short-duration pulses of water availability; (b) shrubs, on the contrary, take up most of the water from the lower layers of the soil and utilize infrequent and long-duration pulses of water availability. Grasses and shrubs were removed selectively and the performance of plants and the availabilty of soil resources were monitored. Results supported the overall hypothesis that grasses and shrubs in the Patagonian steppe use mainly different resources. Removal of shrubs did not alter grass production but removal of grasses resulted in a small increase in shrub production which was mediated by an increase in deep soil water and in shrub leaf water potential. The efficiency of utilization of resources freed by grass removal was approximately 25%. Shrubs used water exclusively from lower soil layers. Grasses took up most of the water from upper layers but they were also capable of absorbing water from deep layers. This pattern of water partitioning along with the lack of response in leaf nitrogen to the removal treatments suggested that shrubs may be at a disadvantage to grasses with respect to nutrient capture and led to questions about the role of nutrient recirculation, leaching, and nitrogen fixation in the steppe.W0*Sala, O. E. Lauenroth, W. K. Parton, W. J. 1992<6Long-Term Soil Water Dynamics in the Shortgrass SteppeEcology734 1175-1181To assess the temporal and spatial dynamics of soil water at a shortgrass steppe site in northcentral Colorado, we evaluated the precipitation regime for a 33-yr period and ran a simulation model for this period. Small precipitation events accounted for a large fraction of the total number of events and represented a source of water with small interannual variability. The difference between wet and dry years was related to the occurrence of a few large events. Average daily precipitation was concentrated during the warmest months of the year with a maximum in late spring. Water in the surface soil layers had a shorter residence time and no seasonal pattern. Intermediate layers reflected the seasonal pattern of precipitation. Maximum soil water availability occurred in the late spring, but this was also the period with the highest interannual variability. The wettest layer was at 4-15 cm of depth. The frequency of wet conditions decreased above this layer because of the strong influence of evaporation and below because recharge was infrequent. No deep percolation events were recorded. During dry year distribution of soil water was very shallow and during wet years wet conditions reached to depths of 120-135 cm. This shallow distribution of soil water matches the distribution of processes and structural elements in the steppe suggesting that there is a cause-effect relationship between them. We speculate that there is a cause-effect relationship between them. We speculate that the pattern of water availability interacts with biotic constraints and determines the rate of ecosystem processes. The depth distribution of water in dry and wet years is compared to the root distribution fo grasses, shrubs,herbs, and succulents to suggest the response of each group to modal and extreme conditions. Comparison of long-term soil water patterns and traits of the major species allows us to suggest why Bouteloua gracilis is the dominant species in the shortgrass steppe.4-Sala, O. E. Lauenroth, W. K. Golluscio, R. A.  1997<5Plant functional types in temperate semi-arid regions7 0*Smith, T.M. Shugart, H. H. Woodward, F. I.XQPlant functional types: Their relevance to ecosystem properties and global change  New York, USA Cambridge University Press217-233012,Kemp, P. R. Cornelius, J. M. Reynolds, J. F. 1994VOTemporal discontinuities in precipitation in the central North American prairie*$International Journal of Climatology145 539-557dUnderstanding the potential for future climate change to affect ecosystems or agriculture in a region will depend, in part, on understanding how variable the present climate is and what its present effects are. Because the central prairie region of North America undergoes short-term climate shifts (particularly drought), and appears sensitive to these changes, we were interested in characterizing the duration and nature of precipitation fluctuations. We used split, moving-window dissimilarity analysis to locate transition points between periods of relatively homogeneous rainfall over the Kansas region. We identified statistically significant discontinuities in precipitation that appear to represent shifts in the regional climate during the last 115 years. All of the transitions were associated with changes in May, June, and July rainfall. Drought and drought cycles were the dominant fluctuations over decade-long periods. Over somewhat longer periods (20-30 years) there were transitions, varying in abruptness, that may also be related to drought or perhaps larger scale climatic fluctuations. The relatively strong periodicity shown by the decadal discontinuities supports the contention that drought climates are triggered, or ended, by a cyclic phenomenon. The use of dissimilarity analysis allowed us to identify fluctuations in climate of the central North American prairie that were not previously described, and that may have been significant enough to influence natural and agricultural ecosystems.>7Kemp, P. R. Reynolds, J. F. Pachepsky, Y. Chen, Jia Lin 1997PIA comparative modeling study of soil water dynamics in a desert ecosystemWater Resources Research331 73-90C>7Mabry, C. M. Jasienski, M. Coleman, J. S. Bazzaz, F. A. 1997lfGenotypic variation in Polygonum pensylvanicum: Nutrient effects on plant growth and aphid infestation Canadian Journal of Botany754a546-551aVariation in the performance of 20 genotypes of Polygonum pensylvanicum under two nutrient treatments was studied in a garden experiment. Nutrient fertilization enhanced vegetative biomass and fruit biomass production, but did not result in significant genotype-environment interactions, suggesting that nutrient variation of the range used in the experiment has little potential as a microevolutionary factor in this species. Leaf nitrogen concentration was not affected by a nutrient pulse. The degree of transient aphid infestation that occurred during the experiment had a weak positive correlation with final reproductive biomass of plants. Mean density of aphids per leaf was negatively correlated with percent leaf nitrogen and was not affected by genotypic identity of host plants.n Magnani, F. Borghetti, M.s 1995lfInterpretation of seasonal changes of xylem embolism and plant hydraulic resistance in Fagus sylvatica Plant Cell and Environment186689-69630)The annual course of xylem embolism in twigs of adult beech trees was monitored, and compared to concurrent changes of tree water status and hydraulic resistances. Xylem embolism was quantified in 1-year-old apical twigs by the hydraulic conductivity as a percentage of the maximum measured after removal of air emboli. Tree and root hydraulic resistances were estimated from water potential differences and sap flux measurements. The considerable degree of twig embolism found in winter (up to 90% loss of hydraulic conductivity) may be attributed to the effect of freeze-thaw cycles in the xylem. A partial recovery from winter embolism occurred in spring, probably because of the production of new functional xylem. Xylem embolism fluctuated around 50% throughout the summer, without significant changes. Almost complete refilling of apical twigs was observed early in autumn. A significant negative correlation was found between xylem embolism and precipitation; thus, an active role of rainfall in embolism reversion is hypothesized. Tree and root hydraulic resistances were found to change throughout the growing period. A marked decrease of hydraulic resistance preceded the refilling of apical twigs in the autumn. Most of the decrease in total tree resistance was estimated to be located in the root compartment.l)"Sperry, J. S. Pockman, W. T. 1993f`Limitation of transpiration by hydraulic conductance and xylem cavitation in Betula occidentalis Plant Cell and Environment163279-287The extent to which stomatal conductance (g-s) was capable of responding to reduced hydraulic conductance (k) and preventing cavitation-induced xylem pressures was evaluated in the small riparian tree, Betula occidentalis Hook. We decreased k by inducing xylem cavitation in shoots using an air-injection technique. from 1 to 18d after shoot injection we measured midday transpiration rate (E), g-s, and xylem pressure (PSI-p-xylem) on individual leaves of the crown. We then harvested the shoot and made direct measurements of k from the trunk (2-3 cm diameter) to the distal tip of the petioles of the same leaves measured for E and g-s. The k measurement was expressed per unit leaf area (k-1, leaf-specific conductance). Leaves measured within 2d of shoot injection showed reduced g-s and E relative to non-injected controls, and both parameters were strongly correlated with k-1. At this time, there was no difference in leaf PSI-p-xylem between injected shoots and controls, and leaf PSI-p-xylem was not significantly different from the highest cavitation-induced pressure (PSI-p-cav) in the branch xylem (-1.43 +- 0.029 MPa, n = 8). Leaves measured 7-18d after shoots were injected exhibited a partial return of g-s and E values to the control range. This was associated with a decrease in leaf PSI-p-xylem below PSI-p-cav and loss of foliage. The results suggest the stomata were incapable of long-term regulation of E below control values and that reversion to higher E caused dieback via cavitation.Ein summer. The number of beetles in different pools tended to vary synchronously within and among years, but there were many exceptions. Beetles survived drought periods by transferring to neighbouring, permanent refuge pools, or by seeking refuge for up to a week under a boulder in the desiccating pool. This illustrates the great importance of environmental heterogeneity for local population survival, i.e. normally unimportant elements in the habitat may assume critical importance under stressful periods.nasin of the Western United States led to a striking pulse of available soil nitrate in a field plot, but available phosphate. was not affected. This is the first field demonstration of this phenomenon in the Great Basin as far as we are aware. This pulse was only apparent for a few days, probably due to microbial immobilization of the nitrogen. Root ammonium uptake capacity increased within one day of the water pulse, but new root growth was not apparent until 3 days after the water pulse. Thus, to capture this ephemeral release of nitrouen, enhanced uptake capacity of existing roots was probably more important than development of new roots. Mycorrhizal infection was not affected by the water pulse treatments. However, since the water pulse only affected nitrogen availability and mycorrhizae are generally most effective in facilitating acquisition of less mobile nutrients such as phosphate, mycorrhizae likely do not play an important role in taking advantage of this opportunity provided by the pulse of water.^>8Temporal changes in a Chihuahuan desert rodent community Brown, J. H. Heske, E. J.e Oikosn593t290-302y 1990We used time series analysis of ten years of monthly census data to assess the responses of both individual species and an entire community of rodents to a fluctuating desert environment. Autocorrelation analysis revealed different patterns of intra-annual fluctuation among the 11 species; Dipodomys spectabilis and Perognathus flavus had pronounced annual cycles; D. ordii, D. merriami, Chaetodipus penicillatus, Onychomys torridus, O. leucogaster, and Neotoma albigula exhibited annual cycles modified by interannual variation; and Peromyscus eremicus, Pm. maniculatus, and Reithrodontomys megalotis showed little evidence of annual periodicity. The timing at annual cycles and the pattern of inter-annual fluctuations also differed among species. However, two results suggest taht several species responded similarly to long-term environmental variation: 1) population densites of four species and total rodent biomass and numbers were positively correlated with the densities of annual plants; and 2) many pairs of species exhibited positively correlated population dynamics over the ten years. Clustering of pairwise cross-correlation coefficients was used to identify sets of species with similar populatoin dynamics. These clusters did not necessarily contain closely related or ecologically similar species. Detrended Correspondence Analysis identified three independent patterns of variation in species composition: a long-term trend; a four- to five-year repeated pattern that appeared to correspond to the climatic effects of the El Nino Southern Oscillation: and an annual cycle. In general, species appeared to respond individualistically to environmental variation. There was no evidence of an equilibrium community composition or of alternative stable configurations. Some competing species had negatively correlated population dynamics, but the majority of competitors exhibited positive correlations that apparently reflected similar responses to fluctuating resources.Vhihuahuan Desert of the southwestern United States we monitored responses of both winter and summer annual plant communities to natural environmental variation an|uResource competition in a variable environment: Phytoplankton growing according to the variable-internal-stores model9 Grover, J. P.American Naturalist 1384811-835  1991yMany studies in population ecology and competition theory are based on models in which the consumption rate of a resource is some function of resource availability and in which the yield of new consumers is constantly proportional to the amount of resource consumed. For such models, the best competitors at equilibrium have low resource requirements, and the best competitors in nonequilibrium habitats have high maximal growth rates. In this study, I abandon the assumption of constant yield and allow consumption rates to be a function of the consumer's internal state with respect to a resource, as well as function of external resource availability. Specifically I study functions and parameters describing the competition of planktonic algae for a dissolved nutrient, phosphorus, that is supplied in peroidic pulses. In this model, trade-offs between competitive abilities in equilibrium and nonequilibrium habitats can arise in several ways. The most important trade-offs are likely to involve the capacity to store nutrient and the relation between consumption rate and internal nutrient state. It is possible that a species may have unimpressive rates of growth and consumption as functions of resource availability but nevertheless be a superior competitor. It is also possible that no trade-offs between competitive abilities in equilibrium and nonequilibrium habitats arise: that the same species is the best competitor in all habitats. Which of these possibilities is prevalent in natural phytoplankton can only be determined empirically, by adequate knowledge of the constraints on and correlations among the physiological properties of algae. er supply to the existing plant cover. Although water was accessible to roots in deeper soil layers in all habitats, as demonstrated by high soil moisture, earlier rain events were not fully utilized by the current plant cover during summer drought. The role of seedling establishment in determining species composition and vegetation type, and the indirect effect of seedling establishment on the use of water by fully developed plant cover, are discussed in relation to climate change and vegetation modelling.n in the sources of water used by tree species has important ram(!Milchunas, D. G. Lauenroth, W. K.s 2001`ZBelowground primary production by carbon isotope decay and long-term root biomass dynamics Ecosystemc4n2r139-150sThe isotope decay method of estimating belowground net primary production (BNPP) has the potential to overcome the assumptions and biases associated with traditional methods. Isotope loss through in situ decomposition after pulse-labeling is considered the inverse of production, and turnover times are estimated by regression to time of zero remaining isotope. Method development and estimates of production were previously published using 4 years of data, which showed a clear linear loss rate over time. A slow, distinctly different phase in isotope loss developed 5-10 years postlabeling. We assess reasons for the two-phase loss functions and the implications for estimates of BNPP and compare the isotope decay method with standard coring methods over a 13-year period. Reasons for the two-phase dynamics of carbon 14 (14C) loss could include various biological and/or methodological factors. Results suggest that 14C in soil embedded in roots as they grow, a small proportion of roots that live for a much longer time than the majority of roots, and method of separating roots from soil organic matter may influence estimates of BNPP by isotope methods. Remobilization of label in structural tissue or reuptake of label from the soil did not appear to be responsible for the slow, second phase of loss dynamics. Isotope decay produced more reliable estimates than standard coring methods. Estimates using harvest sum of increments were zero in 6 of 13 years. Thirteen years of root biomass data showed no predictable trend over winter or consistent seasonal pattern, although longer-term cycles were evident. Aboveground:belowground ratios were generally smaller during dry periods, but root biomass was not as responsive as aboveground biomass to annual precipitation.regions, the effects of grazing or sparing management on natural communities of long-lived plants generally take decades to become evident. Event-driven dynamic behavior, unpredictable and low rainfall and complicated interactions between species make it difficult to assess probabilities and time scales of vegetation change. To gain a better understanding of the main processes and mechanisms involved in vegetation change, we have developed a spatially explicit individual based model that simulates changes in plant communities over long time spans. The model, based on life-history attributes of the five dominant component plant species of a typical Karoo shrub community, follows the fate of each individual plant within the commun:4Whitford, Walter G. Anderson, John Rice, Patricia M. 1997^WStemflow contribution to the 'fertile island' effect in creosotebush, Larrea tridentatat"Journal of Arid Environments3530451-4572,Stemflow, throughfall and bulk precipitation were collected on six creosotebushes (Larrea tridentata) during 18 events in the summer rainy season in the northern Chihuahuan Desert. The average stemflow was 16.8 +- 1.9%; throughfall averaged 64.7 +- 3.2%. The concentration of all ions measured were significantly higher in stemflow than in the bulk precipitation. Total nitrogen, sulfate, and calcium concentrations were more than an order of magnitude higher in the stemflow than in the bulk precipitation. Concentration of ions in the upper 10 cm of soil were generally higher in soils under shrubs than in soils between shrubs. Measured quantities of ions in dry-fall were of sufficient magnitude to account for the increased concentration in stemflow water of most ions. Increases in nitrogen in stemflow water may be due to biological activity of stem crust micro-organisms in addition to dry-fall. Dry-fall that collects on the leaves and stems of this desert shrub may contribute to the 'fertile island' effect on the soils under the canopies of creosotebushes.ation of woody plants ("Bowman, D. M. J. S. Minchin, P. R. 1987`ZEnvironmental Relationships of Woody Vegetation Patterns in the Australian Monsoon Tropics"Australian Journal of Botany352i151-170.Indirect gradient analysis was applied to 48 vegetation samples taken from a mosaic of woody vegetation at Berry Springs in the Northern Territory of Australia. Compositional variation among the samples was effectively summarized by a two-dimensional ordination by non-metric multidimensional scaling. Subsequent rotational correlation analysis revealed marked relationships between the vegetation pattern and edaphic variables which reflect two aspects of the moisture regime: water availability during the dry season and the degree of inundation during the wet season. Moisture availability is principally determined by topographic position, through it relationship with soil texture and water table depth. Poor drainage during the wet season appears to separate Melaleuca communities from those dominated by eucalypts. Shrubby and grassy open forests appear to be differentiated by the intensity of the winter drought. The grassy understoreys, which occur on unslope positions well above the water table, die off shortly after the end of the wet season thus providing fuel for fires. A closed Carpentaria forest, located on the slopes above a spring, was found to have relatively organic-rich, fertile, fine-textured soils, possibly reflecting the superior nutrient cycling of the closed forest compared with the frequently burnt surrounding open communities. We suggest that the dense evergreen vegetation presents a barrier to fires from the open communities. This would account for the greater proportion of woody, closed forest species that regenerate exclusively from seed. The fires in the eucalypt forests are of low intensity and plants have vegetative mechanisms to recover from damage. We conclude that the edaphically determined vegetation controls fire regime rather than the coverse.c X =>7Roundy, Bruce A. Abbott, Laurie B. Livingston, Margaret 1997NGSurface soil water loss after summer rainfall in a semidesert grassland ,%Arid Soil Research and Rehabilitations111r 49-62 Surface soil water contents were analyzed for bare and litter- or gravel-covered soils for 20 drying periods after summer rainfall in southeastern Arizona. Water contents were measured every minute by fiberglass cells calibrated for the sandy-loam soil, and 30- to 60-min averages were stored by microloggers. Water loss to equivalent matric potentials of -1.5 MPa for bare surface soils (1-3 cm) was fast (1.2 +- 0.5 days), moderate (2.6 +- 0.9 days), or slow (5.9 +- 1.4 days) when the upper 15 cm of soil at the start of the drying period had 13.3, 27.1, and 36.1 cm of water, respectively. Those water contents are near saturation and near field capacity for slow and moderate drying rates, respectively. For fast drying rates, water content was near field capacity at 1-3 cm but dry below 8 cm. Litter or gravel mulches increased the time of surface soil water availability by 0.7 to 1.9 days for fast and moderate drying periods, respectively. Drying from the surface down into the soil profile averaged 3.6 cm day-1. Short summer rainstorms may germinate warm-season grasses which will desiccate if subsequent rainfall is not persistent enough to maintain surface water availability and allow adventitious root development. Estimates of time that the drying front reaches the depth of seminal roots of warm-season grasses were used to suggest rain-rainless day patterns that might result in seedling mortality. Sowing during the summer rainy season when the probability of these patterns is minimal may increase revegetation success. High correlation of available water periods with meteorological variables suggests that soil water dynamics could be accurately estimated using physically based models, and may permit more detailed modeling of seedling establishment.*#Ruxton, Graeme D. Humphries, Stuartn 1999>8Multiple ideal free distributions of unequal competitors$Evolutionary Ecology Research15635-640JDWe studied an individual-based model of a number of competitors each able to move freely between two resources. If individuals move only so as to improve their resource-gathering rate, then the system settles to one of a finite number of equilibria (often called 'ideal free distributions'). The addition of occasional switching of a randomly selected individual between resources does not (contrary to the predictions of Hugie and Grand, 1998) lead to this distribution of equilibria collapsing to a single point. In fact, it can induce the population to shift periodically between equilibria, thereby increasing spatio-temporal variation in competitor numbers. Furthermore, we show that the probability of the system reaching a given equilibrium is critically dependent on the fine detail of the rules describing individual movements.2,Ryel, R. J. Caldwell, M. M. Manwaring, J. H. 1996lfTemporal dynamics of soil spatial heterogeneity in sagebrush-wheatgrass steppe during a growing seasonPlant and Soil 184h2s299-309Variability in five soil resources essential for plant growth (NH-4+, NO-3-, P, K and soil moisture) was quantified using univariate, multivariate and geostatistical techniques in a sagebrush-grass steppe ecosystem at three times (early April, June and August) during the 1994 growing season. Samples were collected every meter in a 10 times 10-m 'macrogrid', every 20 cm within nested 1 times 1-m 'minigrids', and every 3 cm within additionally nested 15 times 15-cm 'microgrids'. Strong autocorrelation for all variables in the three sample periods was only found over distances less than 2 m, indicating that patches of high internal uniformity in this soil were smaller than 2 m during the growing season. Differences in semivariograms between sample periods were most pronounced for NO-3-, NH-4+ and soil moisture, variables that we consider to primarily limit plant growth in this system. The distance over which sample points were autocorrelated for NO-3-, NH-4+ and soil moisture increased from April to June. In contrast P and K, which are relatively more abundant at the study site, exhibited relatively constant semivariance patterns over the three sample periods. Weak correlation was found between samples collected in the three sample periods for N and soil moisture indicating that the spatial pattern of these limiting resources changed between sample periods. However, P and K had highly significant correlations (p lt 0.00001) among sample periods, indicating that the distributional patterns of these relatively more abundant resources remained rather constant. There were strong negative correlations between P and K and distance from the base of shrubs for all sample times (p lt 0.001), indicating an increase in P and K close to shrubs. Similar strong negative correlations were not found between distance from the shrubs and levels of NH-4+, NO-3-, or soil moisture, nor for any soil variable and distance from perennial tussock grasses. Changes in patterns of nutrient and soil moisture variability within a growing season suggest that not only must plants acquire soil resources that vary in time and space, but that they may also have to adjust to different scales of resource patchiness during the season.cB;R. J. Ryel M. M. Caldwell C. K. Yoder D. Orr A. J. Leffler 2001Hydraulic redistribution in a stand of Artemisia tridentata: evaluation of benefits to transpiration assessed with a simulation model1 ( <  OecologiaafHydraulic lift/redistribution - Soil-water model - Root distribution - Water use - Artemisia tridentat R VThe significance of soil water redistribution facilitated by roots (an extension of "hydraulic lift", here termed hydraulic redistribution) was assessed for a stand of Artemisia tridentata using measurements and a simulation model. The model incorporated water movement within the soil via unsaturated flow and hydraulic redistribution and soil water loss from transpiration. The model used Buckingham-Darcy's law for unsaturated flow while hydraulic redistribution was developed as a function of the distribution of active roots, root conductance for water, and relative soil-root (rhizosphere) conductance for water. Simulations were conducted to compare model predictions with time courses of soil water potential at several depths, and to evaluate the importance of root distribution, soil hydraulic conductance and root xylem conductance on transpiration rates and the dynamics of soil water. The model was able to effectively predict soil water potential during a summer drying cycle, and the rapid redistribution of water down to 1.5 m into the soil column after rainfall events. Results of simulations indicated that hydraulic redistribution could increase whole canopy transpiration over a 100-day drying cycle. While the increase was only 3.5% over the entire 100-day period, hydraulic redistribution increased transpiration up to 20.5% for some days. The presence of high soil water content within the lower rooting zone appears to be necessary for sizeable increases in transpiration due to hydraulic redistribution. Simulation results also indicated that root distributions with roots concentrated in shallow soil layers experienced the greatest increase in transpiration due to hydraulic redistribution. This redistribution had much less effect on transpiration with more uniform root distributions, higher soil hydraulic conductivity and lower root conductivity. Simulation results indicated that redistribution of water by roots can be an important component in soil water dynamics, and the model presented here provides a useful approach to incorporating hydraulic redistribution into larger models of soil processes.   B 7-17$://000084407700002 (!Wu, J. Q. Zhang, R. D. Gui, S. X.o>7Modeling soil water movement with water uptake by rootssPlant and SoilRKmodeling; plant water uptake; root zone; soil water equation; system; plant Soil water movement with root water uptake is a key process for plant growth and transport of water and chemicals in the soil- plant system. In this study, a root water extraction model was developed to incorporate the effect of soil water deficit and plant root distributions on plant transpiration of annual crops. For several annual crops, normalized root density distribution functions were established to characterize the relative distributions of root density at different growth stages. The ratio of actual to potential cumulative transpiration was used to determine plant leaf area index under water stress from measurements of plant leaf area index at optimal soil water condition. The root water uptake model was implemented in a numerical model. The numerical model was applied to simulate soil water movement with root water uptake and simulation results were compared with field experimental data. The simulated soil matric potential, soil water content and cumulative evapotranspiration had reasonable agreement with the measured data. Potentially the numerical model implemented with the root water extraction model is a useful tool to study various problems related to flow transport with plant water uptake in variably saturated soils. Plant Soil 1999 215r1e268HF PLANT SOILISI:000084407700002  12a 92-103HAOesterheld, Martin Loreti, Juan Semmartin, Maria Sala, Osvaldo E. 2001piInter-annual variation in primary production of a semi-arid grassland related to previous-year productionn$Journal of Vegetation Science1210137-142 Mean annual precipitation accounts for a large proportion of the variation in mean above-ground net primary production (ANPP) of grasslands worldwide. However, the inter-annual variation in production in any grassland site is only loosely correlated with precipitation. The longest record of variation in production and precipitation for a site corresponds to a shortgrass steppe in Colorado, USA. A previous study of this record showed that current-year precipitation accounted for 39% of the inter-annual variation in ANPP. In this note, we show that ca. one third of the unexplained variation is related to previous-year ANPP: ANPP per mm of precipitation was higher in years preceded by wet, more productive years than in years preceded by average years; similarly, ANPP per mm of precipitation was lower in years preceded by dry, less productive years than in years preceded by average years. Since previous-year ANPP was, in turn, associated with precipitation of a year before, current-year ANPP was also explained by precipitation of two previous years. Our finding not only increases our predictive ability, but it also changes our understanding of how ANPP responds to fluctuations in precipitation. If ANPP is thought to vary according to current-year precipitation only, it will simply track annual precipitation in time. According to this new result, however, ANPP fluctuations are buffered if wet, more productive years alternate with dry, less productive years, and they are amplified if wet or dry sequences of several years take place.*#Ostfeld, Richard S. Keesing, Felici 2000TNPulsed resources and community dynamics of consumers in terrestrial ecosystems&Trends in Ecology and Evolution156232-237<PJ6@:Sperry, J. S. Adler, F. R. Campbell, G. S. Comstock, J. P. 1998^XLimitation of plant water use by rhizosphere and xylem conductance: Results from a model"Plant, Cell and Environment214347-359Hydraulic conductivity (K) in the soil and xylem declines as water potential (PSI) declines. This results in a maximum rate of steady-state transpiration (Ecrit) and corresponding minimum leaf PSI (PSIcrit) at which K has approached zero somewhere in the soil-leaf continuum. Exceeding these limits causes water transport to cease. A model determined whether the point of hydraulic failure (where K = 0) occurred in the rhizosphere or xylem components of the continuum. Below a threshold of root:leaf area (AR:AL), the loss of rhizosphere K limited Ecrit and PSIcrit. Above the threshold, loss of xylem K from cavitation was limiting. The AR:AL threshold ranged from > 40 for coarse soils and/or cavitation-resistant xylem to < 0.20 in fine soils and/or cavitation-susceptible xylem. Comparison of model results with drought experiments in sunflower and water birch indicated that stomatal regulation of E reflected the species' hydraulic potential for extracting soil water, and that the more sensitive stomatal response of water birch to drought was necessary to avoid hydraulic failure. The results suggest that plants should be xylem-limited and near their AR:AL threshold. Corollary predictions are (1) within a soil type the AR:AL should increase with increasing cavitation resistance and drought tolerance, and (2) across soil types from fine to coarse the AR:AL should increase and maximum cavitation resistance should decrease. Sperry, J. S. 20002+Hydraulic constraints on plant gas exchangei*#Agricultural and Forest Meteorologyr 1041 13-23Squeo, Francisco A. Olivares, Nancy Olivares, Sandra Pollastri, Alberto Aguirre, Evelyn Aravena, Ramon Jorquera, Carmen Ehleringer, James R. 1999^XFunctional groups in north Chilean desert shrub species, based on the water sources usedGayana Botanican561e 1-15.h"Primary productivity and vegetation structure in arid ecosystems are determined by water availability. In studies conducted in the coastal dryland of north-central Chile (29degree43'S; 71degree14'O, 300 m), the mechanisms to use different water sources by shrubs species, in two contrasting rainfall years were compared. Information on phenological studies, root architecture and water sources used by shrubs through the use of stable isotopes is are discussed. Six functional groups based on water uptake and water use are recognized. The functional groups were defined based on their habits (deciduous and evergreen), their root systems (shallow, dimorphic and deep), and their ability to use different water sources (surficial and/or deep). Because of the differential impact of the goat over-grazing on different functional groups, this would result on a lower utilization of surficial waters. A management and/or restoration plan should maximize the use of all water sources available to recover the primary productivity and the system stability.i to current-year precipitation only, it will simply track annual precipitation in time. According to this new result, however, ANPP fluctuations are buffered if wet, more productive years alternate with dry, less productive years, and they are amplified if wet or dry sequences of several years take place.*#Ostfeld, Richard S. Keesing, Felici 2000TNPulsed resources and community dynamics of consumers in terrestrial ecosystems&Trends in Ecology and Evolution156232-2372\ Steinberger, Y. Sarig, S.s 1993zsResponse by soil nematode populations and the soil microbial biomass to a rain episode in the hot, dry Negev desert$Biology and Fertility of Soils163188-1928tnSince the amount, intensity, and frequency of rainfall in desert regions vary strongly over space and time, the response by soil biota to this variability is of great importance. We conducted a study in the Negev desert in order to examine the immediate response by the soil nematode populations and the microbial biomass to varying amounts of water applied in a single pulse. Soil samples from the 0-10-cm depth were collected from areas undergoing four different wetting treatments, comprising 5, 10, 15, and 20 mm of water, and from a non-irrigated control soil. There was a correlation between diurnal variations in nematode populations and the diurnal fluctuations in soil moisture. The greatest abundance of nematodes was found in the soil treated with 20 mm water (970 individuals 100 g-1 dry soil) which was 2, 4, 5, and 14 times larger than that found in the soil treated with 15, 10, 5, and 0 mm of water, respectively. Bacterial-feeding and fungal-feeding nematodes accounted for approximately 95% of the total nematode population found in all treatments. The microbial biomass examined in the current study exhibited an immediate response to the wetting which was greater in soil treated with, 10, 15, and 20 mm of water compared with 0 and 5 mm. However, after 4 days (96 hours) the microbial biomass stabilized again at the basic level of the 0-mm control. However, our results indicated that the major trigger for changes in the nematode populations, and in the microbial biomass, was diurnal fluctuations in soil moisture, since peaks in nematode populations and in the microbial biomass were observed at various times of the day."Stephens, G. Whitford, W. G. 1993piResponses of Bouteloua eriopoda to irrigation and nitrogen fertilization in a Chihuahuan desert grassland"Journal of Arid Environments244r415-421aThe response of Bouteloua eriopoda, black grama, to irrigation and nitrogen fertilization was examined to test the hypothesis that, in desert grasslands, nitrogen availability is greater than in adjacent shrublands. Tiller growth was greater in irrigated plots (P lt 0.05) while nitrogen had no significant effect on growth (P gt 0.05). Nitrogen induced early flowering but water had a greater effect during the various phenological stages. Water affected flowering, mortality, insect damage, and biomass production. Nitrogen and nitrogen plus water reduced root length of the experimental plants (P lt 0.05). The short and mat-like rooms of fertilized plants which received only irrigation (46 out of 61 precipitation events were lt 6 mm in size). There were no differences in the total biomass production of nitrogen-treated or water-nitrogen-treated subplots compared to controls. There was a higher nitrogen content of stems and leaves on plants from the irrigated-fertilized plots, but no significant differences among other treatments. Based on the large effects of irrigation in a third wet year ( gt 300 mm year-1) and marginal nitrogen effects, we conclude that productivity in Chihuahuan Desert Bouteloua eriopoda grasslands is less nitrogen limited than that of the Larrea shrublands. Further, productivity in this perennial desert grassland appears to be closely linked to rainfall with no temporal lags due to nitrogen immobilization.d201-208$://A1996TW19100008i(!Stirzaker, R. J. Passioura, J. B.a4.The water relations of the root-soil interface Plant Cell and Environmenthydraulic resistance; hydrostatic pressure; osmotic pressure; root water uptake; water transport hydraulic resistance; maize plants; lupin; accumulation; conductance; transport; barley; modeliF@The difference in hydrostatic pressure between the xylem of the leaf and the soil depends, for a given transpiration rate, on the series of hydraulic resistances encountered along this pathway, Many studies have shown that the sum of the resistances in the plant and the soil is too small to account for the fall in water pressure between the leaf xylem and the soil, especially when plants are growing in sandy soils, which are prone to dry rapidly, A resistance at the root-soil interface, caused possibly by poor contact between the roots and the soil, has been proposed to account for the discrepancy, We explored the resistance in the pathway from soil to leaf using a technique that allows precise and continuous non- destructive measurement of the hydrostatic pressure in the leaf xylem, When the soil was leached with water, the fall in leaf water status as the soil dried was reasonably well described by a simple physical model without the need to invoke an interfacial resistance, However, when the soil was flushed with a nutrient solution with an osmotic pressure of 70 kPa, the hydrostatic pressure in the leaf xylem fell several times faster than that in the soil, We suggest that solutes accumulated either in the root or just outside it, creating large osmotic pressures, which gave the appearance of an interfacial resistance.Plant Cell Environ.  1996 Febe192tTW191 PLANT CELL ENVIRONISI:A1996TW19100008pSvensson, Bo W. 1999Environmental heterogeneity in space and time: Patch use, recruitment and dynamics of a rock pool population of a gyrinid beetle Oikos . Feb.842a227-238A population of the whirligig beetle Gyrinus opacus inhabiting a cluster of 108 rock pools on a small peninsula in south-central Sweden was studied over a 3-year period. In terms of recruitment of new adults and population size, the study period included two good years, one dry (1990) and one wet (1991), and one bad, dry year (1992). Mean occupancy of pools was around 40% during the two good years and 18% during the bad year, i.e. most pools were vacant. As expected from theory, turnover rates were highest for pools with the lowest average population size. While new adults emerged from about half the pools, only 7.4% were successful in all three years. Beetles attempted to hibernate in about half the pools and succeeded in 41%. Most activities, such as local movements, recruitment of new adults and hibernation, involved the 19 major pools, but no pool was outstanding. Dispersal between pools was frequent and, as expected, breeding pools were in contact with a higher number of pools than those where no successful breeding took place. Surprisingly, during the dry good year, both recruitment number and reproductive success were positively related to the length of the dry period of pools, which may possibly reflect an escape from predators on eggs and larvae. During the two first years when the population was stable, pools that dried out in 1990 but not in 1991 produced significantly more new adults in the dry 1990. The size of the total population showed a seasonal bimodal pattern with a minimum in summer. The number of beetles in different pools tended to vary synchronously within and among years, but there were many exceptions. Beetles survived drought periods by transferring to neighbouring, permanent refuge pools, or by seeking refuge for up to a week under a boulder in the desiccating pool. This illustrates the great importance of environmental heterogeneity for local population survival, i.e. normally unimportant elements in the habitat may assume critical importance under stressful periods.n6\VHeritability of carbon isotope discrimination in Gutierrezia microcephala (Asteraceae)LESchuster, W. S. F. Phillips, S. L. Sandquist, D. R. Ehleringer, J. R. American Journal of Botany792m216-221s 1992D=The carbon isotope composition (13C/12C) of C-3 plant tissues provides a long-term, integrated measure of photosynthetic metabolism. Quantitative genetic methods were used to study the inheritance of carbon isotope composition and several morphological characters in Guiterrezia microcephala, a short-liver desert perennial. Open-pollinated seed was collected from a population located near Lee's Ferry, Arizona, in an area that was disturbed approximately 20 years ago. Seeds were germinated and seedlings grown in a common greenhouse environment. Carbon isotope discrimination (DELTA) and all morphological characters varied significantly among maternal families. Heritability of DELTA was estimated as 81% in the greenhouse and a 92% using the regression of maily mean DELTA on parent DELTA from field samples. Using both field and greenhouse data, we estimated a lower bound for heritability in nature of 54%. Offspring size and biomass both showed significant, positive correlations with DELTA. The corresponding genetic correlations were similar in magnitude and direction, but did not differ significantly from zero. These results imply the existence of heritable differences in physiology associated with carbon assimilation and water loss within populations, and the potential for microevolutionary change through natural selection.iion of semiarid ecosystems is a major environmental problem worldwide, characterized by a reduction in the ratio of herbaceous to woody plant biomass. These ecosystems can be described as a set of canopy patches comprising woody plants and the intercanopy patches that separate them, yielding an overstory with intermediate closure. Field measurements of microclimate at the scale of canopy patches, particularly for near-ground solar radiation and soil moisture, are largely lacking from,&Bowman, William D. Bilbrough, Carol J. 2001^XInfluence of a pulsed nitrogen supply on growth and nitrogen uptake in alpine graminoidsPlant and Soil 233P2283-290 F?The supply of N in alpine soils is influenced by environmental factors (freeze-thaw, drying-rewetting, release of N from winter snowpack) which lead to a pulsed nature in plant N availability. To address the ability of alpine species to acquire N and grow when N is supplied in a pulsed manner, six alpine graminoid species, 3 sedges (Cyperaceae) and 3 grasses (Poaceae), were grown under 3 treatments: low and high N supply applied 3 times weekly, and a pulsed N supply applied once weekly at the same concentration as the high N treatment, but with the same total N supply as the low N treatment. Growth, biomass allocation, and N uptake were the same in all species for plants grown under a pulsed N treatment relative to a constant N supply with the same amount of total N. Root:shoot ratios and uptake of experimentally applied 15N indicated there were no adjustments in growth allocation or root uptake capacity in the plants to enhance the uptake of N when supplied in a pulsed relative to a more constant supply. The fertility of the site from which the graminoids were collected did not influence the plants' ability to respond to a high versus a low N supply, but instead growth form was more important. Grasses exhibited variation in growth, biomass allocation, and N uptake in response to changes in N supply, while sedges did not. <F* ..(Bilbrough, Carol, J. Caldwell, Martyn M. 1997TNExploitation of springtime ephemeral N pulses by six Great Basin plant speciesEcology781231-243tnThe ability to exploit short-duration nutrient pulses may be an important factor in the competitive balance of plants and in shaping plant community structure. We investigated the growth responses and biomass production of six Great Basin plant species growing in monocultures in the field following a single pulse of nitrogen applied in early, mid, or late spring. As a control, we applied the same total quantity of N that was in each of the individual pulses as a continuous series of applications at twice-weekly intervals over 10 wk in the spring. Surprisingly, most of the species grown under the control, continuous N supply had lower growth rates, fewer tillers, and less biomass production than plants receiving N in a pulse. At least one of the pulse treatments increased biomass production relative to controls in all but one species. The exception to this pattern was the shrub Chrysothamnus, which responded to all pulse treatments and the control supply with equivalent growth rates and biomass production. Each species responded differently to the set of pulses, with the greatest response occurring early in the growth phase when plants were small and growth rates were high. Thus, phenological stage determined the timing of maximum response. Four of six species not only responded to the early-spring pulse, but also had their greatest response to this pulse, suggesting that the cold-season-adapted species of the Great Basin system are well suited to take advantage of this pulse. The combination of rapid plant growth rates and predictable pulses following snowmelt would likely result in intense competition for nutrients at this time. Our study demonstrates that plants are remarkably capable of utilizing pulses of N, and that pulsed nutrients are potentially important in natural systems. In addition, it suggests that studies conducted under constant nutrient supply may not reflect the responses of plants growing under pulsed nutrient conditions. The plants, instead of benefitting from a season-long continuous supply of N, at certain times during the growing season were able to use pulses of N for significant gains in biomass.NGBorghetti, Marco Cinnirella, Sergio Magnani, Federico Saracino, Antonio 1998XQImpact of long-term drought on xylem embolism and growth in Pinus halepensis Mill Trees124187-195sNHThe present study was carried out to elucidate the response mechanisms of 50-year-old Pinus halepensis Mill. trees to a long-term and severe drought. The amount of water available to trees was artificially restricted for 12 months by covering the soil with a plastic roof. Over the short term a direct and rapid impact of drought was evident on the water relations and gas exchanges of trees: as the soil dried out in the Spring, there was a concurrent decrease of predawn water potential; transpiration was strongly reduced by stomatal closure. Seasonal changes in the water volume fractions of twig and stem xylem were observed and interpreted as the result of cavitation and refilling in the xylem. When droughted trees recovered to a more favourable water status, refilling of embolized xylem was observed; twig predawn water potentials were still negative in the period when the embolism was reversed in the twig xylem. A few months after the removal of the covering, no differences in whole plant hydraulic resistance were observed between droughted and control trees. Needle and shoot elongation and stem radial growth were considerably reduced in droughted trees; no strategy of trees to allocate carbon preferentially to the stem conducting tissues was apparent throughout the experiment. An after-effect of the drought on growth was observed. Bowers, M. A. 1987zPrecipitation and the Relative Abundances of Desert Winter Annuals a 6-Year Study in the Northern Mohave Desert Nevada Usa"Journal of Arid Environments122a141-150e\UThe relationship between winter annual plants and the amount and timing of precipitation was studied for 6 years in Rock Valley, Nevada, U.S.A. Sixty-two annual species, whose year-to-year densities commonly varied 100-fold, were encountered in sampling quadrats. While dominance-diversity curves varied markedly between years, the similarity in relative abundances of annuals between years was correlated with differences in September-October precipitation of the previous fall; thus, in a comparison of years, the more similar the amount of precipitation in the early fall months, the more similar the relative abundances of annuals. Between-year differences in the total amount of fall and spring precipitation, or in precipitation in November-December or January-February, were not related to compositional similarity. Annual composition was not related to that of the previous year. The proportional abundance of species varied less than did their absolute abundances. These results suggest that compositional dynamics of annual plants in the Mohave Desert are keyed to processes that affect germination.i: `Dawson, Todd E. 1993vpHydraulic lift and water use by plants: Implications for water balance, performance and plant-plant interactions Oecologia954565-574r During drought periods, sugar maple (Acer saccharum) demonstrates "hydraulic lift", nocturnal uptake of water by roots from deep soil layers that is released from shallow roots into upper soil layers. Using standard water relations methods and stable hydrogen isotope analysis of both source-water and plant-water, I investigated (1) the magnitude and radial extent of hydraulic lift by mature, relatively open-grown trees, of A. saccharum, (2) the proportion of hydraulically-lifted water (HLW) used by shallow-rooted neighbors growing at different distances from target trees, and (3) the influence that this water source had on stomatal conductance to water vapor (g), water balance and groth of these neighbors. Soil water potentials (PSI-s) at -20 and -35 cm showed a distinct diel fluctuation. Soil pits dug beneath three mature trees revealed a distinct hard-pan (e.g. fragipan) layer at a depth of approximately 50 cm. Examination of root distributions obtained from soil cores and soil pits revealed that some larger diameter roots (1.9-3.7 cm) did penetrate the fragipan and were established in the ground water table. The presence of the fragipan indicated that the "rewetting" of the upper soil layer during the night could not be explained by capillary rise from the shallow water table; it was the trees that were taking up ground water and then redepositing it at night into the upper 35 cm of soil, above the fragipan. The greatest fluctuations in PSI-s occurred within 2.5 m of trees and only extended out to approximately 5 m. Application of a two-end-member linear mixing model which used stable hydrogen isotopic data obtained from environmental water sources and xylem-sap demonstrated that all neighbors used some fraction (3-60%) of HLW supplied by sugar maple trees. Plants that used a high proportion of HLW (e.g. rhizomatous or stoloniferous perennials) maintained significantly higher leaf water potentials and g, and showed greater aboveground growth when compared with (i) neighbors that used little or no HLW or (ii) conspecifics found growing at distances greater than about 3 m away from maple trees. Three important conclusions can be drawn from the results of this investigation that have not been demonstrated before: (1) hydraulic lift need not only occur in arid or semi-arid environments where chronic water deficits prevail, but can be important in relatively mesic environments when subjected to periodic soil water deficits, (2) that plants neighboring trees which conduct hydraulic lift can use a significant proportion of this water source, and (3) that the HLW source can effectively ameliorate the influence of drought on the performance and growth of neighboring vegetation. The results are also discussed in terms of their influence on plant nutrient relations (including plant-mycorrhizal associations), the nature of plant-plant interactions and the water balance of individuals, communities and floristic regions.i:?dp 93-102$://000166750800012c"Duke, S. E. Caldwell, M. M. `ZNitrogen acquisition from different spatial distributions by six Great Basin plant species(!Western North American Naturalistuztspatial heterogeneity; soil nitrogen distribution; sagebrush steppe; Great Basin; Aegilops cylindrica; Agropyron desertorum; Artemisia tridentata; Bromus tectorum; Chrysothamnus nauseosus; Pseudoroegneria spicata fertile-soil microsites; desert ecosystems; evolutionary- theory; temporal patterns; root growth; community; steppe; precipitation; heterogeneity; exploitation Plants Of different growth form may utilize soil nutrients in various spatial distributions through different scales of foraging. In this study we evaluated the ability of 6 species commonly found in the Great Basin to utilize nitrogen (N) distributed in different patterns. Three growth forms were represented by these 6 species. We applied N-15-labeled nitrogen in concentrated patches and over broader uniform areas (at approximately 1% the concentration of the patches) in large, outdoor sand-culture plots. Six weeks after N was applied, 2 plants adjacent to the patch (Patch Treatment) and 2 plants within the uniform application (Uniform Treatment) were harvested. One plant 35-45 cm from both applications (Distant Treatment) was also harvested. The proportion of application- derived N in the leaf N pool was calculated and the mass of N this represented was estimated. Winter annual species Aegilops cylindrica and Bromus tectorum utilized the concentrated patches to a greater extent than did perennial species. The mass of N acquired by Patch-Treatment annual plants was significantly greater than by Uniform- and Distant-Treatment plants. Annual plants in the Distant Treatment had very little application-derived N in their leaf tissue. The perennial tussock grasses Agropyron desertorum and Pseudoroegneria spicata differed in utilization of the N applications. Agropyron acquired a greater quantity of N from patches than from uniform applications, and Distant-Treatment plants acquired very little from treatment applications. On the other hand, Pseudoroegneria utilized N in the 3 treatments equally. The shrub species Artemisia tridentata sap, vaseyana and Chrysothamnus nauseosus also differed in their pattern of N acquisition. There were no differences in quantity of N acquired by plants from different treatments for Chrysothamnus; all treatment plants acquired appreciable amounts of N from the applications. In contrast, Artemisia tridentata was very effective at acquiring large quantities of N from patches relative to Uniform- and Distant-Treatment plants, and yet there was still appreciable acquisition of applied N by Distant-Treatment Artemisia plants. We compared our results for these species in utilizing N patches with their ability to utilize N pulses (Bilbrough and Caldwell 1997). The annual grasses, Artemisia, and Agropyron were capable of effectively acquiring N from both pulses and patches, whereas Pseudoroegneria was effective in exploiting pulses but not patches. Chrysothamnus was generally not responsive to either patches or pulses. Our results suggest that the 2 shrubs and 2 perennial grasses differed in the scale at which they foraged for nutrients. Some species exhibited a coarse-scale utilization of nutrients while others were clearly capable of fine-scale utilization of spatially distributed nutrient sources. This suggests the potential for at least some spatial niche separation among these species. West. North Am. Naturalist 2001 Jan611$398HL WEST NORTH AM NATURALISTISI:000166750800012@:Edwards, W. R. N. Jarvis, P. G. Grace, J. Moncrieff, J. B. 1994^XReversing cavitation in tracheids of Pinus sylvestris L. under negative water potentials Plant Cell and Environment174389-397i|uXylem cavitation is a frequent event, but since resistance to flow does not generally increase in vivo, reversal must also occur even under negative potentials. We demonstrated that this can occur in excised wood. Our results suggest that refilling of cavitated tracheids at negative water potentials may result from a change in equilibrium between gas concentrations, water potential and surface tension at the embolism interface. Excised branch-wood specimens from small trees of Pinus sylvestris were dried on the bench to a range of relative water contents and then rehydrated in a permeability apparatus using ultrafiltered, de-aerated water as permeant. Water inflow and outflow were measured gravimetrically by recording the gain or loss from two reservoirs held on balances. Flow was induced through the specimen by holding the balances at different levels, while an overall negative water potential could be imposed by raising the specimen above the inflow/outflow reservoirs. Changes in water content of the specimen were calculated as the difference between inflow and outflow. The time-course data for both relative water content and permeability were fitted to an exponential function to give initial and final estimates and a time constant. Rehydration occurred at all imposed water potentials, but the speed of recovery was affected at lower potentials. Where drying of the specimen was more protracted, permeability was initially lower but also recovered during permeation. Both flow and de-aeration were necessary for complete rehydration. A model requiring new information on gas concentrations and transport coefficients is suggested.sJames R. Ehleringera 1984aIntraspecific competitive effects on water relations, growth and reproduction in Encelia farinosa Q  Oecologia 63153-158i@9Differential utilization of summer rains by desert plantsLEEhleringer, J. R. Phillips, S. L. Schuster, W. S. F. Sandquist, D. R.i Oecologiai883430-434 1991tmSeasonal changes in the hydrogen isotope ratios of xylem waters were measured to determine water sources used for growth in desert plants of southern Utah. While all species used winter-spring recharge precipitation for spring growth, utilization of summer rains was life-form dependent. Annuals and succulent pereninals exhibited a complete dependence on summer precipitation. Herbaceous and woody perennial species simultaneously utilized both summer precipitation and remaining winter-spring precipitation, with herbaceous species much more reliant on the summer precipitation component. Several of the woody perennials exhibited no response to summer precipitation. Currently, precipitation in southern Utah is evenly partitioned between winter and summer time periods; however, global circulation models predict that summer precipitation will increase in response to anticipated climate change. Our data indicate that components within the community will differentially respond to the change in precipitation patterns. These results are discussed in relation to impact on competition and possible changes in community structure.  dvLEInjection of nitrogen-15 into trees to study nitrogen cycling in soil2+Horwath, W. R. Paul, E. A. Pregitzer, K. S.o.'Soil Science Society of America Journal561316-319  1992Most 15N dilution techniques disturb either the soil or N-pool size. The objective of this study was to develop a method of labelling the roots of Populus trees with 15N without physically disturbing the soil. Such a method would enable the direct measurement of the flux of 15N from dead roots into the soil organic matter. Leaf and root biomass were labeled by injection of 15N directly into the vessel elements of hybrid Populus trees during their second growing season. The 15N was uniformly distributed throughout the canopy and root system. The rate and amount of 15N turnover from plant tissue can be determined by pool transfer or through differences in plant 15N concentrations. The 15N was detected in the dead-root pool 8 wk after injection, indicating root turnover. Results demonstrate the ability to measure the contribution of fine-root litter to N-cycling processes without disturbing the soil experiment. 1595-1616s$://000089675600012 Hsiao, T. C. Xu, L. K.xrSensitivity of growth of roots versus leaves to water stress: biophysical analysis and relation to water transport$Journal of Experimental Botany6/expansive growth; yield threshold; hydraulic isolation of growth zone; leaky cable model of root water uptake maize primary root; vitis-vinifera l; abscisic-acid accumulation; cell-wall extension; osmotic adjustment; spatial- distribution; zea-mays; potential gradients; yield threshold; plant responsesiWater transport is an integral part of the process of growth by cell expansion and accounts for most of the increase in cell volume characterizing growth. Under water deficiency, growth is readily inhibited and growth of roots is favoured over that of leaves. The mechanisms underlying this differential response are examined in terms of Lockhart's equations and water transport. For roots, when water potential (Psi) is suddenly reduced, osmotic adjustment occurs rapidly to allow partial turgor recovery and re-establishment of Psi gradient for water uptake, and the loosening ability of the cell wall increases as indicated by a rapid decline in yield-threshold turgor. These adjustments permit roots to resume growth under low Psi. In contrast, in leaves under reductions in Psi of similar magnitude, osmotic adjustment occurs slowly and wail loosening ability either does not increase substantially or actually decreases, leading to marked growth inhibition. The growth region of both roots and leaves are hydraulically isolated from the vascular system, This isolation protects the root from low Psi in the mature xylem and facilitates the continued growth into new moist soil volume. Simulations with a leaky cable model that includes a sink term for growth water uptake show that growth zone Psi is barely affected by soil water removal through transpiration. On the other hand, hydraulic isolation dictates that Psi of the leaf growth region would be low and subjected to further reduction by high evaporative demand. Thus, a combination of transport and changes in growth parameters is proposed as the mechanism co-ordinating the growth of the two organs under conditions of soil moisture depletion. The model simulation also showed that roots behave as reversibly leaky cable in water uptake. Some field data on root water extraction and Vertical profiles of Psi in shoots are viewed as manifestations of these basic phenomena. Also discussed is the trade-off between high xylem conductance and strong osmotic adjustment. J. Exp. Bot. 2000 Seph51 350eSI 360PC J EXP BOTISI:000089675600012u(!Humphrey, L. David Pyke, David A. 1998leDemographic and growth responses of a guerrilla and a phalanx perennial grass in competitive mixturessJournal of Ecology865a854-865e4-1. The advantages of guerrilla and phalanx growth for the guerrilla Elyrnus lanceolatus ssp. lanceolatus and phalanx E. 1. ssp. wawawaiensis were evaluated over 2 years in two taxon mixtures with a range of densities of each subspecies and under two levels of watering. 2. Ramet numbers and biomass of the guerrilla subspecies were higher than those of the phalanx grass in the first year but in the second year declined greatly, while the phalanx grass showed no change in biomass and an increase in ramet numbers. High neighbour densities affected the phalanx subspecies more strongly than the guerrilla subspecies in the first year, but in the second year there were few differences between subspecies. Biomass of the guerrilla grass remained greater than that of the phalanx grass but ramet numbers were similar in the second year. 3. For both subspecies in both years, probability of flowering decreased at higher neighbour densities, indicating adaptation for competitive ability. In the first year, biomass was more strongly reduced by densities than flowering was, but in the second year, when crowding was apparently greater, flowering was more severely affected. 4. Genet survival was high and similar for both subspecies. 5. The presumed advantage of guerrilla subspecies in exploiting open space was supported. The guerrilla grass exploited resources more quickly in the first year by faster growth and greater ramet production, but its biomass, ramet numbers and rhizome growth, and thus its advantage, were reduced in the second year. 6. The phalanx subspecies had slower growth, produced more ramets in later years, and delayed flowering until later years. Although less able to exploit open resources, it appeared adapted to more stressful conditions, and may be able to exploit temporal resource pulses more effectively.oH4-Brown, Joel S. Kotler, Burt P. Bouskila, Amos 2001VPEcology of fear: Foraging games between predators and prey with pulsed resources Annales Zoologici Fennici381i 71-87aWe model the foraging game between a prey and predator when the prey experiences a temporally pulsed resource (e.g., seed-eating gerbils). Animals have the options of foraging or remaining inactive. Prey harvest resources and incur a mortality risk only while foraging. ESS levels of prey and predator activity have three distinct phases over the time course of a resource pulse. During the first phase, resources are sufficiently abundant to permit profitable foraging by all prey and predators. During the second phase, only a fraction of prey and predator are active. The fraction of active prey is sufficient to allow profitable foraging by the predators. Resource abundances and activity level of predators decline synchronously, balancing the prey's needs for food and safety. During the third phase, resources decline to where both prey and predator cease activity. These adaptive behaviors of prey and predator to resources and to each other promote the stability of the predator-prey dynamics.60Buchard, Corinne McCully, Margaret Canny, Martin 1999\VDaily embolism and refilling of root xylem vessels in three dicotyledonous crop plants Agronomie19297-106.Embolisms were observed directly in the xylem vessels of roots of field-grown buckwheat (Fagopyrum esculentum Moench.), sunflower (Helianthus annuus L.) and soybean (Glycine max (L.) Merr.) by cryo-microscopy. No vessels were embolized at dawn, but in all three species some embolisms had formed by 0900 hours. The percentage of vessels embolized reached 40-60 % between 0900 and 1200 hours, then declined to near 0 % by sunset. The plants were still transpiring when embolisms were refilling. There was no correlation of percentage embolism with rate of transpiration, but a small positive correlation with balance pressure. Daily embolism and refilling of root vessels were observed previously in maize, and also appear to be a normal process in dicotyledonous crop plants.olned biotic soil crust cover and composition at nine shrub-steppe sites in central and eastern Oregon, U.S.A. One pair of livestock-grazed and excluded transects was established at each site. Data were collected on the cover of biotic soil crust and vascular plant species,@:Lauenroth, W. K. Sala, O. E. Coffin, D. P. Kirchner, T. B. 1994d^The importance of soil water in the recruitment of Bouteloua gracilis in the shortgrass steppeEcological Applications04V4o741-749In the shortgrass steppe region of North America there is a controversy about the ability of the dominant species to recruit from seedlings. The prevailing view is that Bouteloua gracilis is incapable of recruitment from seedlings in areas receiving lt 380 mm of annual precipitation. A common explanation for this situation is that environmental conditions permitting seedling establishment are infrequent. To assess the frequency of environmental conditions appropriate for the recruitment of B. gracilis we used a soil water simulation model and long-term climatic data in conjunction with detailed information about the ecophysiological requirements for seed germination and growth of seminal and adventitious roots. We found that recruitment events occur as frequently as every 30-50 yr on silty clay, silty clay loam, and silty loam soils, but less than once in 5000 yr on sandy soils. Simulated frequencies of recruitment were sufficient to account for the observed abundance of B. gracilis in 7 of 11 soil textures evaluated. The differences in silt content and available water holding capacity accounted for the difference among soil textures in the probability of occurrence of recruitment events. Therefore, soil texture variability may explain the spatial pattern of recruitment and of population recovery after disturbance that occur at the soil type and microsite scales. Annual precipitation explained a large fraction of the temporal variability in recruitment. On average, recruitment occurred in years when precipitation was above the mean. The occurrence of recruitment events in some dry years (precipitation lt mean), and their absence during some wet years (precipitation gt mean), emphasizes the importance of the intraseasonal distribution of precipitation. The sensitivity of recruitment to soil water availability suggests that climate change, particularly changes that increase or decrease the amount or the effectiveness of soil water, could have important effects on the future of populations of B. gracilis.etF?Pelaez, D. V. Distel, R. A. Boo, R. M. Elia, O. R. Mayor, M. D. 1994HAWater relations between shrubs and grasses in semi-arid Argentina"Journal of Arid Environments271m 71-78nThe two-layer hypothesis argues that in shrublands shrubs take up most of the water from the lower layers of the soil whereas grasses take up most of the water from the upper layers of the soil. We tested two predictions from this hypothesis in a shrubland of central semi-arid Argentina: (1) the correlation between plant water potential and soil water potential in the upper layers is stronger for grasses. The apparent pattern of water use by the deciduous shrub (Prosopis caldenia) and the two perennial grasses (Stipa tenuis and Piptochaetium napostanense) under study was consistent with the two-layer hypothesis whereas the evergreen shrub (Condalia microphylla) did not support this model. Our results suggest that the pattern of water use by grasses and shrubs in shrublands may be dependent upon strageties to counteract water stress and the phenological patterns developed by species within each life form.h<6Pendall, Elise Betancourt, Julio L. Leavitt, Steven W. 1999Paleoclimatic significance of deltaD and delta13C values in pinon pine needles from packrat middens spanning the last 40,000 years6/Palaeogeography Palaeoclimatology PalaeoecologyC 147p 1-2e 53-72hWe compared two approaches to interpreting deltaD of cellulose nitrate in pinon pine needles (Pinus edulis) preserved in packrat middens from central New Mexico, USA. One approach was based on linear regression between modern deltaD values and climate parameters, and the other on a deterministic isotope model, modified from Craig and Gordon's terminal lake evaporation model that assumes steady-state conditions and constant isotope effects. One such effect, the net biochemical fractionation factor, was determined for a new species, pinon pine. Regressions showed that deltaD values in cellulose nitrate from annual cohorts of needles (1989-1996) were strongly correlated with growing season (May-August) precipitation amount, and delta13C values in the same samples were correlated with June relative humidity. The deterministic model reconstructed deltaD values of meteoric water used by plants after constraining relative humidity effects with delta13C values; growing season temperatures were estimated via modern correlations with deltaD values of meteoric water. Variations of this modeling approach have been applied to tree-ring cellulose before, but not to macrofossil cellulose, and comparisons to empirical relationships have not been provided. Results from fossil pinon needles spanning the last apprx40,000 years showed no significant trend in deltaD values of cellulose nitrate, suggesting either no change in the amount of summer precipitation (based on the transfer function) or deltaD values of meteoric water or temperature (based on the deterministic model). However, there were significant differences in delta13C values, and therefore relative humidity, between Pleistocene and Holocene..'Phillips, Susan L. Ehleringer, James R.h 1995|Limited uptake of summer precipitation by bigtooth maple (Acer grandidentatum Nutt) and Gambel's oak (Quercus gambelii Nutt) Treesn9e4 214-219cWinter and spring precipitation that saturates to deep soil layers precedes summer droughts in the Intermountain West. Occasional summer convection storms relieve summer drought, but are infrequent and unreliable from year to year, leading to the hypothesis that dominant tree species might not invest limited carbon reserves to surface roots to take up summer precipitation in these regions. We compared the hydrogen (delta-D) and oxygen (delta-18O) isotope ratios of winter, spring and summer precipitation to that of xylem sap water in Acer grandidentatum and Quercus gambelii, two dominant trees of this region. By this method we could identify water sources utilized throughout the growing season. Xylem delta-D and delta-18O values changed significantly when each species leafed-out; this change was not associated with changes in either soil or plant water status (as measured by predawn and midday water potentials). This shift is apparently related increased transpirational flux, which may flush out residual stem water from the previous growing season. delta-D values of xylem sap of both species matched winter precipitation input values throughout most of the summer, indicating a reliance on deep-soil moisture sources throughout the growing season. Mature Q. gambelii did not take up summer precipitation, whereas A. grandidentatum responded slightly to the largest summer rain event. Small trees of both species, particularly A. grandidentatum, showed a limited uptake of summer rains.o[<5Paruelo, Jose M. Sala, Osvaldo E. Beltran, Adriana B. 2000rlLong-term dynamics of water and carbon in semi-arid ecosystems: A gradient analysis in the Patagonian steppe Plant Ecology 150f 1-2o133-143 We used a soil water simulation model and remotely sensed data to study the long-term dynamics of transpiration, evaporation, drainage and net primary production across a precipitation gradient in Northwestern Patagonia (Argentina). The proportion of precipitation transpired, the precipitation use efficiency and the transpiration use efficiency were constant across the gradient that covered a range of 150 to 600 mm. The proportion of water evaporated was higher than the proportion drained at the driest extreme of the gradient. The opposite relationship was observed at the wet extreme. Two important characteristics of arid-semiarid systems dominated by winter precipitation emerged from our analyses: the importance of drainage losses and the asynchrony between evaporation and transpiration fluxes. These characteristics of the water dynamics influence the relative abundance of plant functional types and are crucial to generate heterogeneity at the landscape level. The coefficient of variation (CV) of transpiration, evaporation and ANPP was, in general, lower than the CV of annual precipitation. This pattern suggests a buffering capacity of the ecosystem. The ecosystem would be able to damp at the functional level inter-annual changes in the availability of resources."LZTSpacing patterns in Mojave Desert (USA) plant communities: Nearest-neighbor analyses Cody, M. L."Journal of Arid Environments113199-218 1986The identities, sizes and densities of woody shrubs gt 10 cm high were recorded at four sites in the Mojave Desert of comparable elevation and topography, all located on flat, sandy granitic alluvium. For each commoner species, the identity, size and distance of c. 50 first and 50 second near-neighbors were recorded. Using the known densities of species in the quadrats, the observed neighbor frequencies were compared to those expected from random disposition of species. The comparison showed that some species are the 'preferred' neighbors of others, while yet other species are 'avoided' as neighbors. Some species 'prefer' conspecifics as neighbors, but the larger dominant shrubs almost invariably 'avoid' conspecifics and tend to a uniform spacing. Positive interactions (preferred neighbors) are 2-3 times commoner than negative interactions (involving avoided neighbors). It is hypothesized that the likely mechanism of these spacing patterns is differential compatibility of root systems, although specificity of germination sites may also play a role; this mechanism is hypothesized to be of importance in the maintenance of diversity in desert shrubs.eD>Growth-form diversity and community structure in desert plants Cody, M. L."Journal of Arid Environments172199-210 1989("Niche theory and plant growth form Cody, M. L.n VegetatioN971 39-55 1991lfPlant growth form diversity (GFD) is high in the vegetation of North American deserts, and increases from north (Great Basin Desert) to south (Sonoran Desert). While abiotic features (annual temperature, precipitation, and seasonality) appear to limit the range of desert plant GFD, biotic features associated with the coexisting plants at a site, and their FG distribution, add further constraints. Climate may constrain the GF options at certain sites and select for some degree of GF convergence there, but within sites other species in the vegetation select for GF segregation that fosters the local coexistence of species. In this paper GF variation is viewed along structural niche axes, and related to classical niche theory; several corollaries of the theory are examined in the light of plant GF patterns. These are: a) regular spacing of species on the structural niche axis, and the concept of limiting similarity; b) niche axis complementarity, such that species dissimilar in position on one axis, e.g. GF, are similar in postion on other axes, e.g. habitat or substrate, and vice versa; (c) niche shifts in GF within species are expected, and occur, as the suite of coexisting species varies among sites with similar climate; d) in some desert plant guilds species with very similar GF do not coexist at a site, but act as geographical replacements in different sites. Cohen, D. 1970nhThe expected efficiency of water utilization in plants under different competition and selection regimesIsrael Journal of Botany19 50-54s$Colasanti, R. L. Grime, J. P.r 1993piResource dynamics and vegetation processes: A deterministic model using two-dimensional cellular automataFunctional Ecology72a169-176eRules of resource capture, utilization and release derived from CSR theory have been used to construct a cellular automation model simulating secondary succession and spatial patterns of vegetation development on gradients in resource concentration and intensity of disturbance. It is confirmed that simple differences in the resource dynamics of founder populations are sufficient to generate a succession in which there are phases of dominance and decline consistent with Egler's initial floristic composition hypothesis, Odum's theory of ecosystem maturation and Connell and Slatyer's inhibition model of secondary succession. Differences in resource dynamics may also permit a stable coexistence of populations in the presence of gradients of vegetation disturbance. Patterns of vegetation development on a resource gradient are strongly affected by the prevailing levels of disturbance and the resource storage capacity of component populations. #4.Advances in Water Resources Adv. Water Resour.,&American Journal of Botany Am. J. Bot.D?Annual Review of Ecology and Systematics Annu. Rev. Ecol. Syst.82Biology and Fertility of Soils Biol. Fertil. SoilsPJBulletin of the American Meteorological Society Bull. Amer. Meteorol. Soc.(#Ecological Applications Ecol. Appl.$!Ecological Modelling Ecol. Model.(#Ecological Monographs Ecol. Monogr.<8Environmental and Experimental Botany Environ. Exp. Bot. Flora Flora40Forest Ecology and Management For. Ecol. Manage.$Functional Ecology Funct. Ecol.,'Hydrological Processes Hydrol. Process.@!HPu$Eissenstat, D. M. Yanai, R. D. 1997$The ecology of root life span&Advances in Ecological Research27 1-60(!Ely, L. L. Enzel, Y. Cayan, D. R.r 1994piAnomalous North Pacific atmospheric circulation and large winter floods in the southwestern United StatesJournal of Climate7a6977-987voSpecific anomalous atmospheric circulation conditions over the North Pacific are conducive to the occurrence of the largest winter floods ( less than or equal to 10-yr return period) on rivers in six hydroclimatic subregions of Arizona and southern Utah, Nevada, and California. Composite maps of anomalies in daily 700-mb heights indicate that floods in all of the subregions are associated with a low pressure anomaly off the California coast and a high-pressure anomaly in the vicinity of either Alaska or the Aleutian Islands. Of these two major circulation features, the presence of the low is the controlling factor in determining whether large floods will occur. Shifts in the locations of the low and high pressure anomalies over the North Pacific appear to control which subregions experience floods, with high-elevation topographic features and proximity to air masses forming a major influence over the specific atmospheric circulation conditions that generate large floods in each hydroclimatic region. Concerning the interannual variability of flooding in the Southwest, there is an increased frequency of large winter floods during multiple-year periods dominated by negative SOI and a virtual absence of large floods during the intervening periods. This suggests that global-scale climatic anomalies exert a strong influence on the occurrence of severe regional winter floods.v@:Epstein, H. E. Lauenroth, W. K. Burke, I. C. Coffin, D. P. 1996tnEcological responses of dominant grasses along two climatic gradients in the Great Plains of the United States$Journal of Vegetation Science716777-788fFew empirical data exist to examine the influence of regional scale environmental gradients on productivity patterns of plant species. In this paper we analyzed the productivity of several dominant grass species along two climatic gradients, mean annual precipitation (MAP) and mean annual temperature (MAT), in the Great Plains of the United States. We used climatic data from 296 weather stations, species production data from Natural Resource Conservation Service rangeland surveys and a geographic information system to spatially integrate the data. Both MAP and MAT were significantly related to annual above-ground net primary production (ANPP). MAP explained 54% to 89% of the variation in ANPP of two C-4 short-grasses, Bouteloua gracilis and Buchloe dactyloides, and two C-4 tallgrasses, Andropogon gerardii and Schizachyrium scoparium (= Andropogon scoparius). MAT explained 19% to 41% of the variation in ANPP of two C-4 grasses, B. gracilis and B. dactyloides, and 41% to 66% of the variation in ANPP of two C-3 grasses, Agropyron smithii and Stipa comata. ANPP patterns for species along both gradients were described by either linear, negative exponential, logistic, normal or skewed curves. Patterns of absolute ANPP (g/m-2) for species differed from those of relative ANPP (%) along the MAP gradient. Responses were similar for species with common functional characteristics (e.g. short-grasses, tall-grasses, C-3, C-4). Our empirical results support asymmetric responses of species to environmental gradients. Results demonstrate the importance of species attributes, type of environmental gradient and measure of species importance (relative or absolute productivity) in evaluating ecological response patterns.nRLProductivity patterns of C3 and C4 functional types in the U.S. Great Plains2,Epstein, H. E. Lauenroth, W. K. Burke, I. C. 1997EcologyL 1997 722-31(!Using Smart Source Parsing 78 AprrThe productivity of C3 and C4 grasses in the Great Plains of the U.S. was examined. Models of relative and absolute production were derived in terms of mean annual temperature (MAT), mean annual precipitation (MAP), percentage sand (SAND), and percentage clay (CLAY). Both the relative and absolute production of C3 grasses were negatively related to MAT and SAND and positively related to CLAY, whereas relative production decreased and absolute production increased with MAP. The production of C4 grasses was positively associated with MAT, MAP, and SAND and negatively associated with CLAY. MAT was the most explanatory variable for all models except the absolute production of C4, which was best explained by MAP. These models indicate that, under present climate conditions, C3 grasses dominate 35 percent of the Great Plains, mostly north of Colorado and Nebraska. A 2[degree]C increase in MAT would cause the C3 grasses to recede northward and dominate only 19 percent of the region.Biological productivity Plants; Grassland ecology Great Plains; Soil texture Effect on plants; Vegetation and climate Great Plains@9Epstein, Howard E. Burke, Ingrid C. Lauenroth, William K. 1999JDResponse of the shortgrass steppe to changes in rainfall seasonality Ecosystems22139-150azsStudies in temperate grassland ecosystems have shown that differences in composition of C3 and C4 plant functional types can have important influences on ecosystem pools and processes. We used a plant community dynamics model (STEPPE) linked to a biogeochemical cycling model (CENTURY) to determine how ecosystem properties in shortgrass steppe are influenced by plant functional type composition. Because of phenological differences between C3 and C4 plants, we additionally simulated the effects of precipitation seasonality on plant communities and examined how C3 and C4 composition interacts with precipitation to affect ecosystems. The model output suggests that differences in C3 and C4 composition can lead to differences in soil organic carbon (C) and nitrogen (N) within 1000 simulation years. Soil organic C and N (g C and N m-2 to 0.2-m depth) were least in a 100% C4 community compared with a 100% C3 community and a mixed C3-C4 community. A change in the time of maximum precipitation from summer to spring in a simulated shortgrass steppe slightly favored C3 plants over C4 plants. The proportion of total net primary production accounted for by C3 plants increased from 21% to 25% after 200 years, when 90 mm of precipitation was switched from summer to spring. Soil organic matter (SOM) was relatively stable in the C4-dominated communities with respect to changes in precipitation seasonality, whereas SOM in the C3 community was sensitive to precipitation seasonality changes. These results suggest an important interaction between plant community composition and precipitation seasonality on SOM, with phenology playing a key role.mi,&Hendrick, Ronald L. Pregitzer, Kurt S. 1993jcThe dynamics of fine root length, biomass, and nitrogen content in two northern hardwood ecosystems*#Canadian Journal of Forest Researchc2312 2507-25205The dynamics of fine ( lt 2.0 mm) roots were measured in two sugar maple (Acer saccharum Marsh.) dominated ecosystems (northern and southern sites) during 1989 and 1990 using a combination of minirhizotrons and destructive harvests of fine root biomass and N content. Greater than 50% of annual length production occurred before midsummer in both ecosystems, while the period of greatest mortality was from late summer through winter. About one third of annual fine root production and mortality occur simultaneously, with little observable change in total root length pools. Using fine root length dynamics to derive biomass production and mortality, we calculated annual biomass production values of approximately 8000 and 7300 kg cntdot ha-1 cntdot year-1, respectively, at the southern and northern sites. Corresponding biomass mortality (i.e., turnover) values were 6700 and 4800 kg cntdot ha-1 cntdot year-1, and total nitrogen returns to the soil from fine root mortality were 72 kg cntdot ha-1 cntdot year-1 at the southern site and 54 kg cntdot ha-1 cntdot year-1 at the northern site. Fine roots dominated total biomass and N litter inputs to the soil in both ecosystems, accounting for over 55% of total biomass and nearly 50% of total N returns. In both ecosystems, roots lt 0.5 mm comprised the bulk of fine root biomass and N pools, and the contribution of these roots to northern hardwood ecosystem carbon and nitrogen budgets may have been underestimated in the past.b,&Hendrick, Ronald L. Pregitzer, Kurt S. 1996\VTemporal and depth-related patterns of fine root dynamics in northern hardwood forestsJournal of Ecology842167-176p 1 The dynamics of fine ( lt 2.0 mm) roots growing in two northern hardwood forests were quantified to a depth of 1 m in 10-cm increments. We hypothesized that patterns of root production and mortality would be broadly synchronous at all depths, but that deep roots would be comparatively less dynamic than shallow roots. 2 Our data showed that shallow roots were responsible for the majority of total annual fine root production and mortality. Nearly half of all roots growing and dying in the 1-m profile occurred in the upper 20 cm of the soil, while roots located at depths of 75 cm or more accounted for only 11% of annual production and 4% of annual mortality. 3 Fine root production prior to, or coincident with, canopy expansion was significant at all depths. The relative importance of early fine root growth generally increased with depth, with between 50 and 80% of annual production occurring prior to mid-June at depths exceeding 50 cm. Episodic deep root production during the growing season appeared to be related to periods of high water demand. 4 Patterns of fine root mortality were more variable among depths. Mortality was distributed rather evenly throughout the year near the soil surface, but mid-season mortality was generally low at depths greater than 50 cm.ogyne ramosissima (blackbrush), Salvia mohavensis, Y. brevifolia and Y. whipplei (Whipple yucca) expanded their ranges southward and to lower elevations. Dominants of arid central Baha California communities such as Fouquieria columnaris (boojum tree) and Pachycormus discolor (elephant tree) were conspicuously absent from low-elevation Lower Colorado River Valley and Gran Desierto midden assemblages. Arid desert scrub communities have probably been in these Sonoran Desert lowlands throughout the Pleis ocene.GF?Varnamkhasti, A. S. Milchunas, D. G. Lauenroth, W. K. Goetz, H. 1995piProduction and rain use efficiency in short-grass steppe: Grazing history, defoliation and water resourcea$Journal of Vegetation Science66-787-796tGrassland in the semiarid shortgrass steppe, subjected to 50 years of heavy, light, and no grazing intensity, was clipped to simulate the natural pattern and intensities of defoliation by cattle or not clipped. A level of water resource treatment was superimposed upon the grazing and clipping treatments. Half of the plots were supplemented with additional water to simulate a wet year and hall were not supplemented in a year of average precipitation. All three treatments interactively determined above-ground production. Water treatment had the largest overall effect oil above-ground production. Current-year defoliation had no direct significant effect on production, but mediated differences between both long-term grazing and watering treatments. Long-term ungrazed compared to grazed grassland was capable of responding to high amounts of precipitation, but was also most affected by low amounts of precipitation and, therefore, displayed greater variability in above-ground production and rain use efficiency. Only in the year of average precipitation, defoliation increased rain use efficiency in long-term lightly, but not heavily, grazed treatment. This suggests a water conservation mechanism of defoliation that is reduced with heavy grazing.  tAdiku, S. G. K. Allen, C. D. Barnes, F. J.BassiriRad, H.Boisvert, J. B. Bond, B. J. Bouten, W.Braddock, R. D.Breshears, D. D. Bruen, M.Brunell, M. H.Caldwell, M. M. Campbell, K. Canadell, J. Chesson, P Chesson, P.Cioccale, M. A. Clark, J. L.Clothier, B. E.Colello, G. D. Cook, E.Coughlan, J. C.Davenport, D. W. Dawson, T. Dawson, T. E. De Jong, R. de Rosnay, P.de Soyza, A. G.Dickinson, R. E. Dirmeyer, O.Donovan, L. A. Duke, S. E.Ehleringer, J. R. Ewers, B. E. Feddes, R. A. Field, C. B.Francovizcaino, E.Friedlingstein, P.Gibbens, R. P. Green, S. R. Gui, S. X.Hakonson, T. E. Heil, C. E. Heimann, M. Hibbard, K. Hoff, H.Hopmans, J. W. Horton, J. L. Hsiao, T. C. Hunt, E. R.Jackson, R. B.Jobbagy, E. G.Johansen, M. P.Johnson, S. R. Joseph, R. Kabala, Z. J. Kabat, P. Kalma, J. D. Katul, G. Kemp, P. R.Kicklighter, D. W. Kinzig, A Kleidon, A. Kremer, R. G. Kumar, P. Lai, C. T. Lall, U.Langridge, J. L.Leffler, A. J. Lenz, J. M. Lhomme, J. P. Li, K. Y. Lilly, A.Lindemann, W. C. Linton, M. J. Liu, X. Y. MacDonald, L.Martens, S. N.McMichael, B. L. Meyer, C. W. Moen, R. A.Musters, P. A. D. Myers, O. B.Neilson, R. P. Neuhauser, C Nowak, R. S.Nungesser, M. K. Nyhan, J. W.Ojha, C. S. P. Or, D. Oren, R.Ozier-Lafontaine, H. Pacala, SPachepsky, Y. A. Parton, W. J.Passioura, J. B. Pataki, D. Phillips, N. Pitman, A. J.Pockman, W. T. Rai, A. K.Rajagopalan, B. Ray, B. K. Reid, K. D.Reynolds, J. F. Rich, P. M.Richards, J. H. Rose, C. W.Running, S. W. Ryan, M. G. Ryel, R. J. Sala, O. E. Schenk, H. J. Shein, E. V. Simunek, J.Sosebee, R. E. Sperry, J. S.Steiner, R. L.Stirzaker, R. J. Sykes, M. T.Tenhunen, J. D. Tilman, D Ting, M. F. Todd, P.Tremmel, D. C.van Wijk, M. T.Virginia, R. A. Vrugt, J. A. Walker, B. H. Walker, J. P. Wan, C. G.Whitford, W. G. Wilcox, B. P.Willgoose, G. R.Williams, D. G. Williams, M. Wu, J. Q. Xu, L. K.Yeakley, J. A. Yoder, C. K. Zhang, R. D.\EBF?Jackson, L. E. Strauss, R. B. Firestone, M. K. Bartolome, J. W. 1988JCPlant and Soil Nitrogen Dynamics in California Usa Annual GrasslandPlant and Soil 1101m 9-18vpSeasonal chanegs in soil water and nitrogen availability were related to the phenology and growth of plants in California annual grassland. Plant accumulation of nitrogen was mainly confined to two shorts period of the year: fall and early spring. At these times, plants were in the vegetative growth phase, roots were growing rapidly and soil moisture was high. During these periods, soil nitrate was low or depleted. High flux of nitrogen in this ecosystem, however, is indicated by the rapid disappearance of the previous year's detrital material, high microbial biomass, and high mineralizable nitrogen and nitrification potential. At the end of the summer drought, significant amounts of the previous year's detrital material had disappeared, chloroform-labile N (expressed as microbial biomass N) was at its seasonal maximum, and soil inorganic nitrogen pools were high. This suggest inorganic nitrogen flux during the drought period. The 'drought escaper' life history characteristics of annual grasses in California annual grassland, however, may prevent plants from utilizing available nitrogen during a large part of the year.$Jackson, R. B. Caldwell, M. M. 1989ngThe Timing and Degree of Root Proliferation in Fertile-Soil Microsites For Three Cold-Desert Perennials Oecologia812149-153oRoot proliferation in nutrient-rich soil patches is an important mechanism facilitating nutrient capture by plants. Although the phenomenon of root proliferation is well documented, the specific timing of this proliferation has not been investigated. We studied the timing and degree of root proliferation for three perennial species common to the Great Basin region of North America: a shrub, Artemisia tridentata, a native tussock grass, Agropyron spicatum, and an introduced tussock grass, Agropyron desertorum. One day after we applied nutrient solution to small soil patches, the mean relative growth rate of Agropyron desertorum roots in these soil patches was two to four times greater than for roots of the same plants in soil patches treated with distilled water. Most of the increased root growth came from thin, laterally branching roots within the patches. This rapid and striking root proliferation by Agropyron desertorum occurred in response to N-P-K enrichment as well as to P or N enrichment alone. A less competitive bunchgrass, Agropyron spicatum, showed no tendency to proliferate roots in enriched soil patches during these two-week experiments. The shrub Artemisia tridentata proliferated roots within one day of initial solution injection in the N-enrichment experiment, but root proliferation of this species was more gradual and less consistent in the N-P-K- and P-enrichment experiments, respectively. The ability of Agropyron desertorum to proliferate roots rapidly may partly explain both its general competitive success and its superior ability to exploit soil nutrients compared to Agropyron spicatum in Great Basin rangelands of North America.$Jackson, R. B. Caldwell, M. M. 1993lfThe scale of nutrient heterogeneity around individual plants and its quantification with geostatisticsEcology742612-614n]EJY WJ2J1Xad'<gB9i_uvmvm`{qq _X~$\a$%!!!!zp) 7B7=[YL[Y;98+rihh._nggg__B@*%Ep XC|||09L*l .5 hX>> DD WbP&E=6RKXiao, Xiangming Wang, Yifeng Jiang, Shu Ojima Dennis, S. Bonham Charles, D.a 1995Interannual variation in the climate and above-ground biomass of Leymus chinensis steppe and Stipa grandis steppe in the Xilin river basin, Inner Mongolia, China5"Journal of Arid Environments313y283-299spiWe evaluated the relationship between variability in climate and variability in primary production and efficiencies of water use of Leymus chinense steppe and Stipa grandis steppe during 1980-89. On average, annual precipitation was 313.3 mm, while peak above-ground live biomass (PALB) and peak standing crop (PSC) were 182.68 g.m-2 and 193.48 g.m-2 for L. chinense steppe, 144.43 g.m-2 and 152.12 g.m-2 for S. grandis steppe. The coefficient of variation (CV) in annual precipitation was 22%, while the CV in PALB and PSC were 29% and 26% for L. chinense steppe, 24% and 25% for S. grandis steppe. Rain-use efficiency was 6.3 kg.DM.ha-1mm-1year-1 for L. chinense steppe and 4.9 for S. grandis steppe, using PSC as the estimate of ANPP. Monthly and seasonal patterns of precipitation were as important as annual precipitation in determining responses of these two steppes. HAXiao, Xiangming Chen, Du Peng, Yumei Cui, Xianyi Ojima Dennis, S.B 1996pjObservation and modeling of plant biomass of meadow steppe in Tumugi, Xingan League, Inner Mongolia, China Vegetatio 127s2t191-201B://000087656400007t"Duke, S. E. Caldwell, M. M.opjPhosphate uptake kinetics of Artemisia tridentata roots exposed to multiple soil enriched-nutrient patches Flora phosphate uptake; root; soil; Artemisia shrub-steppe ecosystem; mineral-nutrition; growth; plants; nitrate; responses; system; geostatistics; acquisition; capture Root phosphate (P) uptake capacity in a nutrient-rich soil patch (termed the Primary Patch) was assessed as the rest of the root system was exposed to different mineral nutrient conditions in the form of other nutrient-rich patches (Additional Patches) in a factorial experiment. The experimental design involved different numbers of Additional Patches with different phosphate concentrations, [P-i]. In addition to testing the effect of the total quantity of added P on root uptake capacity in the Primary Patch, the influence of the distribution of the added P was also tested. The [P-i] and number of Additonal Patches were chosen such that the total quantity of P added was the same among some treatments which afforded a test of the influence of the concentration and distribution of additional Patches on the uptake capacity of the roofs in the Primary Patch. As the number and the concentration of Additional Patches increased, the upake capacity of Primary Patch roots was expected to be reduced relative to roots from untreated Control Patches. Averaged across all treatments, roots from the Primary Patch exhibited greater uptake capacities than the Control Patch roots (i.e., roots from unamended soil). With substantial enrichment of P-i in the Additional Patches, the uptake capacity of roots was reduced in both the Primary and Control Parch roots. However, the concentration and number of Additional Patches had no clear influence on P uptake capacities in the Primary Patch. The P uptake capacity of roots from the Additional Patches with low and moderate [P-i] did not differ from the uptake capacity of roots in the Control Patches. However, for roots in the Additional Patches with high [P-i] (equivalent [P-i] to that of the Primary Patch) the uptake capacities were significantly greater that those of the Control Patch roots. In an ancillary P acquisition experiment, plants procured less P from the Primay Patch if then were four Additional Patches than if there were only two Additional Patches. This suggests that P acquisition occurred in the Additional Patches and that regulation at the whole-plant level reduced the uptake of P from the Primary Patch. In contrast, the root uptake kinetics did not seem to be as responsive to added P-i presented in the Additional Patches. Flora 2000 Apr 1952 325BL FLORAISI:000087656400007 ical correspondence analysispinon-juniper woodland\VMandujano, Mara C. Miguel Franco Jordan Golubov Carlos Montaa Arturo Flores-Martnez 2001NGIntegration of demographic annual variability in a clonal desert cactusEcology782344359yCactaceae; Chihuahuan Desert, Mexico; clonal propagation; demographic parameters; demographic plasticity; elasticity; life history; Opuntiaperiodic matrix; prickly pear; sexual reproduction; stochastic environments   kSpatial and temporal variability influence the structure and dynamics of perennial plant populations. In order to investigate the consequences of this environmental heterogeneity on population and life history traits of a perennial plant with a complex life history, four size-classified population matrix models were employed (i.e., annual, mean, periodic, and stochastic simulations) in a clonal cactus, Opuntia rastrera, in the Chihuahuan Desert, Mexico. This species was studied over a seven-year period in two contrasting, neighboring habitats (nopalera and grassland). The specific aims were: (a) to assess the effect of annual environmental variability on the long-term dynamics of both populations; and (b) to estimate the relative contribution of sexual reproduction and clonal propagation to the populations' rates of increase. Projections from each model provided complementary information on population dynamics and life history in the two habitats. The finite rate of population growth () varied among matrix projections. Mean, periodic, and stochastic projections yielded > 1 for nopalera and 1 for grassland. The relative contributions of size classes and demographic processes to changes in differed widely between years and habitats for annual matrices. In contrast, elasticities of periodic matrices showed a stable habitat-dependent pattern. The proportional change in produced by sexual recruitment and clonal propagation showed wide spatial variation in which the most distinctive difference between habitats was the predominance of clonal recruitment at the nopalera and seedling recruitment at the grassland. Elasticities also showed temporal variation whereby clonal propagation decreased as precipitation increased, while sexual reproduction and growth tended to increase with precipitation. The striking spatial and temporal differences found in the structure, dynamics, and life history traits of O. rastrera are aptly summarized by the periodic analyses. These differences reflect both the varying influence of the selective pressures operating on this species and the ways in which demographic plasticity deals with them.     higher root/shoot ratio (P lt 0.05), but only 17% hydraulic conductance of those from Tahoka, Texas (TA) seed source. Consequently, transpiration surface area of the ID seedlings was 17% that of the TA seedlings. There was no difference in total root length between the seedlings, but hydraulic conductance per unit root length was 3.8 fold greater (P lt 0.05) in the TA seedlings, which was accompanied by a lower specific lateral root length (P lt 0.05), thus greater lateral root diameters. A parallel experiment with adult plants from New Mexico (NM) and Plains, Texas (PL) and the above two populations showed that whole-plant hydraulic conductance was ordered as NM gt TA=PL gt ID. The NM and TA populations had lower specific lateral root length (P lt 0.05), thus larger-diameter laterals, than the ID population. The NM population had similar transpiration surface area and total root length as that of the TA population, but a greater lateral root biomass (P lt 0.05) than the latter. Because of higher hydraulic conductance in the NM population, its xylem water potential declined more slowly than that of the Texas and Idaho populations as soil moisture deficit developed. Stem hydraulic conductance was positively correlated with xylem water potential. Therefore, the NM population was able to maintain a higher stem conductance during drought, which was associated with its higher whole-plant hydraulic conductance and greater carbon partitioning in the stems. The drought avoidance mechanism in the NM plants was characterized by an efficient water transport system due to greater lateral root biomass and more stems per unit leaf area. The higher root/shoot ratio in desert populations (NM and ID) as compared with that of the semiarid rangeland populations (TA and PL) does not ensure more effective water acquisition. Rather, root morphological modifications such as accelerated secondary thickening of lateral roots may play an important role in enhancing water-acquisition capability of the root systems of Gutierrezia sarothrae. gooddingii(!Hooper, David U. Johnson, Lorettau 1999rlNitrogen limitation in dryland ecosystems: Responses to geographical and temporal variation in precipitationBiogeochemistry46 1-3e247-293 We investigated the relationship between plant nitrogen limitation and water availability in dryland ecosystems. We tested the hypothesis that at lower levels of annual precipitation, aboveground net primary productivity (ANPP) is limited primarily by water whereas at higher levels of precipitation, it is limited primarily by nitrogen. Using a literature survey of fertilization experiments in arid, semi-arid, and subhumid ecosystems, we investigated the response of ANPP to nitrogen addition as a function of variation in precipitation across geographic gradients, as well as across year-to-year variation in precipitation within sites. We used four different indices to assess the degree of N limitation: (1) Absolute Increase of plant production in response to fertilization (the slope of ANPP vs. amount of added N at different levels of annual precipitation); (2) Relative Response (the percent increase in fertilized over control ANPP at different levels of N addition); (3) Fertilizer Use Efficiency (FUE, the absolute gain in productivity per amount of fertilizer N), and (4) Maximum Response (the greatest absolute increase in ANPP at saturating levels of N addition). Relative Response to fertilization did not significantly increase with increasing precipitation either across the geographic gradient or across year-to-year variation within sites. Nor did the Maximum Response to fertilization increase with increasing precipitation across the geographic gradient. On the other hand, there was a significant increase in the Absolute Increase and FUE indices with both geographical and temporal variation in precipitation. Together, these results indicate that there is not necessarily a shift of primary limitation from water to N across the geographic water availability gradient. Instead, our results support the hypothesis of co-limitation. The apparently contradictory results from the four indices of N limitation can best be explained by an integration of plant ecophysiological,community, and ecosystem mechanisms whereby plants are co-limited by multiple resources, species shifts occur in response to changing resource levels, and nitrogen and water availability are tightly linked through biogeochemical feedbacks.^[|JCXiao, Xiangming Jiang, Shu Yifeng, Wang Ojima, D. S. Bonham, C. D. 1996Temporal variation in aboveground biomass of Leymus chinense steppe from species to community levels in the Xilin River Basin, Inner Mongolia, China Vegetatio 1231 1-12`YWe analyzed the long-term dynamics of aboveground biomass of Leymus chinense steppe in relation to interannual variation of precipitation and temperature during 1980-1989 at levels of community, growth form and species in the Xilin river basin, Inner Mongolia Autonomous Region, China. Annual aboveground net primary production (ANPP) varied from 154.00 g m-2 yr-1 in 1980 to 318.59 g m-2-1 in 1988, with a mean of 248.63 g m-2 yr-1 and the coefficient of variation of 25%. ANPP was not significantly correlated to annual precipitation and total precipitation during April-September at p ltoreq 0.05 level, but precipitation in May and August accounted for 69% of interannual variation of ANPP. The means of rain use efficiency and water use efficiency of L. chinense steppe were 8.1 kg DM ha-1 mm-1 yr-1 and 0.89 mg DM g-1 H-20 respectively. Aboveground biomass of various growth forms and species had different response patterns to interannual variation of precipitation and temperature. Monthly and seasonal distribution of precipitation and temperature were the key controls of aboveground biomass of species.hbYan, Shunguo Wan, Changgui Sosebee, Ronald E. Wester, David B. Fish, Ernest B. Zartman, Richard E. 2000Responses of photosynthesis and water relations to rainfall in the desert shrub creosote bush (Larrea tridentata) as influenced by municipal biosolids"Journal of Arid Environments464t397-412rResponses of photosynthesis (Pn), stomatal conductance (gs), pre-dawn leaf water potential (PSIlp) and leaf water content (omegal) of creosote bush to 10 rainfall events in the Chihuahuan Desert were investigated. Infiltration of rainwater was manipulated by applying municipal biosolids. The responses of Pn and water relation parameters to rainfall (> 10 mm) were mainly dependent upon drought severity: (1) following a moderate drought, Pn, gs, PSIlp and omegal recovered to corresponding values of irrigated plants within 2 days after a 23-mm rainfall; (2) PSIlp and gs responded to a 15-mm rainfall within 2 days, following a 25-day drought, whereas responses of Pn and omegal were delayed for several days; (3) responses of Pn, gs, PSIlp and omegal to a 14.7-mm rainfall were all delayed for several weeks following a 110-day drought, but the delay was longer in Pn, gs and omegal than in PSIlp. Creosote bush responded to small rainfall events (approximately 6 to 8 mm) with an increase in PSIlp, but without noticeable changes in gs and Pn, suggesting a strong stomatal control of water loss even though xylem embolism was reduced. Biosolids applied at high rates (3.4 and 9 kg m-2) decreased the soil water by 2 to 4 mm following rainfall events, and this in turn delayed and decreased the responses of Pn and water relation parameters to rainfall. Pn and gs were linearly related to omegal and exponentially related to PSIlp. With the generally coincidental responses of Pn or gs and omegal to rainfall, we concluded that the responses of Pn and gs to rainfall were dependent on leaf rehydration which resulted from restored hydraulic conductance following drought.epjResponse of North American ecosystem models to multi-annual periodicities in temperature and precipitationLFYeakley, J. Alan Moen Ron, A. Breshears David, D. Nungesser Martha, K. 1994Landscape Ecology 9o4r249-260c Using Smart Source Parsing249-260$://A1994PX89500002dB7Yoder, C. K. Thurow, T. L. Carlson, D. H. Caesar, B. L.o 1995Southwestern Naturaliste403P273-280e Using Smart Source ParsingPJRoot distribution was determined in terms of biomass, length and root hairs/mm root for populations of sideoats grama (Bouteloua curtipendula) and common curlymesquite (Hilaria belangeri) in the Edwards Plateau and Rolling Plains resource regions of Texas. There were few significant differences in incremental (100 mm increments to a depth of 1 m) or total root biomass or root length between species at either site. Sideoats grama had a significantly greater root hair density than curlymesquite from the soil surface to a depth of 600 mm at both sites. Root hair density was greatest, for both species, at the Rolling Plains site. Greater root hair density of sideoats grama may be an adaptation allowing greater and more rapid access to water, as suggested by concurrent studies of water use within lysimeters with sideoats grama or curlymesquite. This suggests that greater root hair density of sideoats grama confers an adaptive advantage with respect to soil water use compared to curlymesquite, and may be partially responsible for its regional dominance in advanced succession communities.  1988 drought 1993 floods abscisic-acid accumulation acquisition activityAegilops cylindricaagravitropic pea mutantAgropyron desertorumair-amazonian deforestationAmbrosia dumosa anomaliesapoplastic solutesapparent competition aquaporins architecturearidarid rangeland arizona ArtemisiaArtemisia tridentataartemisia-tridentata atmospheric australia availability balance barleybasinbelowground processes and global change, biogeochemistry, ecosystem models, global change, plant life forms, roots, shrub encroachment, soil carbon and nutrients, water balancebetula-occidentalis biogeography biomassBouteloua eriopodaBouteloua gracilisBromus tectorumbroom snakeweedC-13 calibration$!canonical correspondence analysis canopy canopy and intercanopy gaps capture carbon carbon isotope discriminationcatastrophe theorycatastrophe-theorycell-wall extensionchaparral shrubsChihuahuan DesertChrysothamnus nauseosusclimate cyclesclimate variabilityclimate-change coexistence communities communitycomparative approach competition conductance conductivity continuum$creosotebush larrea-tridentatacropdecadal variabilitydeciduous shrubs deep rootsdelta delta D delta N-15 dependence desertdesert ecosystems desert shrub desert shrubsdesert succulentsdesert xerophytedesertification determinationdifferential use distribution diversitydouglas-fir-forest droughtdrought responsedrought tolerancedryas-octopetala ecotypes drying soil dynamics ecological ecosystem ecosystems ecotype edulis efflux el-ninoelectromagnetic embolismenvironmental gradient equation erodibility erosion ess germination strategies establishmentestimating soil-water evaporationevapotranspiration evergreen evolutionary- exchangeexpansive growthexperimental ecology exploitation extractionextraction patternsfertile-soil microsites fertilizerfield fine rootsfire floodplainflourensia-cernuaflowforbs forest$forest architecture hypothesisforest ecologyfremont cottonwoodfrequency analysis gap models gas-exchange general-model geostatistics germination global-scale gradientgrass grasses grasslandgrassland ecologygrassland/forest grassland/forest continuum grazing Great Basin great- great-basin growth growth zonegutierrezia-sarothrae halophyteshardwood forest ecosystem$ hawaiian metrosideros-polymorphaheat-flux estimationherbaceous vegetation heterogeneity hydraulichydraulic conductancehydraulic conductivityhydraulic isolation ofhydraulic lift hydraulic lift/redistributionhydraulic resistance hydrologic hydrologic characteristics hydrologyhydrostatic pressureidentification infiltrationinfiltration rates interannual intercanopyinterdecadal variabilityinterspecific competition$intraseasonal extreme rainfall irrigation Jornada Basin Jornada LTER juniper Juniperus kiwifruitlagsland degradationland-surface schemes LarreaLarrea tridentatalarrea-tridentataleaf leaf nitrogenleaf water potential leaf-area,&leaky cable model of root water uptakeliftlimiting similarity linkageslitter quantities long-termlupin Lyciummaize maize plantsmaize primary rootmicroarthropods microclimatemicroorganismsmineral-nutritionmodel modeling modelling models moisturemoisture transport moisture use mojave desert monosperma multicomponent decompositionmycorrhizal fungin-15of Q. emoryi recruitment within grassland below tree line are relatively low and are constrained by low rates of seed dispersal coupled with a low probability of seedling emergence. In contrast, large numbers of acorns are dispersed directly beneath Q. emoryi trees, where they have a higher probability of emergence than in adjacent grassland. Survival rates of emerged seedlings were low, regardless of landscape position. Thus, observed patterns of seedling distribution on the landscape resulted from interactions between seed dispersal and habitat-specific response of seedlings to environmental variation. Results of this and complementary research suggest that the lower tree line in southern Arizona is stabilized by self-enhancing feedback mechanisms of overstory shade, seed dispersal, and seedling establishment, coupled with strong abiotic constraints beyond the current ecotone. These processes stabilize the woodland-grassland ecotone both spatially and temporally, consistent with Wilson and Agnew's one-sided positive feedback switch. Although this switch would not produce an indefinitely stable vegetation mosaic, upslope or downslope shifts in lower tree line are apparently resistant to decadal or even century-scale climatic pe rturbation. The observed shift in tree line in the last millennium was less likely the result of slow, spatial progression of autogenic safe sites than the result of episodic and infrequent allogenic processes that simulated or negated the importance of conspecific, biogenic safe sites.$t Ehleringer1984! Ehleringer1990( Ehleringer1990 Ehleringer1991(" Ehleringer19919 Ehleringer1992 Ehleringer1992(f Ehleringer1992~ Ehleringer1992 Ehleringer1992l Ehleringer1992l Ehleringer1992l Ehleringer1993( Ehleringer1993* Ehleringer1993 Ehleringer1994. Ehleringer1994 Ehleringer1994 Ehleringer1994 Ehleringer1994l Ehleringer1995 Ehleringer1995 Ehleringer1995 Ehleringer1995lD Ehleringer1996( Ehleringer1996{ Ehleringer1996 Ehleringer1996 Ehleringer1996ly Ehleringer1997z Ehleringer1997l Ehleringer1997l Ehleringer1998l Ehleringer1998l< Ehleringer1999 Ehleringer2000 Ehleringer2000l Ehleringer2000l Ehleringer2000 Ehleringer2001 Ehleringer2002 Eissenstat1985 Eissenstat1988 Eissenstat1989u Eissenstat1997 Elia19949 Elia19966 Ellsworth2000 Ely1994 Engen1997 Enzel1994 Epstein1996! Epstein1997 Epstein1998 Epstein1999~ Ernst1996, Esler1994 Esler1999 Esler Karen2000 Evans1991 Evans1992 Evans1992 Evans1993 Evans1993 Evans1994 Evans1999 Evans2001 Ewers1998 Ewers2000z Ezcurra1996{ Ezcurra1998M Farrar19949 Feddes2001 Felger19900 Fellows1992 Fernandez1975  Fernandez2000Fernandez-Illescas2001 Field2000E Firestone1988 Fish20000 Fisher1985T Fitter1994c Fitter19999yFlanagan1997Flanagan19989Flores-Martnez2001h Fogel1993j Fogel1993k Foley2001 Fonteyn1987 Forseth1990' Fowler1995( Franco19969 Franco20010Francovizcaino1994Friedlingstein2000 Fuji1997 Garcia1998 Garcia-Pichel2001Gardener1995 Gebauer2000 Gebauer2000 Georgakakos1999 Gibbens2001 Gill19989 Gill1999h Giller Ken20000G Goetz1995 Gold19954Goldberg1986Goldberg19972Goldberg200103Goldberg200105Goldberg2001x Golluscio1989d Golluscio1997^ Golluscio1998r Golluscio1998 Golubov20017 Gozlan19989> Grace1992? Grace1994 Graham19989 Green1997c Griffiths1999& Grime1989" Grime1993% Grime2000 Grimm1985C Groeneveld1988Groeters1987;Grosjean1998ce Gross1992g Gross1993 Grover1988 Grover1990 Grover1991 Gui1999@ Guo1993B Guo1995E Guo1996 Gutierrez1992 Gutierrez19967 Gutterman1998 Hacke2000 Hakonson2001 Handley1999 Harper2001 Heaton19991 Heggem1994 Heil19988 Heimann2000iHendrick1993jHendrick19933mHendrick1996X Henkin1998('Hennessy1995 Herr19955 Heske1990@ Heske1993 Hibbard2000c Hodge1999 Hoff20010Holbrook1986NHolland Elisabeth1995 Holm19979, Honig1994 Hook19989 Hooper1999h Hooper David20000 Hopmans2001 Horton2001d Horwath1992 Hsiao2000Humphrey1998 Humphries1999K Hunt1987L Hunt1987M Hunt19877N Hunt1987 Hunt19969 Huntly19888 Huntly19899 Huntly19933 Huntly199776 Huston1994 Hutchings1999@ Inanaga1999  IPCC1996  Ives1997F Jackson1986E Jackson1988 Jackson1989 Jackson1993 Jackson1994D Jackson1996{ Jackson1996 Jackson1996 Jackson2000 Jackson2000 Jackson2001> Jarvis19922? Jarvis19944 Jasienski1997 Jeltsch1998 Jeltsch2000 Jeltsch2000 Jeltsch2001 Jiang1995 Jiang1996D Jobbagy1996^ Jobbagy1998q Jobbagy1998 Jobbagy2000 Johansen2001 Johnson1994 Johnson1997 Johnson1999 Jones2001<Jorquera19999 Joseph2000w Joyce1988 Joyner1997 Kabala1997 Kabat2001 Kachi1990 Kadmon1990( Kadmon1990 Kadmon1993 Kadmon19971 Kadmon20002X Kafkafi19989 Kalin1988; Kalin1998 Kalma2001z Kao Wen1997 Katul1997 Katul1998 Katul2000 Keesing2000 Kellner1998 Kelly1998+ Kemp19921 Kemp19940 Kemp1997 Kemp19991 Kemp20002 Kicklighter2000lKirchner19944 Kleidon2000 Kleidon2001 Klopfer1997 Knapp2001J Knoop19858 Kolb19997 Kolb1999 Kotler2001L Kravitz1998 Kremer1996k Kubiske1995 Kumar2000 Lai2000 Laio2001 Laio2001 Laio2001 Laio2001 Lall2000a Lande1997 Lane1998 Lane2000 Lange1997 Lange2001 Lange2001 Langridge1996o Laskowski1998v Lathrop1987 Latorre2000 Lauenroth1982j Lauenroth1985v Lauenroth1987w Lauenroth1988x Lauenroth1989t Lauenroth1992l Lauenroth1994G Lauenroth1995 Lauenroth1995 Lauenroth1996} Lauenroth1996 Lauenroth1996 Lauenroth1996! Lauenroth1997 Lauenroth1997 Lauenroth1997 Lauenroth1997 Lauenroth1997! Lauenroth1997! Lauenroth1997Lauenroth1997h1996! Lauenroth1997 Lauenroth1997 Lauenroth1997 Lauenroth199719991 Heggem1994f Heimann2000X Henkin1998('Hennessy1995 Heske1990@ Heske Edward1993f Hibbard2000Holbrook1986NHolland Elisabeth1995, Honig1994 Hook19989 Hooper1999h Hooper David20000Humphrey1998 Humphries1999 Huntly199776 Huston1994 Hutchings1999@ Inanaga1999  IPCC1996  Ives1997F Jackson1986E Jackson1988 Jackson1989 Jackson1993 Jackson1994D Jackson1996f Jackson2000> Jarvis19922? Jarvis19944 Jasienski1997 Jeltsch1998 Jeltsch2000 Jeltsch2000 Jeltsch2001 Jiang1995 Jiang1996D Jobbagy1996^ Jobbagy1998q Jobbagy1998f Jobbagy2000 Johnson1994 Johnson1999 Jones2001<Jorquera19999w Joyce1988 Joyner1997 Kachi1990 Kadmon1990( Kadmon1990 Kadmon1993 Kadmon19971  Kadmon20002X Kafkafi19989 Kalin1988; Kalin Arroyo1998 Keesing2000 Kellner1998 Kelly1998+ Kemp19921 Kemp19940 Kemp1997) Kemp19991* Kemp20002f Kicklighter2000lKirchner19944f Kleidon2000  Klopfer1997 Knapp2001J Knoop19858 Kolb19997Kolb Kimberley1999 Kotler2001L Kravitz1998 Lande1997 Lane1998 Lane2000 Lange1997 Lange2001 Lange2001v Lathrop1987 Latorre2000 Lauenroth1982j Lauenroth1985v Lauenroth1987w Lauenroth1988x Lauenroth1989t Lauenroth1992l Lauenroth1994G Lauenroth1995 Lauenroth1995 Lauenroth1996} Lauenroth1996 Lauenroth1996 Lauenroth1996! Lauenroth1997 \BB309-323$://A1994PC24300001Francovizcaino, E.d]Water Regime in Soils and Plants Along an Aridity Gradient in Central Baja-California, Mexico"Journal of Arid EnvironmentsJ. Arid. Environ.6 1994 Aug274QPC243 J ARID ENVIRONISI:A1994PC2430000190*Gebauer, Renate L. E. Ehleringer, James R. 2000^WWater and nitrogen uptake patterns following moisture pulses in a cold desert communitytEcologyF815 1415-1424aztVariation in the ability to utilize pulses of both water and nitrogen (N) is one possible mechanism allowing the coexistence of species in the cold desert community on the Colorado Plateau. We simulated 25-mm precipitation events and used stable isotope tracers (2H and 15N) to follow water and N uptake patterns in six dominant perennials (Artemisia filifolia, Coleogyne ramosissima, Cryptantha flava, Ephedra viridis, Quercus havardii, and Vanclevea stylosa) at different times of the growing season. Water pulse utilization varied on a seasonal basis and was to some extent different among species during the summer. Carbon isotope discrimination was negatively related to both plant use of moisture in upper soil layers and foliar N concentration. Species that were similar in water pulse utilization patterns differed in the natural abundances of 15N, suggesting partitioning in N sources. All species were able to utilize N pulses after rain events, but there were temporal differences in the responses among species. We also found that water and N uptake in shallow roots do not necessarily occur simultaneously. Artemisia, Cryptantha, and Quercus showed significant uptake of both water and N from the upper soil layers. In contrast, Coleogyne and Ephedra showed the capacity to utilize the water pulse, but not the N pulse. Vanclevea only took up N. The results indicate that different parts of the root system may be responsible for the acquisition of water and N. Our results also suggest that N and water partitioning could contribute to the coexistence of species in highly variable environments such as the Colorado Plateau desert system.221-263$://000171533000001a Gibbens, R. P. Lenz, J. M.4-Root systems of some Chihuahuan Desert plants"Journal of Arid EnvironmentsChihuahuan Desert; root systems; shrubs; forbs; grasses; competition; soil resources creosotebush larrea-tridentata; hardwood forest ecosystem; agravitropic pea mutant; hydraulic lift; water relations; gutierrezia-sarothrae; mycorrhizal fungi; flourensia-cernua; fine roots; soile$Root systems of 11 shrub or shrub-like species, 11 grass species, 19 perennial forb species and four annual forb species were excavated on the Jornada Experimental Range in the northern Chihuahuan Desert in southern New Mexico, U.S.A. Maximum radial horizontal spread of shrub root systems usually occurred above calcic and petrocalcic horizons at depths above 1 m. With one exception, all shrub species roots were traced through calcic and petrocalcic horizons to depths down to 5 m. Upward growing roots reaching very shallow depths ( < 10 cm) were common for most shrub species. Thus, the shrubs can readily access soil water and nutrients from both surface and deep soil horizons. Grass root systems on sandy soils extended radially up to 1.4 m, perhaps an adaptation to capture more soil water from the frequent small rainfall events. Grass roots did not extend through calcic or petrocalcic horizons and none penetrated deeper than 1.6 m. Perennial forb root systems varied in depth of branching but often penetrated into or through calcic and petrocalcic horizons and, like shrubs, have an advantage over grasses during droughts. Root systems of the annual forbs, excavated at the end of a season of above average precipitation, reached depths of 0.5-1.2 m. At each of the 18 excavation sites the roots of all the plant life forms were highly intermingled in the upper soil horizons, indicating that competition for water and soil nutrients is intense. It is believed that the plasticity and architecture of the shrub root systems, enabling them to compete with the grasses for soil water in the upper soil horizons and also access soil water at depths beyond the reach of grass roots, has been a major reason for the increase of shrubs during the historical period in this and environment. (C) 2001 Academic Press.J. Arid. Environ. 2001 Oct492481RG J ARID ENVIRONISI:000171533000001h F ("Yoder, Carolyn K. Nowak, Robert S. 1999vpSoil moisture extraction by evergreen and drought-deciduous shrubs in the Mojave Desert during wet and dry years"Journal of Arid Environments422I 81-96iAnnual and seasonal evapo-transpiration (ET) were compared among Mojave Desert shrubs with different leaf phenologies over a 3-year period during which annual precipitation varied from well below average to more than twice average. During the wet year, soil wetting fronts reached maximum depths of 0.75 m to > 1.95 m, depending on soil texture at the study sites. The evergreen shrubs Larrea tridentata and Ephedra nevadensis, and the drought-deciduous shrub Ambrosia dumosa, were able to extract soil water in a uniform manner to depths > 1 m. For stands of the deciduous shrub Lycium pallidum, a soil texture change at c. 0.75 m impeded percolation of water below that depth. There were no significant differences (p < 0.05) in annual ET between the evergreen shrubs Larrea and Ephedra relative to the drought-deciduous shrubs Ambrosia and Lycium during the 3 years of the study. Early in the growing season, extraction of soil water from beneath plant canopies was slightly greaterthan from shrub interspaces for Ambrosia, Ephedra, and Lycium, but not for Larrea. For all species, annual soil water extraction from beneath plant canopies was not significantly different than that from shrub interspaces. The lower limit of soil water extraction (Le) for the study sites varied from 4 to 10 volumetric per cent, depending on soil texture, and did not differ significantly among species. For all species, Le was reached within 6 to 12 months following twice average precipitation during the period of November 1994 to March 1995. We conclude that ET in the Mojave Desert is dependent largely on winter precipitation and the amount of soil water available during the growing season rather than on species composition.685-692$://000166401300004a Yoder, C. K. Nowak, R. S.atnPhosphorus acquisition by Bromus madritensis ssp rubens from soil interspaces shared with Mojave Desert shrubsFunctional EcologyAmbrosia dumosa; competition; Larrea tridentata; Lycium pallidum; Red Brome root communication; nitrogen availability; southern-california; larrea-tridentata; deciduous shrubs; water relations; plants; competition; growth; evergreen1. Bromus madritensis ssp. rubens (L.) Husnot (Red Brome) is an invasive annual grass that is associated with increases in fire frequency and decreases in perennial plant diversity. The success of Red Brome in the Mojave Desert has been attributed to its competitive ability, but competition between Red Brome and native shrubs for below-ground resources has not been investigated previously. In this study we present a modification of previous dual-isotope methods that assesses competitive interactions and responses among plants under field conditions. We then use this method to (i) determine if direct competition for phosphorus (P) occurs between Red Brome and native shrubs, and (ii) evaluate the effectiveness of phosphate acquisition by Red Brome from soil interspaces shared with different Mojave Desert shrub species. 2. Clipping Red Brome to remove approximate to 85% of its foliage on the day prior to labelling soil interspaces with P isotopes did not have the desired effect of inhibiting phosphorus uptake by Red Brome. Therefore we were unable to verify that direct competition for P occurred between Red Brome and native shrubs. 3. Nonetheless, by sampling plant tissue from unclipped strips of Red Brome that were centred between two shrubs, we were able to evaluate the effectiveness of phosphate acquisition by Red Brome from interspaces shared with different shrubs. There were no differences in the amount of phosphorus acquired by Red Brome from soil interspaces shared with the evergreen shrub Larrea tridentata (D.C.) Cov. versus soil interspaces shared with the drought deciduous shrub Lycium pallidum Miers. However, Red Brome acquired more than six times as much P from soil interspaces shared with Larrea than from soil interspaces shared with the drought deciduous shrub Ambrosia dumosa Payne. Less P uptake by Red Brome from soil interspaces shared with Ambrosia suggests greater effectiveness of Ambrosia compared with Larrea to deplete soil P. Funct. Ecol. 2000 Dec146392FT FUNCT ECOLISI:000166401300004jcZak, Donald R. Pregitzer, Kurt S. Curtis, Peter S. Terri, James A. Fogel, Robert Randlett, Diana L. 1993ZSElevated atmospheric carbon dioxide and feedback between carbon and nitrogen cyclesPlant and Soil 15119105-117o B ;We tested a conceptual model describing the influence of elevated atmospheric CO-2 on plant production, soil microorganisms, and the cycling of C and N in the plant-soil system. Our model is based on the observation that in nutrient-poor soils, plants (C-3) grown in an elevated CO-2 atmosphere often increase production and allocation to below ground structures. We predicted that greater below ground C inputs at elevated CO-2 should elicit an increase in soil microbial biomass and increased rates of organic matter turnover and nitrogen availability. We measured photosynthesis, biomass production, and C allocation of Populus grandidentata Michx. grown in nutrient-poor soil for one field season at ambient and twice-ambient (i.e., elevated) atmospheric CO-2 concentrations. Plants were grown in a sandy subsurface soil i) at ambient CO-2 with no open top chamber, ii) at ambient CO-2 in an open top chamber, and iii) at twice-ambient CO-2 in an open top chamber. Plants were fertilized with 4.5 g N m-2 over a 47 d period midway through the growing season. Following 152 d of growth, we quantified microbial biomass and the availabilities of C and N in rhizosphere and bulk soil. We tested for a significant CO-2 effect on plant growth and soil C and N dynamics by comparing the means of the chambered ambient and chambered elevated CO-2 treatments. Rates of photosynthesis in plants grown at elevated CO-2 were significantly greater than those measured under ambient conditions. The number of roots, root length, and root length increment were also substantially greater at elevated CO-2. Total and below ground biomass were significantly greater at elevated CO-2. Under N-limited conditions, plants allocated 50-70% of their biomass to roots. Labile C in the rhizosphere of elevated-grown plants was significantly greater than that measured in the ambient treatments; there were no significant differences between labile C pools in the bulk soil of ambient and elevated-grown plants. Microbial biomass C was significantly greater in the rhizosphere and bulk soil of plants grown at elevated CO-2 compared to that in the ambient treatment. Moreover, a short-term laboratory assay of N mineralization indicated that N availability was significantly greater in the bulk soil of the elevated-grown plants. Our results suggest that elevated atmospheric CO-2 concentrations can have a positive feedback effect on soil C and N dynamics producing greater N availability. Experiments conducted for longer periods of time will be necessary to test the potential for negative feedback due to altered leaf litter chemistry.oreased by variability, but that of Synedra was not. During pulses, Fragilaria had a higher uptake rate per cell than Synedra, but the two species had similar specific uptake rates. Previous studies (Tilman 1981) suggest that Synedra is a better steady-state competitor for phosphorus than Fragilaria. Although Fragilaria may be better able to exploit phosphorus pulses than Synedra, this advantage is apparently too weak to prevent the dominance of Synedra under the regime of pulsed phosphorus supply used here. |j2*#Belnap, J. Prasse, R. Harper, K. T. 2001RLInfluence of biological soil crusts on soil environments and vascular plants "Belnap, Jayne Lange, Otto L.B;Biological soil crusts: Structure, function, and managementB "Heidelber, Berlin, New York Springer Verlag281-300 "Belnap, Jayne Lange, Otto L. 2001B;Biological soil crusts: Structure, function, and management "Heidelberg, Berlin, New York Springer Verlag}Carbon and mineral element accumulation and allocation in two annual plant species in response to timing of nutrient addition"Benner, B. L. Bazzaz, F. A.Journal of Ecology761 19-40 1988(1) The effects of the timing of a large 'pulse' addition of mineral nutrients on accumulation and allocation of C, N, P, K and Ca, and on seed production and element status, were examined in the annual plants Abutilon theophrasti and Datura stramonium. (2) Nutrient addition increased total plant weight and element content, but the increase occurred over a period of several weeks rather than within the week after the addition. (3) The various mineral elements were allocated independently of carbon and of each other in both species. A. theophrasti started reproduction later than did D. stramonium and had lower proportional allocations of weight, N and P to reproduction at the final harvest. (4) Although nutrient pulse addition produced a temporary increase in proportional allocation of weight, N and P to leaves shortly after the addition, pulse timing had little effect on final proportional allocations. Plant age had a greater effect on allocation patterns than did the nutrient treatments. (5) Differences in seed production observed among nutrient treatments appeared to result primarily from differences in total accumulation of carbon and other elements and hence total plant growth, rather than from differences in proportional allocations. (6) Element concentrations of seeds and capsules, and mineral element contents per seed, did not differ among the treatments, and variation in mean seed weight among treatments was relatively small. $Berndtsson, Ronny Chen, Heshen 1994JCVariability of soil water content along a transect in a desert area"Journal of Arid Environments272 127-139ESpatial variation patterns of soil water content are investigated along a bare soil transect covering a crest-to-crest spacing (about 60 m) in a shifting sand dune area. Soil water content analyses from gravimetrical soil sampling and neutron probe observations are shown to give different information regarding variability patterns. Gravimetrical sampling displays a large effect of a small rainfall (4.5 mm) on spatial and temporal variability patterns. These effects are not clearly distinguishable from the neutron sond observations. Both gravimetrical and neutron sond observations display a small-scale variation that needs to be further investigated in order to be explained.2,Bertiller, M. B. Beeskow, A. M. Coronato, F. 1991VPSeasonal Environmental Variation and Plant Phenology in Arid Patagonia Argentina"Journal of Arid Environments211 1-12:4The phenological development of the main plant components of an arid ecosystem of north-eastern patagonia and their relationships with some environmental variables were investigated. Maximum and minimum air temperatures, precipitation, soil moisture (%) and (NO3-N) and (NH4-N) in the equilibrium solution at different soil depths were measured at weekly intervals. Thirteen phenological phases were described for each of the following species: Larrea divaricata, Chunquiraga avellanedae, Lycium chilense, Poa, ligularis, Stipa speciosa, and Stipa tenuis. The depth of penetration of each species' root system was also measured. Two groups of species were identified based in the timing of the phenological events and the depth of the root systems. One group represented by L. chilense, P. ligularis, S. speciosa, and S. tenuis included shallow-rooted species. These exhibited biological activity early in the growing season (winter-late spring) in accordance with high soil moisture and high (NO3-N) in the upper soil (0-0.8m). The other group consisted of L. divaricata and C. avellanedae, which are deep-rooted species. Phenology in these species progressed during the dry period (summer-late summer) when soil moisture and (NO3-N) in the upper soil were depleted and the deep soil (0.9m or more) maintained high values of soil moisture and high (NO13-N). As in other arid ecosystems, plant species of north-eastern Patagonia showed phenological asynchrony among species in accordance with the ability of each species to exploit water and nutrients stored in different soil sub-spaces.JCBetancourt, J. L. Latorre, C. Rech, J. A. Quade, J. Rylander, K. A. 2000ZTA 22,000-year record of monsoonal precipitation from northern Chile's Atacama DesertScience 289a 1542-1546aFossil rodent middens and wetland deposits from the central Atacama Desert (22degree to 24degreeS) indicate increasing summer precipitation, grass cover, and groundwater levels from 16.2 to 10.5 calendar kiloyears before present (ky B.P.). Higher elevation shrubs and summer-flowering grasses expanded downslope across what is now the edge of Absolute Desert, a broad expanse now largely devoid of rainfall and vegetation. Paradoxically, this pluvial period coincided with the summer insolation minimum and reduced adiabatic heating over the central Andes. Summer precipitation over the central Andes and central Atacama may depend on remote teleconnections between seasonal insolation forcing in both hemispheres, the Asian monsoon, and Pacific sea surface temperature gradients. A less pronounced episode of higher groundwater levels in the central Atacama from 8 to 3 ky B.P. conflicts with an extreme lowstand of Lake Titicaca, indicating either different climatic forcing or different response times and sensitivities to climatic change.I&K &>7Lauenroth, William K. Burke, Ingrid C. Paruelo, Jose M. 2000Patterns of production and precipitation-use efficiency of winter wheat and native grasslands in the central Great Plains of the United States Ecosystems34i344-351/The Great Plains of the United States is characterized by a large west-east gradient in annual precipitation and a similar large north-south gradient in annual temperature. Native grasslands and winter wheat are found over a large portion of the precipitation and temperature gradients. In this article, we use long-term data to analyze the differences in the patterns in aboveground net primary production and precipitation-use efficiency between wheat and native grassland ecosystems in the central portion of Great Plains, and their relationships to potential water availability (precipitation). Aboveground net primary production of native grasslands shows a large response to precipitation. Aboveground net primary production of winter wheat has a smaller response to changing precipitation. Annual precipitation-use efficiency of native grasslands is unaffected by increases in average annual precipitation, but precipitation-use efficiency of summer-fallow wheat ecosystems decreases substantially with increased average precipitation. Our results suggest that in the wetter portion of the central Great Plains, summer-fallow wheat management is relatively inefficient, because increased water availability results in diminishing returns. Comparisons with data from continuously cropped wheat confirmed this result. Shifts across the region to continuous cropping of wheat potentially could have significant impacts on regional wheat yield, carbon balance, and economic status.haHow species with different regeneration niches coexist in patchy habitats with local disturbances$Lavorel, Sandra Chesson, Peter 1995 Oikos 741v103-114s Using Smart Source Parsing D =We used a two-species simulation model to study mechanisms of coexistence of annual plants in patchy habitats with local disturbances. In habitats with nested scales of patchiness, short dispersal is advantageous because favorable habitat tends to be aggregated. The invasion of a resident population with short dispersal distance by a species with longer-range dispersal was simulated for combinations of habitat pattern, disturbance frequency and germination strategies. A germination strategy was defined by the type of response to disturbance ("disturbance-broken" when disturbances trigger germination, "risk-spreading" when germination is insensitive to disturbance) and the dormancy fraction at dispersal. Simulations estimated the long-term low-density growth rate of the invader, the mean local crowding (number of competing seeds per invader seed at each site) and the effective fecundity of each species (the mean number of seeds successfully dispersed per adult plant). Crowding increased with habitat suitability and decreased with increasing dormancy fractions for the resident. Effective fecundity in a landscape can be taken as a measure of competitive ability. The short-dispersing resident invariably had higher effective fecundity, but this difference decreased with increasing suitability, i.e. competitive differences decreased. Coexistence depended on both habitat suitability and disturbance frequency. Maximum coexistence was obtained for habitats of intermediate suitability with moderately frequent disturbances. General linear modelling of the long-term low-density growth rate showed that coexistence results from a reduction in local crowding. This growth rate also increased for increasing habitat suitability and connectivity, and for a higher dormancy fraction of the resident species. The effects of disturbance frequency and of invader's dormancy fraction depended on the type of dormancy of the resident species. The analysis showed that 2 different mechanisms are involved in the coexistence of species with different niches. Differences in regeneration niches permit coexistence through competitive equivalency with trade-offs between dispersal and germination traits, but for a limited range of habitat pattern and disturbance conditions. On the other hand, coexistence through density fluctuations of a disturbance-broken species and storage effects can be achieved for a broad range of environmental conditions and species germination strategies. Species coexistence thus results from the combination of two mechanisms. Evidence from natural communities is discussed. Our results also demonstrate the importance of detailed attention to spatial patterns and dispersal because of the complexity of spatial effects. Further, spatial pattern and disturbance frequencies need to be considered jointly to understand the dynamics of diversity.Le Houerou, H.N. 1984BCaldwell, M. M. Eissenstat, D. M. Richards, J. H. Allen, M. F. 1985xqCompetition For Phosphorus Differential Uptake From Dual-Isotope-Labeled Soil Interspaces Between Shrub and GrassScience5 229 4711384-386hTwo species of Agropyron grass differed strikingly in their capacity to compete for phosphate in soil interspaces shared with a common competitor, the sagebrush Artemisia tridentata. Of the total phosphorus-32 and -33 absorbed by Artemisia, 86% was from the interspace shared with Agropyron spicatum and only 14% from that shared with Agropyron desertorum. Actively absorbing mycorrhizal roots of Agropyron and Artemisia were present in both interspaces, where competition for the labeled phosphate occurred. The results have important implications about the way in which plants compete for resources below ground in both natural plant communities and agricultural intercropping systems.i"Campbell, B. D. Grime, J. P. 1989lfA Comparative Study of Plant Responsiveness to the Duration of Episodes of Mineral Nutrient EnrichmentNew Phytologist 11222261-268eGrowth, dry matter partitioning and nitrogen capture were compared in two grasses of contrasted ecology, subjected to treatments involving pulses of nutrient enrichment of various durations supplied every 6 d over a period of 6 weeks. The potentially fast-growing Arrhenatherum elatius ssp. bulbosum (Willd.) Schubler and Martens was superior in rates of nitrogen capture and dry matter production when exposed to long (> 10 h) nutrient pulses whereas the slow-growing Festuca ovina L. enjoyed an advantage in treatments providing nutrient pulses of between 0.1 and 10 h duration. These results are consistent with differences in mechanisms of resource foraging predicted by the C-S-R model of plant strategies and related models. It is concluded that the lower rates of turnover of tissues and the capacity of roots to remain viable under chronic nutrient stress are important components of the ability of Festuca ovina to exploit brief pulses of mineral nutrient enrichment. The range in duration of the pulse advantageous F. ovina corresponds to which might be expected to arise from the death and recovery of microbial populations in infertile soils.  u &f'N\[  =-  x  A hn *?  k +{ M > 7 q   a :  ` 2EV7 ;/     R^ FgV63;C X:#xf }HU M+kQ^0  ;%deep@2 LB;Saliendra, Nicanor Z. Sperry, John S. Comstock, Jonathan P. 1995Influence of leaf water status on stomatal response to humidity, hydraulic conductance, and soil drought in Betula occidentalis  Planta 19623357-366o,%Whole-canopy measurements of water flux were used to calculate stomatal conductance (g-s) and transpiration (E) for seedlings of western water birch (Betula occidentalis Hook.) under various soil-plant hydraulic conductances (k), evaporative driving forces (DELTA-N; difference in leaf-to-air molar fraction of water vapor), and soil water potentials (PSI-S). As expected, g-s dropped in response to decreased k or PSI-S, or increased DELTA-N ( gt 0.025). Field data showed a decrease in mid-day g-s with decreasing k from soil-to-petiole, with sapling and adult plants having lower values of both parameters than juveniles. Stomatal closure prevented E and PSI from inducing xylem cavitation except during extreme soil drought when cavitation occurred in the main stem and probably roots as well. Although all decreases in g-s were associated with approximately constant bulk leaf water potential (PSI-L), this does not logically exclude a feedback response between PSI-L and g-s. To test the influence of leaf versus root water status on g-s, we manipulated water status of the leaf independently of the root by using a pressure chamber enclosing the seedling root system; pressurizing the chamber alters cell turgor and volume only in the shoot cells outside the chamber. Stomatal closure in response to increased DELTA-N, decreased k, and decreased PSI-S was fully or partially reversed within 5 min of pressurizing the soil. Bulk PSI-L remained constant before and after soil pressurizing because of the increase in E associated with stomatal opening. When DELTA-N was low (i.e., lt 0.025), pressurizing the soil either had no effect on g-s, or caused it to decline; and bulk PSI-L increased. Increased PSI-L may have caused stomatal closure via increased backpressure on the stomatal apparatus from elevated epidermal turgor. The stomatal response to soil pressurizing indicated a central role of leaf cells in sensing water stress caused by high DELTA-N, low k, and low PSI-L. Invoking a prominent role for feedforward signalling in short-term stomatal control may be premature.h Using Smart Source ParsingTMSalih, A. A. Ali, I. A. Lux, A. Luxova, M. Cohen, Y. Sugimoto, Y. Inanaga, S. 1999`YRooting, water uptake, and xylem structure adaptation to drought of two sorghum cultivars Crop Science391 168-173hSeveral mechanisms have been proposed to explain the high tolerance of sorghum (Sorghum bicolor (L.) Moench) to drought. This paper reports a field study on the effects of soil moisture stress on the rooting habits, transpiration rate, and xylem anatomy of two sorghum cultivars, Tabat (drought susceptible) and Gadambalia (drought tolerant). Two levels of water stress, -0.02 MPa (wet) and -0.75 MPa (dry), were applied. Tabat had a higher root length density (RLD), higher late metaxylem (LMX) vessels per nodal root, higher leaf area, and higher transpiration rate than Gadambalia. In Tabat, soil moisture stress reduced RLD by 30%, nodal roots by 31%, number of LMX vessels in the root by 42%, leaf area by 13%, and transpiration rate by 11%. In Gadambalia soil moisture stress did not affect RLD at depths gtoreq 0.2 m, number of nodal roots, or number of LMX vessels per nodal root. However, leaf area and transpiration rate were reduced by 3 and 11%, respectively. Under dry conditions, Gadambalia displayed a higher water extraction efficiency than Tabat throughout the profile (0-0.9 m). In Gadambalia, unlike Tabat, the stem was highly sclerified. A 1- to 3-cell-thick layer of schlerenchyma was observed beneath the epidermis. The peripheral vascular bundles were surrounded with a 3- to 6-cell-thick schlerenchyma sheath. However, in roots anatomical differences were less prominent. Drought tolerance fewer nodal roots per plant, fewer LMX vessels per nodal root, a smaller leaf area, and a well developed sclerenchyma.5|4 approachSonoran desert vegetation:4Pockman, William T. Sperry, John S. O'Leary, James W 1995@:Sustained and significant negative water pressure in xylem Nature 378 6558715-716tnDespite two centuries of research, the mechanism of water transport in plants is still debated. The prevailing cohesion-tension theory, which states that water is pulled upwards by capillarity in cell-wall pores, remains vulnerable to challenge because its corollary is difficult to prove: that large negative pressures exist in xylem conduits. Recent xylem pressure-probe and z-tube experiments suggest that cavitation limits xylem pressures to above -0.5 MPa, despite the much more negative pressures predicted by the cohesion-tension theory and measured with the standard pressure-chamber method. Here we show, using centrifugal force to induce negative pressure between -0.5 and -3.5 MPa in intact stems, that xylem conduits remained water-filled and conductive to species-specific pressures ranging from -1.2 to below -3.5 MPa. Results were consistent when stems were air-dried or injected with air. Agreement among these techniques demonstrates that xylem can support large negative pressures, that the pressure chamber reliably measures these pressures, and that cavitation is nucleated by air entry through conduit wall pores.Using Smart Source Parsing ( FTXT: Journals@OVID http://gateway.ovid.com/ovidweb.cgi?T=JS&PAGE=fulltext&D=ovft&NEWS=n&DBC=n&AN=00006056-199512140-000244*#Pockman, William T. Sperry, John S. 1997TMFreezing-induced xylem cavitation and the northern limit of Larrea tridentata Oecologia 1091 19-27eHBWe investigated the occurrence of freezing-induced cavitation in the evergreen desert shrub Larrea tridentata and compared it to co-occurring, winter-deciduous Prosopis velutina. Field measurements indicated that xylem sap in L. tridentata froze at temperatures below c. -5 degree C, and that this caused no measurable cavitation for minimum temperatures above -7 degree C. During the same period P. velutina cavitated almost completely. In the laboratory, we cooled stems of L. tridentata to temperatures ranging from -5 to -20 degree C, held them at temperature for 1 or 12 h, thawed the stems at a constant rate and measured cavitation by the decrease in hydraulic conductivity of stem segments. As observed in the field, freezing exotherms occurred at temperatures between -6.5 and -9 degree C and as long as temperatures were held above -11 degree C there was no change in hydraulic conductivity after thawing. However, when stems were cooled to between -11 degree C and -20 degree C, stem hydraulic conductivity decreased linearly with minimum temperature. Minimum temperatures between -16 and -20 degree C were sufficient to completely eliminate hydraulic conductance. Record ( gt 20 year) minimum isotherms in this same range of temperatures corresponded closely with the northern limit of L. tridentata in the Mojave and Sonoran deserts.|uUsing Smart Source Parsing ( FTXT: EBSCO Online http://www.ebsco.com/online/direct.asp?ArticleID=DY743Q4QKV46FWHXP0L5 z F2,Buchmann, Nina Kao Wen, Yuan Ehleringer, Jim 1997Influence of stand structure on carbon-13 of vegetation, soils, and canopy air within deciduous and evergreen forests, in Utah, United States OecologiaA 110o1109-119y D >Carbon isotope ratios (delta-13C) were studied in evergreen and deciduous forest ecosystems in semi-arid Utah (Pinus contorta, Populus tremuloides, Acer negundo and Acer grandidentatum). Measurements were taken in four to five stands of each forest ecosystem differing in overstory leaf area index (LAI) during two consecutive growing seasons. The delta-13C-leaf (and carbon isotope discrimination) of understory vegetation in the evergreen stands (LAI 1.5-2.2) did not differ among canopies with increasing LAI, whereas understory in the deciduous stands (LAI 1.5-4.5) exhibited strongly decreasing delta-13C-leaf values (increasing carbon isotope discrimination) with increasing LAI. The delta-13C values of needles and leaves at the top of the canopy were relatively constant over the entire LAI range, indicating no change in intrinsic water-use efficiency with overstory LAI. In all canopies, delta-13C-leaf decreased with decreasing height above the forest floor, primarily due to physiological changes affecting c-i/c-a ( gt 60%) and to a minor extent due to delta-13C of canopy air ( lt 40%). This intra-canopy depletion of delta-13C-leaf was lowest in the open stand (1 permill ) and greatest in the denser stands (4.5 permill ). Although overstory leaf-delta-13C did not change with canopy LAI, delta-13C of soil organic carbon increased with increasing LAI in Pinus contorta and Populus tremuloides ecosystems. In addition, delta-13C of decomposing organic carbon became increasingly enriched over time (by 1.7-2.9 permill ) for all deciduous and evergreen dry temperate forests. The delta-13C-canopy of CO-2 in canopy air varied temporally and spatially in all forest stands. Vertical canopy gradients of delta-13C-canopy, and (CO-2)-canopy were larger in the deciduous Populus tremuloides than in the evergreen Pinus contorta stands of similar LAI. In a very wet and cool year, ecosystem discrimination (DELTA-e) was similar for both deciduous Populus tremuloides (18.0 +- 0.7 permill ) and evergreen Pinus contorta (18.3 +- 0.9 permill ) stands. Gradients of delta-13C-canopy and (CO-2)-canopy were larger in denser Acer spp. stands than those in the open stand. However 13C enrichment above and photosynthetic draw-down of (CO-2)-canopy below tropospheric baseline values were larger in the open than in the dense stands, due to the presence of a vigorous understory vegetation. Seasonal patterns of the relationship delta-13C-canopy versus 1/(CO-2)-canopy were strongly influenced by precipitation and air temperature during the growing season. Estimates of DELTA-e for Acer spp. did not show a significant effect of stand structure, and averaged 16.8 +- 0.5 permill in 1933 and 17.4 +- 0.7 permill in 1994. However, DELTA-e, varied seasonally with small fluctuations for the open stand (2 permill ), but more pronounced changes for the dense stand (5 permill ).@9Burke, Ingrid C. Lauenroth, William K. Parton, William J.a 1997jcRegional and temporal variation in net primary production and nitrogen mineralization in grasslandsEcology785g 1330-1340 Spatial variability that occurs at large scales has long been used by ecologists as a tool to examine the controls over ecosystem structure and function. Correlations of control variables such as climatic factors and response variables such as vegetation and soil carbon storage across broad regions have played a crucial role in predicting the response of ecosystems to global climate change. Despite the importance of these large-scale space-for-time substitutions, there are substantial limitations. One of these limitations is that many of the possible control factors covary with one another, and only some of the important control factors actually exist in large-scale databases. Thus, the true proximal controls may be difficult to identify. A second limitation is that models of spatial variability may not be appropriately applied to temporal variability. In this paper, we utilize a new approach to determine the extent to which N availability may constrain aboveground primary productivity in the Central Grassland region of the U.S. The strong relationship between average annual primary production and average annual precipitation found in spatial patterns in ecosystems globally has often been interpreted as evidence of a fundamental water limitation. However, temporal variation in annual aboveground net primary production (ANPP) indicates that other factors constrain production. We generated a spatial and temporal database for annual aboveground net primary production and annual net N mineralization by linking a database of input variables (precipitation, temperature, and soils) with predictive models. We generated independent data sets of aboveground net primary production and net N mineralization by using regression models to predict aboveground net primary production, and the Century model to simulate net N mineralization. Our analyses indicate that net primary production and net N mineralization both increase with mean annual precipitation; thus, it is not possible to separate the extent to which ANPP is controlled by water or N availability. Nitrogen use efficiency (NUE) increased with increasing precipitation across the region. Aboveground net primary production decreased with increasing temperature across the region, while N mineralization increased slightly, leading to decreasing (NUE) with increasing temperature. At high precipitation levels, aboveground net primary production increased and N mineralization decreased slightly with increasing soil fineness. Nitrogen use efficiency generally increased with increasing pools of soil organic matter, likely because in grasslands, the proportion of recalcitrant organic matter increases with the total organic matter pools. A comparison of interannual variation in net N mineralization with average spatial variation indicated a high degree of inertia in the response of N availability to precipitation levels. Our simulation results as well as field results of Lauenroth and Sala (1992) raise important questions about the applicability of space-for-time substitutions when dealing with ecosystem function. The structure of the systems appears to provide important constraints on the temporal variability that are not evident in an analysis of spatial variability.`Wiegand, Kerstin Ward, David Thulke, Hans Herman Jeltsch, Florian 2000b[From snapshot information to long-term population dynamics of Acacias HAWiegand, Kerstin Ward, David Thulke, Hans Herman Jeltsch, Florian 2000b[From snapshot information to long-term population dynamics of Acacias by a simulation model Plant EcologyK 150t 1-2 97-114voThe African Acacia species A. raddiana is believed to be endangered in the Negev desert of Israel. The ecology of this species is not well understood. The main idea of our study is to learn more about the long-term population dynamics of these trees using snapshot information in the form of size frequency distributions. These distributions are highly condensed indices of population dynamics acting over many years. In this paper, we analyse field data on recruitment, growth, and mortality and use an existing simulation model of the population dynamics of A. raddiana (SAM) to produce contrasting scenarios of these live history processes that are based on the analysed field evidence. The main properties of simulated as well as observed tree size frequency distributions are characterised with Simpson's index of dominance and a new permutation index. Finally, by running the SAM model under the different scenarios, we study the effect of these different processes on simulated size frequency distributions (pattern) and we compare them to size distributions observed in the field, in order to identify the processes acting in the field. Our study confirms rare recruitment events as a major factor shaping tree size frequency distributions and shows that the paucity of recruitment has been a normal feature of A. raddiana in the Negev over many years. Irregular growth, e.g., due to episodic rainfall, showed a moderate influence on size distributions. Finally, the size frequency distributions observed in the Negev reveal the information that, in this harsh environment, mortality of adult A. raddiana is independent of tree size (age).t78 Kolb, K. J. Sperry, J. S. 1999QTransport constraints on water use by the Great Basin shrub, Artemisia tridentata =  Plant Cell and Environment228e925-935iAs soil and plant water status decline, decreases in hydraulic conductance can limit a plant's ability to maintain gas exchange. We investigated hydraulic limitations for Artemisia tridentata during summer drought. Water use was quantified by measurements of soil and plant water potential (PSI), transpiration and leaf area. Hydraulic transport capacity was quantified by vulnerability to water stress-induced cavitation for root and stem xylem, and moisture release characteristics for soil. These data were used to predict the maximum possible steady-state transpiration rate (Ecrit) and minimum leaf xylem pressure (PSIcrit). Transpiration and leaf area declined by apprx 80 and 50%, respectively, as soil PSI decreased to -2.6 MPa during drought. Leaf-specific hydraulic conductance also decreased by 70%, with most of the decline predicted in the rhizosphere and root system. Root conductance was projected to be the most limiting, decreasing to zero to cause hydraulic failure ifEcrit was exceeded. The basis for this prediction was that roots were more vulnerable to xylem cavitation than stems (99% cavitation at -4.0 versus -7.8 MPa, respectively). The decline in water use during drought was necessary to maintain E and PSI within the limits defined by Ecrit and PSIcrit.("Kolb, Kimberley J. Sperry, John S. 1999^XDifferences in drought adaptation between subspecies of sagebrush (Artemisia tridentata)Ecology807 2373-2384PIThree subspecies of Artemisia tridentata occupy distinct habitats in the Great Basin of North America: ssp. wyomingensis in low, arid elevations; ssp. vaseyana in high, mesic elevations; and ssp. tridentata in intermediate zones. We evaluated differences in the drought experienced and drought tolerance among the subspecies. Drought tolerance was measured by two traits: the xylem pressure (PSIx) causing xylem cavitation and PSIx causing loss of leaf turgor (PSItlp). As expected from habitat, ssp. wyomingensis experienced a more severe summer drought than ssp. vaseyana (minimum PSIx = -7.5 MPa and -3.8 MPa, respectively). Despite the large difference in drought exposure, the subspecies exhibited similar drought responses, including a reduction in transpiration (E) below 0.5 mmolcntdots-1cntdotm-2, a shedding of 60-65% of foliage, and >95% decline in soil-to-leaf hydraulic conductance. The similarity in the drought response was consistent with pronounced differences in PSIx, causing 50% loss in xylem conductivity by cavitation (PSI50). The PSI50 was -4.9 MPa in ssp. wyomingensis vs. -3.0 MPa in ssp. vaseyana; ssp. tridentata was intermediate (PSI50 = -3.9 MPa). Differences in cavitation resistance were preserved in a common garden, suggesting that they arose by genetic differentiation. A water transport model indicated that the greater cavitation resistance in ssp. wyomingensis was a necessary adaptation for its more arid habitat, and that the similar restrictions of E among subspecies were required to avoid hydraulic failure. The PSItlp was also lower in ssp. wyomingensis than in ssp. vaseyana. Although PSItlp decreased in both subspecies during drought, the adjustment was not sufficient to maintain turgor. Turgor loss may have been adaptive in minimizing shoot growth and stomatal conductance under hydraulically limiting circumstances.a to almost 600%. Roots produced in response to the additions of water and water plus nitrogen lived longer than roots in the control treatments. Thus, additions of water and water plus nitrogen influenced both the proliferation of new roots and their longevity, with both proliferation and longevity related to the type and duration of resource supply. Results suggest that root longevity and mortality may be plastic in response to changes in soil resource availability, as is well known for root proliferation.{\VCanadell, J. Jackson, R. B. Ehleringer, J. R. Mooney, H. A. Sala, O. E. Schulze, E. D. 1996D=Maximum rooting depth of vegetation types at the global scale Oecologia 108-4583-594aThe depth at which plants are able to grow roots has important implications for the whole ecosystem hydrological balance, as well as for carbon and nutrient cycling. Here we summarize what we know about the maximum rooting depth of species belonging to the major terrestrial biomes. We found 290 observations of maximum rooting depth in the literature which covered 253 woody and herbaceous species. Maximum rooting depth ranged from 0.3 m for some tundra species to 68 m for Boscia albitrunca in the central Kalahari; 194 species had roots at least 2 m deep, 50 species had roots at a depth of 5 m or more, and 22 species had roots as deep as 10 m or more. The average for the globe was 4.6+-0.5 m. Maximum rooting depth by biome was 2.0+-0.3 m for boreal forest, 2.1+-0.2 m for cropland, 9.5+-2.4 m for desert, 5.2+-0.8 m for sclerophyllous shrubland and forest, 3.9+-0.4 m for temperate coniferous forest, 2.9+-0.2 m for temperate deciduous forest, 2.6+-0.2 m for temperate grassland, 3.7+-0.5 m for tropical deciduous forest, 7.3+-2.8 m for tropical evergreen forest, 15.0+-5.4 m for tropical grassland/savanna, and 0.5+-0.1 m for tundra. Grouping all the species across biomes (except croplands) by three basic functional groups: trees, shrubs, and herbaceous plants, the maximum rooting depth was 7.0+-1.2 m for trees, 5.1+-0.8 m for shrubs, and 2.6+-0.1 m for herbaceous plants. These data show that deep root habits are quite common in woody and herbaceous species across most of the terrestrial biomes, far deeper than the traditional view has held up to now. This finding has important implications for a better understanding of ecosystem function and its application in developing ecosystem models.pD $Evans, R. D. Ehleringer, J. R. 1993VOA break in the nitrogen cycle in arid lands? Evidence from nitrogen-15 of soilseOecologia Heidelberg943o314-317nWe examined the content and isotopic composition of nitrogen within soils of a juniper woodland and found that a cryptobiotic crust composed of cyanobacteria, lichens, and mosses was the predominant source of nitrogen for this ecosystem. Disturbance of the crust has resulted in considerable spatial variability in soil nitrogen content and isotopic composition; inter-canopy soils were significantly depleted in nitrogen and had greater abundance of 15N compared to intra-canopy soils. Variations in the 15N/14N ratio for inter- and intra-canopy locations followed similar Rayleigh distillation curves, indicating that the greater 15N/14N ratios for inter-canopy soils were due to relatively greater net nitrogen loss. Coverage of cryptobiotic crusts has been reduced by anthropogenic activities during the past century, and our results suggest that destruction of the cryptobiotic crust may ultimately result in ecosystem degradation through elimination of the predominant source of nitrogen input.voGrowth, photosynthesis, and resource investment for vegetative and reproductive modules of Artemisia tridentata"Evans, R. D. Black, R. Alang 1993Ecology Washington D C745  1516-1528  Using Smart Source Parsing Growth of vegetative and reproductive structures in Artemisia rtridentata is temporally separated during the growing season; vegetative growth occurs during spring and early summer when soil moisture is most abundant, while reproductive growth occurs during summer and fall when soil moisture may be limiting. Vegetative and reproductive structures may therefore exhibit contrasting efficiences of resource acquisition and investment resulting from temporal differences in resource availability during their development. We examined the effect of water stress on growth, photosynthesis, and resource investment for vegetative and reproductive modules of Artemisia tridentata by applying supplemental water. No differences were observed in vegetative biomass between shrubs in the two watering treatments. Growth of vegetative structures occurred in the spring when water was not limiting, and shrubs in both watering treatments exerted little stomatal control over water loss. Conversely, reproductive growth occurred through the summer when water was limiting, and supplemental watering increased reproductive growth. Shrubs conserved water during summer by abscising leaves and lowering stomatal conductance of both vegetative and reproductive modules in response to decreases in xylem pressure potential and increases in evaporative demand. Leaf abscission can occur without decreasing the amount of carbon available for reproductive growth because inflorescences are capable of positive photosynthetic rates comparable to vegetative leaves. Water stress did not alter tissue construction costs of carbon and nitrogen contents for either vegetative or reproductive modules. Resource limitations were, however, reflected in the efficiency of water use during tissue construction; floral leaves and floral heads of shrubs not receiving supplemental water were produced with higher water-use efficiency. Conservative use of water during production of vegetative modules would offer no advantages to A. tridentata because neighboring species are also most active at this time. Reproductive growth in A. tridentata occurs during summer when neighboring species are largely dormant, and so efficient use of water may allow development of reproductive structures to continue throughout the summer even with limited supplies of water.$Evans, R. D. Ehleringer, J. R. 19946/Water and nitrogen dynamics in an arid woodland Oecologia Berlin99 3-41233-242lArid environments are characterized by spatial and temporal variation in water and nitrogen availability. Differences in delta-15N and delta-D of four co-occurring species reveal contrasting patterns of plant resource acquisition in response to this variation. Mineralization potential and nitrogen concentration of surface soils associated with plant canopies were greater than inter-canopy locations, and values decreased with increasing depth in both locations. Mineralization potential and nitrogen concentration were both negatively correlated with soil delta-15N. The spatial variation in soil delta-15N caused corresponding changes in plant delta-15N such that plant delta-15N values were negatively correlated with nitrogen concentration of surface soils. Plants occurring on soils with relatively high nitrogen concentrations had lower delta-15N, and higher leaf nitrogen concentrations, than plants occurring on soils with relatively low nitrogen concentrations. Two general temporal patterns of water and nitrogen use were apparent. Three species (Juniperus, Pinus and Artemisia) relied on the episodic availability of water and nitrogen at the soil surface. delta-15N values did not vary through the year, while xylem pressure potentials and stem-water delta-D values fluctuated with changes in soil moisture at the soil surface. In contrast, Chrysothamnus switched to a more stable water and nitrogen source during drought. delta-15N values of Chrysothamnus increased throughout the year, while xylem pressure potentials and stem-water delta-D values remained constant. The contrasting patterns of resource acquisition have important implications for community stability following disturbance. Disturbance can cause a decrease in nitrogen concentration at the soil surface, and so plants that rely on surface water and nitrogen may be more susceptible than those that switch to more stable water and nitrogen sources at depth during drought.@X HAHenkin, Zalmen Seligman, No' am G. Kafkafi, Uzi Noy-Meir, Imanuel 1998'Effective growing days': A simple predictive model of the response of herbaceous plant growth in a Mediterranean ecosystem to variation in rainfall and phosphorus availabilityJournal of Ecology861137-148 N G1. Herbaceous plant production on many Mediterranean soils is severely restricted by nutrient deficiency. On such a shallow, phosphorus-deficient soil in the western Galilee of Israel with an average annual rainfall > 800 mm, a single addition of phosphorus fertilizer consistently increased herbaceous plant production in a community dominated by the dwarf shrub Sarcopoterium spinosum by a factor of 2.8-3.8 over 7 years. Regardless of phosphorus addition, interannual variation of the herbaceous vegetation remained high (C.V. = 0.33). A simple model based on climate data was developed to explain the continuing interannual variation in biomass production. 2. This interannual variation is accounted for by an 'effective growing day' index, G, based on an estimate of the number of days throughout the growing season during which the available water in the rooting zone can balance the current potential evaporation demand. The validity of this approach was tested on data from 7 consecutive years. 3. At given phosphate (P) availability levels, the index G was found to be highly correlated with both seasonal biomass production and P uptake of the herbaceous plant, but the regression lines intersected the G axis far to the right of the origin. 4. When the adverse effect of the winter climate on growing conditions was taken into account, the regression lines passed close to the origin. Annual biomass production and P uptake could then be well described as linear functions of G with the slope related to availability of P. 5. Average intraseasonal trends gave G-values which were better predictors of both biomass production and P-uptake than annual weather (temperature or potential evaporation) fluctuations. 6. Both shoot biomass and P uptake were strongly influenced by growing conditions in autumn (beginning of growing season) and in spring (main growing season), but plant production was more sensitive than P uptake to growing conditions in autumn and vice versa in spring. 7. The close relationship between G and P uptake at different P availability levels indicates that availability of P did not diminish for at least 7 years after a single application of 4.5-9.0 g m-2 P as superphosphate. Nutrient cycling, secondary effects of the dominant legume species, and small losses of P from the ecosystem are proposed as possible reasons for this phenomenon.m|uUsing Smart Source Parsing ( FTXT: EBSCO Online http://www.ebsco.com/online/direct.asp?ArticleID=686DFYKQT408CCDQ8Y9FD4-Heske, Edward J. Brown, James H. Guo, Qinfeng 1993tnEffects of kangaroo rat exclusion on vegetation structure and plant species diversity in the Chihuahuan Desert Oecologiar954520-5248Long-term (1977-90) experimental exclusion of three species of kangaroo rats from study plots in the Chihuahuan Desert resulted in significant increases in abundance of a tall annual grass (Aristida adscensionis) and a perennial bunch grass (Eragrostis lehmanniana). This change in the vegetative cover affected use of these plots by several other rodent species and by foraging birds. The mechanism producing this change probably involves a combination of decreased soil disturbance and reduced predation on large-sized seeds when kangaroo rats are absent. Species diversity of summer annual dicots was greater on plots where kangaroo rats were present, as predicted by keystone predator models. However, it is not clear whether this was caused directly by activities of the kangaroo rats or indirectly as a consequence of the increase in grass cover. No experimental effect on species diversity of winter annual dicots was detected. Our study site was located in a natural transition between desert scrub and grassland, where abiotic conditions and the effects of organisms may be particularly influential in determining the structure and composition of vegetation. Under these conditions kangaroo rats have a dramatic effect on plant cover and species composition.ucific pressures ranging from -1.2 to below -3.5 MPa. Results were consistent when stems were air-dried or injected with air. Agreement among these techniques demonstrates that xylem can support large negative pressures, that the pressure chamber reliably measures these pressures, and that cavitation is nucleated by air entry through conduit wall pores.Using Smart Source Parsing ( FTXT: Journals@OVID http://gateway.ovid.com/ovidweb.cgi?T=JS&PAGE=fulltext&D=ovft&NEWS=n&DBC=n&AN=00006056-199512140-00024. ( Rb\De Soyza, Amrita Franco, Augusto C. Virginia, Ross A. Reynolds, James F. Whitford, Walter G. 1996tnEffects of plant size on photosynthesis and water relations in the desert shrub Prosopis glandulosa (Fabaceae) American Journal of Botany831 99-105 4 .The Jornada del Muerto basin of the Chihuahuan Desert of southern New Mexico, USA, has undergone a marked transition of plant communities. Shrubs such as mesquite (Prosopis glandulosa) have greatly increased or now dominate in areas that were previously dominated by perennial grasses. The replacement of grasses by shrubs requires an establishment phase where small shrubs must compete directly with similar-sized grass plants. This is followed by a phase in which large, established shrubs sequester nutrients and water within their biomass and alter soil resources directly under their canopy, creating "islands" of fertility. We hypothesized that these two phases were associated with shrubs having different physiological response capacities related to their age or size and the resource structure of the environment. As a corollary, we hypothesized that responses of small shrubs would be more tightly coupled lo variation in soil moisture availability compared to large shrubs. To test these hypotheses, we studied gas exchange and water relations of small (establishing) and large (established) shrubs growing in the Jornada del Muerto as a function of varying soil moisture during the season. The small shrubs had greater net assimilation, stomatal conductance, transpiration, and xylem water potential than large shrubs following high summer rainfall in July, and highest seasonal soil moisture at 0.3 m. High rates of carbon assimilation and water use would be an advantage for small shrubs competing with grasses when shallow soil moisture was plentiful. Large shrubs had greater net assimilation and water-use efficiency, and lower xylem water potential than small shrubs following a dry period in September, when soil moisture at 0.3 m was lowest. Low xylem water potentials and high water-use efficiency would allow large shrubs to continue acquiring and conserving water as soil moisture is depleted. Although the study provides evidence of differences in physiological responses of different-sized shrubs, there was not support for the hypothesis that small shrubs are more closely coupled to variation in soil moisture availability than large shrubs. Small shrubs may actually be less coupled to soil moisture than large shrubs, and thus avoid conditions when continued transpiration could not be matched by equivalent water uptake."Derner, J. D. Briske, D. D., 1998|An isotopic (15N) assessment of intraclonal regulation in C4 perennial grasses: Ramet interdependence, independence or both?Journal of Ecology8623305-314  1. A series of three experiments was conducted with three C4 perennial grasses, Panicum virgatum (short rhizomes forming distinct clones), Schizachyrium scoparium (caespitose) and Bouteloua gracilis (caespitose), to determine whether intraclonal regulation was characterized by interdependence, independence or a combination of both processes. Specific hypotheses tested were: (i) all ramets within individual clones are physiologically integrated; (ii) ramet hierarchies (several connected generations) within individual clones are capable of acquiring soil nitrogen equally from localized nutrient-rich pulses; and (iii) nitrogen is retained within individual hierarchies through allocation to sequentially developed ramet generations. 2. Mean 15N atom percent excess values within labelled ramet hierarchies were 10-21-fold greater than in immediately adjacent ramet hierarchies, and 105-137-fold greater than in the remaining ramet hierarchies within clones for all three species. This pattern of 15N distribution demonstrates that physiological integration does not occur among all ramets within individual clones and that ramet hierarchies function as integrated physiological individuals. 3. Ramets in closer proximity to a 15N pulse in soils acquired disproportionately greater amounts of the isotope compared to more distant ramets within clones for all three species. Inequitable resource acquisition among ramet hierarchies establishes the potential for asymmetric intraclonal competition below-ground and the development of size inequalities among hierarchies. 4. Approximately 21.7%, 3.5%, 1.7% and 2.3% of the initial 15N pool was allocated from the first ramet generation to the second and third (spring cohort), third (autumn cohort) and fourth generations, respectively. However, the magnitude of nitrogen allocation from parent to juvenile ramet generations was relatively consistent, with 17-32% of the 15N pool within parental ramets allocated to juvenile ramets. Intergenerational 15N allocation in S. scoparium clones therefore demonstrates a high degree of interdependence within individual ramet hierarchies. 5. Clones of the rhizomatous and two caespitose grasses investigated functioned as assemblages of autonomous ramet hierarchies, but ramet generations within individual hierarchies were effectively integrated. Consequently, intraclonal regulation in these temperate, perennial grasses is characterized by both ramet interdependence and independence. The occurrence of ramet independence throughout much of the life history of perennial grasses demonstrates the ecological importance of clonal fragments to genet and population maintenance and emphasizes that this may be the most relevant scale to investigate further the ecology of this important and widely distributed group of clonal plants.J Knoop, W. T. Walker, B. H. 1985TMInteractions of Woody and Herbaceous Vegetation in a Southern African SavannaJournal of Ecology731235-254HAA number of plots were set up in 2 natural savanna communities, with identical climates but different soils, to examine possible competition between the woody and herbaceous components of the vegetation. The community on the more sandy soil is a broad-leaf woody savanna and the other a more open microphyllous Acacia community. Vegetative growth and soil water were monitored over a 2-yr period in control plots and in plots cleared one of the vegetation components. In the broad-leaf community the effect of the herbaceous vegetation on the woody plants is negligible. In the Acacia community with 7 times more herbaceous biomass, mature woody-plant growth was reduced by competition from the grass-dominated herb layer particularly in the first (wetter) year. The vertical root distributions and soil-water data indicate that the grasses take up topsoil (0-30 cm) water sufficiently rapidly to reduce drainage into the subsoil (30-130 cm), an that they also take up subsoil water directly, thus lowering the amount of subsoil water available to woody plants. The different herbaceous to woody-plant biomass ratios in the 2 sites and the different intensity of competition during the 2 yr can be explained in terms of the effects of the soil properties and of the rainfall intensity on the ratio of water in the topsoil to that in the subsoil.ld climates in western South America from 15 degree S to 29 degree S, extending up to 3,000 m in the northern Chilean Andes, result primarily from the Andes intercepting precipitation from the Intertropical Convergence and the drying effect of the equatorward-running Humboldt Current bathing much of the Pacific coast of South America. Paleobotanical, paleontological, and geological evidence sugf`Competition in a variable environment: An experimental study in a desert annual plant populationKadmon, R. Shmida, A.@9Israel Journal of Botany Basic and Applied Plant Sciencesi39 4-6 403-412c 1990~xTheoretical studies suggest that fluctuations in availability of resources should lead to related fluctuations in the intensity of competition, and that significant competition should occur only in years of severe resource scarcity. This prediction has been supported by a number of studies conducted in animal populations, but has never been tested in plant populations. In this study, we performed neighbor removal experiments in conjunction with rainfall manipulation experiments, to test how fluctuations in rainfall, the major limiting resource for desert annuals, affect the intensity of competition in populations of the desert annual Stipa capensis. In order to test for the effect of spatial heterogeneity on the relationships between rainfall and competition intensity, populations were studied in three types of habitats: slopes, depressions, and wadis. Competition intensity was quantified, for each combination of habitat and rainfall, as the percentage of potential seed yield per germinating plant which was prevented due to competitive effects. Intensities of competition in the studied populations varied considerably between habitats, between seasons, and between rainfall manipulation treatments. In the most favorable habitat, the wadi, competition was always intense, independent of rainfall conditions. In the drier, slope and depressions habitats, natural and experimental changes in the amount of the yearly rainfall had considerable effects on competition intensity, but most pronounced competition was associated with high resource availability, a pattern which is opposite to the pattern that was predicted and documented for animal populations. The overall results suggest that competitive effects may have critical influence on the population dynamics of the species studied. This contradicts previous hypotheses on the relative importance of competition in desert plant populations.vA need for niches? Chesson, P.i$Trends in Ecology & Evolutioni61e 26-28m 1991HBTemporal hierarchies of variation and the maintenance of diversity"Chesson, Peter Huntly, Nancy 1993Plant Species Biology8 2-3195-2069 Using Smart Source ParsingA general model shows how the long-term growth rate of a population can be partitioned into components representing various mechanisms of maintenance of species diversity. One component summarises the effects of fluctuation-independent mechanisms, which include classical resource partitioning and frequency-dependent herbivory. Two other components represent fluctuation-dependent mechanisms, the storage effect and relative nonlinearity of competition. The general model shows how a community will track an equilibrium set by fluctuation-independent mechanisms and the environmental state when community dynamics are faster than the rate of environmental change. Fluctuation-dependent mechanisms can be important for diversity maintenance with or without such tracking, but on long time scales their effects are indistinguishable from those of fluctuation-independent mechanisms. These considerations lead to a hierarchical view of mechanisms of diversity maintenance where the effects of different timescales are partitioned or merged depending on the timescale of observation. These issues are illustrated with model examples involving various combinations of resource partitioning, fluctuations in recruitment rates, variation in the timing of germination, and seasonality. The very long timescales associated with climate change contain many complexities but nevertheless many ideas applicable to shorter timescales may be useful in a modified form.81Multispecies competition in variable environmentsChesson, Peter 1994$Theoretical Population Biology453227-276B8Evans, R. D. Black, R. A. Loescher, W. H. Fellows, R. J. Plant Cell and Environment1512 49-60h 1992zsTurgor maintenance, solute contents and recovery from water stress were examined in the drought-tolerant shrub Artemisia tridentata. Predown water potentials of shrubs receiving supplemental water remained above -2 MPa throughout summer, while predawn water potentials of untreated shrubs decreased to -5 MPa. Osmotic potentials decreased in conjunction with water potentials maintaining turgor pressures above 0 MPa. The decreases in osmotic potentials were not the result of osmotic adjustments (i.e. solute accumulation). Leaf solute contents decreased during drought, but leaf water volumes decreased more than 75% from spring to summer, thereby passively concentrating solutes within the leaves. The maintenance of positive turgor pressures despite decreases in leaf water volumes is consistent with other studies of species with elastic cell walls. Inorganic ion, organic acid, and carbohydrate contents of leaves declined during drought. The only solutes accumulating in leaves of A. tridentata with water stress were proline and a cyclitol, both considered compatible solutes. Total and osmotic potentials recovered rapidly following rewatering of shrubs; solute contents did not change except for a decrease in proline. Maintaining turgor through the passive concentrations of solutes may be advantageous compared to synthesis of new solutes for osmotic adjustment in arid environments. Qzt-segment model was combined with a soil water model to predict water uptake by roots of Agave deserti and Ferocactus acanthodes of four different lengths at 10 orientations from horizontal to vertical after various simulated rainfall events. 2. The soil was divided into 15 strata with specific water potentials (psi-soil) and temperatures; each root segment had specific values ofrlGrowth respiration, maintenance respiration and structural-carbon costs for roots of three desert succulents*$Nobel, P. S. Alm, D. M. Cavelier, J.Functional Ecology6r1n 79-85w 1992Water and nutrient uptake are provided at a cost of the carbon incorporated structurally into roots as well as that used for respiratory activities supporting root growth and maintenance. Gas-exchange and dry weight measurements on growing attached roots of Agave deserti indicated a respiratory CO-2 requirement of 6.3+-0.6 mol CO-2 per kg dry weight for newly elongating nodal roots emanating from the stem and 7.2+-0.9 for lateral roots occurring as fine branches on the nodal roots. Similar gas-exchange measurements indicated a growth respiration of 9.4 +- 1.4 mol CO-2 kg-1 for new roots of Ferocactus acanthodes and 8.7+-0.7 for Opuntia ficus-indica. Root growth respiration was also estimated using the heat of combustion of dried root material, which was assumed to contain 47% carbon in the non-ash fraction. For roots 1-20 weeks of age, growth respiration based on the heat of combustion averaged 9.4, 8.4 and 7.7 mol CO-2 kg-1 for A. deserti, F. acanthodes, and O. ficus-indica, respectively, averaging 0.7 mol CO-2 kg-1 higher for 1-week-old roots. The direct measurements of CO-2 evolution and the indirect estimate of growth respiration both indicate that the roots of desert succulents have low respiratory costs for growth compared with other species. The carbon incorporated into the dry weight of the roots averaged 38 mol kg-1 for the three species. Thus, the carbon costs for new roots of these desert succulents represented mainly carbon incorporated structurally into the roots, not growth respiration, and such total carbon costs can become similar to carbon expended by maintenance respiration after about 3 months under wet conditions at 20 degree C. For A. deserti under conditions appropriate to the Sonoran Desert, the water acquired from the soil per unit carbon expended was over twice as high for the drought-deciduous lateral roots and for nodal roots in their second year compared with first-year nodal roots.V p @ XQEffects of maternal nutrient pulse on reproduction of two colonizing Plantago sppl.(Miao, S. L. Bazzaz, F. A. Primack, R. B.Ecologye722u586-596o 1991 B 8Midwood, A. J. Boutton, T. W. Archer, S. R. Watts, S. E. 1998pjWater use by woody plants on contrasting soils in a savanna parkland: Assessment with delta2H and delta18OPlant and Soil 205v1o 13-24 ~In savanna parklands of southern Texas, patches of grassland and 'discrete clusters' of small trees and shrubs occur on sandy loam surface soils underlain by an argillic horizon (claypan) at 40 cm. Large trees and shrubs in 'groves' occur on deep (2 m) sandy loam soils without an argillic horizon. delta2H and delta18O of rainfall, groundwater, and soil and plant water were measured to: (1) determine if coexistence in woody patches occurs via vertical stratification of soil water uptake; (2) document differences in plant water acquisition on contrasting soil types; and (3) evaluate recharge and evaporative losses of soil moisture from grassland vs. wooded landscape elements. Groundwater was isotopically similar to weighted rainfall, suggesting local recharge at this site. Linear regressions of soil water delta2H on delta18O yielded slopes less than the meteoric water line, indicating significant evaporative losses of soil moisture in all landscape elements. Interspecific differences in root density distribution were significant; some woody species had roots well below 1.6 m, while others had few roots below 0.8 m. delta2H and delta18O values of stem water from all plants in groves were lower than those of soil water in the upper 1.5 m of the profile, suggesting all species obtained their water from depths >1.5 m. Deep roots of trees and shrubs at this savanna parkland site thus appeared to have a functional significance that was not revealed by biomass or density determinations. Root densities of species in discrete clusters (claypan present) were typically greater than those of the same species in groves (claypan absent), especially in the upper 80 cm of the soil profile. Consistent with rooting profiles, delta2H and delta18O values of plant water indicated that trees and shrubs in discrete clusters with fine-textured subsoils obtained most of their water at depths < 1.5 m. As with groves, there was no indication of water resource partitioning between species. In summary, we saw no isotopic evidence that co-occurring woody plants at this savanna parkland site were partitioning soil moisture vertically during late summer/early fall, despite marked differences in their root density distributions. This supports other lines of evidence which indicate that species interactions in tree/shrub clumps are competitive, and that species composition is therefore unstable in those landscape elements.nnhEffects of carbon dioxide and temperature on growth and resource use of co-occurring C-3 and C-4 annuals"Coleman, J. S. Bazzaz, F. A.Ecologyr734, 1244-1259z 1992We examined how CO-2 concentrations and temperature interacted to affect growth, resource acquisition, and resource allocation of two annual plants that were supplied with a single pulse of nutrients. Physiological and growth measurements were made on individuals of Abutilon theophrasti (C-3) and Amaranthus retroflexus (C-4) grown in environments with atmospheric CO-2 levels of 400 or 700 mu-L/L and with light/dark temperatures of 28 degree /22 degree or 38 degree /31 degree C. Elevated CO-2 and temperature treatments had significant independent and interactive effects on plant growth, resource allocation, and resource acquisition (i.e., photosynthesis and nitrogen uptake), and the strength and direction of these effects were often dependent on plant species. For example, final biomass of Amaranthus was enhanced by elevated CO-2 at 28 degree but was depressed at 38 degree . For Abutilon, elevated CO-2 increased initial plant relative growth rates at 28 degree but not at 38 degree , and had no significant effects on final biomass at either temperature. These results are interpreted in light of the interactive effects of CO-2 and temperature on the rates of net leaf area production and loss, and on net whole-plant nitrogen retention. At 28 degree C, elevated CO-2 stimulated the initial production of leaf area in both species, which led to an initial stimulation of biomass accumulation at the higher CO-2 level. However, in elevated CO-2 at 28 degree , the rate of net leaf area loss for Abutilon increased while that of Amaranthus decreased. Furthermore, high CO-2 apparently enhanced the ability of Amaranthus to retain nitrogen at this temperature, which may have helped to enhance photosynthesis, whereas nitrogen retention was unaffected in Abutilon. Thus, at 28 degree , final biomass of Abutilon was not stimulated in a high CO-2 environment whereas the final biomass of Amaranthus was. At 38 degree , Abutilon had slightly reduced peak leaf areas under elevated CO-2 in comparison to ambient CO-2 grown plants, but increased rates of photosynthesis per unit leaf area early in the experiment apparently compensated for reduced leaf area. For Amaranthus at 38 degree , peak leaf area production was not affected by CO-2 treatment, but the rate of net leaf area loss hastened under elevated CO-2 conditions and was accompanied by substantial reductions of whole-plant nitrogen content and leaf photosynthesis. This may have led to the reduced biomass accumulation of high CO-2 grown plants that we observed during the last 30 d of growth. Plants of both species grown in elevated CO-2 exhibited reduced tissue-specific rates of nitrogen absorption, increased plant photosynthetic rate per unit of conductance, and increased initial allocation of biomass to roots, irrespective of temperature. Plants of both species grown under an elevated temperature regime had substantially decreased reproductive allocation, increased allocation to stem biomass, and increased plant water flux at both CO-2 treatments. The age of plants also affected our interpretations of plant responses to CO-2 and temperature treatments. For example, significant effects of CO-2 treatment on the growth of Abutilon were evident early, prior to the initiation of flowering, when nitrogen availability would have been highest and pot space would not have been limited. Nevertheless, the opposite was true for Amaranthus, in which significant effects of CO-2 treatment on plant growth were not detectable until the final 30 d of the experiment. Elevated CO-2 interacted with temperature to affect plant productivity in different ways than would have been predicted from plant responses to elevated CO-2 alone. Furthermore, a majority of the interactive effects of CO-2 concentration and temperature on plant growth could be interpreted in light of their effects on the rates of net leaf area production and loss, nitrogen retention, and, to a lesser degree, photosynthesis and resource partitioning.9Intra-and inter-plant variation in xylem cavitation in Betula occidentalis$Sperry, J. S. Saliendra, N. Z. 1994 Plant Cell and Environment1711 1233-1241i Using Smart Source ParsingA modified version of a method that uses positive air pressures to determine t0*Comstock, Jonathan P. Ehleringer, James R. 1992>8Plant adaptation in the Great Basin and Colorado PlateauGreat Basin Naturalist523195-215Adapative features of plants of the Great Basin are reviewed. The combination of cold winters and an arid to semiarid precipitation regime results in the distinguishing features of the vegetation in the Greatest Basin and Colorado Plateau. The primary effects of these climatic features arise from how they structure the hydrologic regime. Water is the most limiting factor to plant growth, and water is most reliably available in the early spring after winter recharge of soil moisture. This factor determines many characteristics of root morphology, growth phenology of roots and shoots, and photosynthetic physiology. Since winters are typically cold enough to suppress growth, and drought limits growth during the summer, the cool temperatures characteristic of the peak growing season are the second most important climatic factor influencing plant habit and performance. The combination of several distinct stress periods, including low-temperature stress in winter and spring and high-temperature stress combined with drought in summer, appears to have limited plant habit to a greater degree than found in the warm deserts to the south. Nonetheless, cool growing conditions and a more reliable spring growing season result in higher water-use efficiency and productivity in the vegetation of the cold desert than in warm deserts with equivalent total rainfall amounts. Edaphic factors are also important in structuring communities in these regions, and halophytic communities dominate many landscapes. These communities of the cold desert share more species common with warm deserts than do the nonsaline communities. The Colorado Plateau differs from the Great Basin in having greater amounts of summer rainfall, in some regions less predictable rainfall, sandier soils, and streams which drain into river systems rather than closed basins and salt playas. One result of these climatic and edaphic differences is a more important summer growing season on the Colorado Plateau and a somewhat greater diversification of plant habit, phenology, and physiology., and increased plant water flux at both CO-2 treatments. The age of plants also affected our interpretations of plant responses to CO-2 and temperature treatments. For example, significant effects of CO-2 treatment on the growth of Abutilon were evident early, prior to the initiation of flowering, when nitrogen availability would have been highest and pot space would not have been limited. Nevertheless, the opposite was true for Amaranthus, in which significant effects of CO-2 treatment on plant growth were not detectable until the final 30 d of the experiment. Elevated CO-2 interacted with temperature to affect plant productivity in different ways than would have been predicted from plant responses to elevated CO-2 alone. Furthermore, a majority of the interactive effects of CO-2 concentration and temperature on plant growth could be interpreted in light of their effects on the rates of net leaf area production and loss, nitrogen retention, and, to a lesser degree, photosynthesis and resource partitioning.H*$Goldberg, Deborah Novoplansky, Ariel 1997LFOn the relative importance of competition in unproductive environmentsJournal of Ecology854409-4181. Whether or not competition intensity increases or is similar along productivity gradients has been highly controversial for a number of years, but empirical results bearing on this question are quite variable and no consistent answer has yet emerged. We have developed a more general hypothesis that includes these contradictory predictions as special cases that apply under different types of resource dynamics and different types of interactions between the growth and survival components of fitness. 2. The two-phase resource dynamics hypothesis of plant interactions along productivity gradients is based on the fact that soil resources are usually supplied in pulses rather than continuously, as assumed by most formal theory for community dynamics of terrestrial plants. When soil resource supply is temporally variable, individual plants will experience two distinct phases of resource availability: pulse periods when resources are high and most growth and resource accumulation occurs, and interpulse periods when resources are too low for most plants to take up and most mortality due to resource deficits takes place. 3. Competitive effects on growth should occur during pulses at both high and low productivity. In productive environments, interpulse intervals should be relatively mild and infrequent and therefore competitive effects during pulses will usually be important for individual and population persistence. However, as productivity decreases, the frequency of pulses (as well as or in place of their magnitude) often decreases and the duration of interpulse periods increases. Therefore, we suggest that processes occurring during interpulse intervals become increasingly important for individual and population persistence as interpulse intervals become longer. Whether or not competition occurs under low productivity will then depend on (i) the extent to which the asymptotically low resource availability during interpulse periods is determined by plant uptake or by abiotic factors such as leaching, drainage, evaporation and volatization, and (ii) the extent to which decreased growth due to competition during pulses results in decreased survival during interpulse periods. 4. According to the two-phase resource dynamics hypothesis, Grime's hypothesis that competition is unimportant at low productivity will hold when soil resource availability between pulses in unproductive environments is controlled by abiotic factors and when survival during interpulse intervals is independent of or even negatively correlated with growth during pulse periods. In contrast, Newman's and Tilman's hypothesis that competition is equally important along productivity gradients will apply when either of these conditions is not true. We predict that the conditions for Grime's hypothesis to apply are more likely for productivity gradients driven by water than by mineral nutrients and when response to competition is measured for community structure or individual survival rather than for individual growth. 5. We tested the predictions about water vs. nutrient gradients and growth vs. survival or community structure responses using data from a literature survey on field and garden experiments measuring relative competition intensity along productivity gradients. The results are consistent with the prediction that competitive effects on survival but not on growth will increase with productivity. However, the second prediction on nutrient vs. water gradients is not supported, although data on water gradients are extremely limited. 6. We intend this description and preliminary test of the two-phase resource dynamics hypothesis to stimulate experiments that explicitly examine processes occurring during pulse and interpulse phases of resource supply, as welt as more direct comparisons of actual resource dynamics along productivity gradients. Even if the details of the hypothesis do not hold up under such detailed scrutiny, our more general point should still be relevant: that much of the controversy over the role of competition along productivity gradie ts could be resolved by a more careful consideration of the conditions under which apparently contradictory hypotheses are expected to apply.f \$Tielbrger, Katja Ronen Kadmon 2000nhTemporal environmental variation tips the balance between facilitation and interference in desert plantsEcology;816o 15441553 annual plants; desert plant community; environmental gradient; facilitation; interference; plantplant interactions; rainfall; shrubs; temporal variation.Recently, numerous studies have pointed to the importance of positive interactions in natural communities. There is now a broad consensus that the balance between negative and positive interactions should shift along environmental gradients, with competition prevailing under environmentally benign conditions and positive interactions dominating under harsh conditions. A commonly cited example of the importance of facilitation in harsh environments is the preference of desert annual plants for the areas under the canopy of shrubs. The recognition of apparently positive effects of desert shrubs on annuals, however, has been mostly based on density measurements, while fitness parameters of the understory plants have been ignored. Also, the temporal consistency of such effects has not been previously tested. Based on conceptual ideas about the balance between interference and facilitation, we predicted that positive effects of the shrubs on the understory should dominate in dry years, while in favorable years, negative effects would be stronger. We tested our hypothesis by measuring the direction and magnitude of the shrub effect on demographic responses of four desert annual plant species during four consecutive seasons of differing rainfall. The results contradicted our initial hypothesis. Depending on the species, the effect of the shrubs shifted from either negative to neutral or from neutral to positive with increasing annual rainfall. However, this trend was stronger for the effect of shrubs on plant reproductive success than on their densities. Our data highlight the importance of measuring fitness parameters in studies of plantplant interactions. We suggest that the negative effects of shrubs on plant fitness were due to rainfall interception, while positive effects were related to increased nutrient availability beneath shrubs. However, the mechanisms by which the shrubs and annuals interact can only be resolved using an experimental approach. Our results contradict previous hypotheses about the relative importance of positive and negative interactions along environmental gradients. A simple conceptual model summarizing the proposed role of rainfall in determining the direction of shrub effects on their understory annuals is presented.cTMLong-term vegetation change at a fully protected Sonoran (Mexico) desert sitel Turner, R. M. EcologyD712464-477 1990To investigate desert vegetation dynamics, I undertook an open-ended study of a site that offers a combination of multiple observations through time with continuous protection from domestic livestock and other human impacts. The site is MacDougal Crater in the Sierra del Pinacate Reserve, Sonora, Mexico. Three sources of data have been used: a series of exactly matched photographs, begun in 1907; detailed permanent-plot maps, dating from 1959-1960; and an age-distribution analysis of a 170-yr-old population of Carnegiea gigantea. The crater vegetation is dominated by the woody perennials Cercidium microphyllum, Encelia farinosa, Prosopis sp., and Larrea tridentata, and the columnar cactus Carnegiea gigantea. Various populations of Larrea tridentata declined 50-90%, and Cercidium declined 60%, during the first half of this century with little or no recruitment since. Carnegiea numbers increased fourfold over the same period. A 200-fold increase in Prosopis in the playa-like crater center occurred in the early 1970s. Elsewhere on the crater floor, Encelia density increased markedly during the same period from insignificant levels in the early 1960s. Age distribution analysis for the Carnegiea population reveals three major establishment peaks during the 1790-1960 period. Recruitment and mortality records from the three sources of data are compared with regional climate records. The high mortality for some of the species was probably the result of the prolonged drought during 1936-1964. Establishment surges for some appear related to periods of unusually heavy precipitation during certain seasons. Clearly, desert communities are highly responsive to changes in the climate regime under which they grow.i("Wiegand, Thorsten Jeltsch, Florian 2000RKLong-term dynamics in arid and semiarid ecosystems: Synthesis of a workshop Plant Ecology 150 1-2 3-6.'Williams, David G. Ehleringer, James R. 1996`ZCarbon isotope discrimination in three semi-arid woodland species along a monsoon gradient Oecologia 1064455-4602,Leaf carbon isotope discrimination (DELTA) was measured for three dominant, semi-arid woodland species along a summer monsoon gradient in the southwestern United States over a 2-year period. We tested the hypothesis that decreased humidity levels during the growing season along this gradient resulted in lower leaf DELTA values. Sites of similar elevation along the transect were selected and the, range in monsoon contribution to overall annual precipitation varied from 18 to 58%, while total annual precipitation differed by a maximum of only 25% across this gradient. Leaf DELTA values in Quercus gambelii were negatively correlated with omega, a seasonally weighted estimate of the evaporative humidity gradient, suggesting that stomatal conductance declined as transpiration potential increased. For two other trees that co-occurred along this gradient, Pinus edulis and Juniperus osteosperma, DELTA remained relatively constant despite large variation in omega. These woodland species represent the full spectrum of responses of carbon isotope discrimination to increases in evaporative potential; that of decline where c-i/c-a (ratio of internal to ambient CO-2 concentration) and presumably stomatal conductance decrease, and that of constancy where whole plant internal adjustments allow c-i/c-a to remain stable.r5 nRLGoldberg, Deborah E. Turkington, Roy Olsvig Whittaker, Linda Dyer, Andrew R. 2001ZTDensity dependence in an annual plant community: Variation among life history stagesEcological Monographsp713s423-446. | vMost studies of density-dependent demography in plants consider the density only of the single focal species being studied. However, density-dependent regulation in plants may frequently occur at the level of the entire community, rather than only within particular species. In addition, because density dependence may differ considerably (even in direction) among demographic parameters, generalizing about patterns of density dependence and extrapolating to lifetime fitness and to population dynamics require comparisons among life history stages, as well as among types of species and physical environments. We constructed seminatural communities of desert annuals composed of all the constituent species in the same relative proportions as found in the natural habitat. These experimental communities were planted at a range of densities that extended far above and below mean natural field density. We compared among physical environments (irrigation treatments), among communities from different physical environments, and among growth forms (dicot and graminoid) to search for generalizations about the magnitude and direction of density dependence. Strong evidence of community-level density dependence was detected at all three life history stages studied in these desert annuals: emergence, survival, and final size. However, both the direction and degree of consistency of this density dependence varied considerably among the stages. The strongest and most consistent competitive effects were experienced at the emergence stage, where the mechanism is most likely a form of interference competition rather than exploitation competition. At the survival stage, the magnitude of effects was highly variable among physical environments and source communities, but negative effects were relatively rare, with either positive or no significant effects of increasing density. Thus, exploitation competition was also unimportant at the survival stage. In contrast, for growth, exploitation competition appeared to be the primary mechanism of interaction influencing growth. This variation in mechanism, direction, and magnitude of interactions among life history stages suggests that current models of plant community structure that are based largely on exploitation competition as it influences growth (with mortality a simple function of growth) are inadequate for even this simple annual plant community. We also compared growth forms and found that graminoids were superior competitors to dicots at the emergence and survival stages; they also had higher emergence and survival, regardless of density. Consistent with this result, grasses are always the numerical dominants in the source communities. In contrast, the two growth forms did not differ in competitive ability for growth, and dicots were consistently larger individuals, independent of density, even though grasses were also usually the biomass dominants in the source communities. These results suggest the importance of nontrophic mechanisms of interaction in controlling community structure and again emphasize the importance of constructing and testing models that incorporate multiple mechanisms of interactions.4-Golluscio, R. A. Sala, O. E. Lauenroth, W. K. 1998zDifferential use of large summer rainfall events by shrubs and grasses: A manipulative experiment in the Patagonian steppe Oecologia 115l 1-2i 17-25a("In the Patagonian steppe, years with above-average precipitation (wet years) are characterized by the occurrence of large rainfall events. The objective of this paper was to analyze the ability of shrubs and grasses to use these large events. Shrubs absorb water from the lower layers, grasses from the upper layers, intercepting water that would otherwise reach the layers exploited by shrubs. We hypothesized that both life-forms could use the large rainfalls and that the response of shrubs could be more affected by the presence of grasses than vice versa. We performed a field experiment using a factorial combination of water addition and life-form removal, and repeated it during the warm season of three successive years. The response variables were leaf growth, and soil and plant water potential. Grasses always responded to experimental large rainfall events, and their response was greater in dry than in wet years. Shrubs only used large rainfalls in the driest year, when the soil water potential in the deep layers was low. The presence or absence of one life-form did not modify the response of the other. The magnitude of the increase in soil water potential was much higher in dry than in humid years, suggesting an explanation for the differences among years in the magnitude of the response of shrubs and grasses. We propose that the generally reported poor response of deep-rooted shrubs to summer rainfalls could be because (1) the water is insufficient to reach deep soil layers, (2) the plants are in a dormant phenological status, and/or (3) deep soil layers have a high water potential. The two last situations may result in high deep-drainage losses, one of the most likely explanations for the elsewhere-reported low response of aboveground net primary production to precipitation during wet years.vior (changes can happen quickly and are not directly reversible without intensive management). This integration of concepts results because rather than assuming a simple, one-way dependence of plant functional types on soil moisture heterogeneity, our model assumes an interdependence between the two: soil moisture heterogeneity constrains the composition of the plant community, which in turn modifies soil moisture heterogeneity. The four-compartment model that we propose enables, for the first time, an integrated picture of both dimensions of soil moisture heterogeneity - horizontal and vertical - and of the interdependence between soil moisture heterogeneity and the proportions of the plant functional types that make up a given plant community. This unified conceptual model can be applied to provide insight into the individual and the combined effects of climate and land use on semiarid plant communities within the grassland/forest continuum, which vary in the proportions of canopy and intercanopy patches.n of ANPP contributed by different functional types. Forbs and shrubs made up a larger proportion of total ANPP on coarse- compared to fine-textured sites. Shrubs contributed more to ANPP at the drier end of the gradient. Basal cover of live vegetation was not significantly related to precipitation and was similar for both soil textures. Our results revealed that across a regional precipitation gradient, soil texture may play a larger role in determining community composition than in determining total ANPP.7E8B>HBGuo, Qinfeng Thompson, Daniel B. Valone, Thomas J. Brown, James H. 1995nhThe effects of vertebrate granivores and folivores on plant community structure in the Chihuahuan Desert Oikos7325251-259RKWe examined the effects of vertebrate granivores and folivores on winter and summer plant communities in the Chihuahuan Desert by selectively excluding different vertebrate combinations of birds, rodents, and large folivorous mammals from small experimental plots continuously since 1982. Few differences were observed across treatments during the first two yr of the experiment. The influence of vertebrates on winter and summer plant communities became apparent after three yr, and changes continued to occur after 11 yr of exclusion. Multivariate repeated-measures analyses indicated that removal of birds and both birds and rodents caused significant increases in the total density of winter annuals. The winter annual plant density was highest on bird removal plots and this increase was significant by 1985. Canonical Discriminant Analyses revealed that the exclusion of birds and both birds and rodents had the strongest influence on species composition of both winter and summer plant communities, although the winter plant community was more sensitive to vertebrate exclusion than the summer plant community. The exclusion of birds had the greatest effect on composition of summer plant communities followed in importance by the exclusion of rodents and mammalian folivores. Most of the plant species that responded significantly to the treatments were large-seeded winter annuals. Our results both complement earlier experimental work documenting the impacts of the granivorous rodents and folivorous mammals on desert plant communities and now demonstrate the important influence of avian granivores.t$Guo, Quinfeng Brown James, H.  1996PJTemporal fluctuations and experimental effects in desert plant communitiesOecologia Berlin 107&4568-577{In the Chihuahuan Desert of the southwestern United States we monitored responses of both winter and summer annual plant communities to natural environmental variation and to experimental removal of seed-eating rodents and ants for 13 years. Analyses of data on population densities of the species by principal component analysis (PCA) followed by repeated measures analysis of variance (rmANOVA) on PCA scores showed that: (1) composition of both winter and summer annual communities varied substantially from year to year, presumably in response to interannual climatic variation, and (2) community composition of winter annuals was also significantly affected by the experimental manipulations of seed-eating animals, but the composition of the summer annual community showed no significant response to these experimental treatments. Canonical discriminant analysis (CDA) was then applied to the data for winter annuals to more clearly identify the responses to the different classes of experimental manipulations. This analysis showed that removing rodents or ants or both taxa caused distinctive changes in species composition. There was a tendency for large-seeded species to increase on rodent removal plots and to decrease on ant removal plots, and for small-seeded species to change in the opposite direction. In the winter annual community there was a significant time x treatment interaction: certain combinations of species that responded differently to removal of granivores also showed opposite fluctuations in response to long-term climatic variation. The large year-to-year variation in the summer annual community was closely and positively correlated across all experimental treatments. The use of multivariate analysis in conjunction with long-term monitoring and experimental manipulation shows how biotic interactions inter act with variation in abiotic conditions to affect community dynamics.nGutierrez, J. R. 1992Effects of Low Water Supplementation and Nutrient Addition On the Aboveground Biomass Production of Annual Plants in a Chilean Coastal Desert Site Oecologian904c556-559eThe effects of low water supplementation and nutrients on the aboveground biomass production of annual plants was tested in the field by mimicking small rainfall events of 5 mm per month and by adding fertilizers to experimental quadrats. Field measurements were made during an extremely dry year, so the potential additional effects of rainfall probably had no important effect on plant responses. Biomass of non-native species was higher in irrigated than in non-irrigated quadrats. No significant responses to irrigation treatments were detected in native species. This lack of response may be due to higher thresholds of watering being required for either germination and/or growth. Because of the low water inputs, fertilizer additions did not promote any biomass response in either native or non-native species. Responses of non-native species to low and frequent pulses of water, which is characterized of this arid system, may be important for the persistence of these species in this environment.n*#Gutierrez, Julio R. Vasquez, Hernanl 1996The effects of water and nutrient addition on annual aboveground biomass production of Chenopodium petiolare H.B.K. (Chenopodiaceae) in a north-central Chilean old fielda Ecoscience3i2o211-215ehbThe effects of water and nutrient supplementation on the annual aboveground biomass production of Chenopodium petiolare H.B.K. were measured in an old field of north-central Chile by simulating rainfall events of 5 mm per month and by adding fertilizers to experimental quadrats in two years with different precipitation levels. In irrigated quadrats, the biomass of C. petiolare was higher than in non-irrigated quadrats. In the nutrient supplementation experiment, the only fertilizer that elicited a plant response was nitrogen. Biomass yield in the nitrogen-supplemented quadrats was over twice that in the control quadrats. As expected from experimental results, biomass in the wet year (1991) was twice as high as the biomass yield during the dry year (1990). Observations on the spatial distribution of C. petiolare elsewhere support our experimental results. Using Smart Source Parsing*#Gutterman, Yitzchak Gozlan, Sabrina 1998Amounts of winter or summer rain triggering germination and 'the point of no return' of seedling desiccation tolerance, of some Hordeum spontaneum local ecotypes in Israel8Plant and Soil 2042223-2342q^pT:4Wan, Changgui Sosebee, Ronald E. McMichael, Bobby L. 1993~wDoes hydraulic lift exist in shallow-rooted species? A quantitative examination with a half-shrub Gutierrezia sarothraePlant and Soil 1531 11-17n^XHydraulic lift occurs in some deep-rooted shrub and herbaceous species. In this process, water taken up by deep roots from the moist subsoil is delivered to the drier topsoil where it is later reabsorbed by shallow roots. However, little is known about the existence of hydraulic lift in shallow-rooted xeric species. The objectives of this study were 1) to ascertain whether hydraulic lift exists in Gutierrezia sarothrae (broom snakeweed), a widespread North American desert species with a shallow root system, grown in pot and field conditions and 2) if it does, how much water can be transferred from the subsoil to the 30 cm topsoil during the night. Snakeweed seedlings were transplanted in buried pots allowing the deeper roots to grow into the subsoil 30 cm below the surface. Soil water content inside and outside of the pot was measured seasonally and diurnally with time domain reflectometry technique (TDR). An increase in water content was detected in the pot after the plant was covered for 3 h by an opaque plastic bag during the day, suggesting hydraulic lift from deeper depths and exudation of water into the drier topsoil. Root exudation was also observed on native range sites dominated by snakeweed. Water efflux in the pot was 271 g per plant per night. which was equivalent to 15.3% of the extrapolated, porometer-derived whole-plant daily transpiration. Hydraulic lift observed in Gutierrezia improved water uptake during the day when evaporative demand is high and less water is available in the topsoil. We concluded that hydraulic lift might help snakeweed to alleviate the effect of water stress..'Wan, C. Sosebee, R. E. McMichael, B. L.r 1993Growth, photosynthesis, and stomatal conductance in Gutierrezia sarothrae associated with hydraulic conductance and soil water extraction by deep rootsl.'International Journal of Plant Sciencesg 154e1r144-151a82We hypothesize that predominance of Gutierrezia sarothrae in the rangeland plant community depends on its ability to take up water more effectively from soil depths below 30 cm. Gutierrezia seedlings grown in 30-cm-deep pots were allowed to develop deep lateral roots out of pots into subsoil. The deep laterals were then excised and photosynthesis and stomatal conductance of the test plants measured before and after root pruning. Photosynthesis (A) and stomatal conductance (g) were at least 49% greater (P lt .05) before than after root pruning. Photosynthesis and g were more affected by the removal of four deep roots 30 cm below the surface than by the removal of four shallow roots in the top 30 cm of soil. The soil water extraction patterns in Gutierrezia plants grown in 60-cm soil columns were determined for the upper 30 cm and 30 to 60 cm of soil. Water extraction was significantly greater (P lt .05) from the wetter, lower section of the soil column than from the drier, upper column. Higher photosynthesis (138%, P lt .05), transpiration (154%, P lt .01), stomatal conductance (223%, P lt .01), and canopy development (340%, P lt .05) were observed in plants with deep roots growing beyond the pots than well-watered, pot-grown plants without deep roots. This was related to higher hydraulic conductance, which in turn appeared enhanced by the greater ratio of deep root length to total root length. Stomatal conductance, photosynthesis, and canopy development in Gutierrezia may be regulated by hydraulic capacity, which is determined largely by deep root development.ick, David Yang Anthony, P.D 1993voThe influence of fluctuating resources on life history: Patterns of allocation and plasticity in female guppiesmEcologyr747 2011-2019  6 /We investigated how resources are allocated to reproduction and how variations in resource availability influence reproductive allocation, offspring number, and offspring size in guppies (Poecilia reticulata). Our goal was to evaluate how plastic these variables are in response to environmental variation and to characterize the nature of this plasticity. Female guppies which had just given birth (litter 1) were assigned to either high or low levels of food availability until they gave birth to their next litter (litter 2, interval 1). They were then randomly reassigned to either high or low food with the constraint that there be equal numbers of individuals in each of four treatments: high-high, high-low, low-high, and low-low. They were maintained on this level of food availability until they produced their next litter (litter 3, interval 2). We analyzed variables that characterized the female after the birth of the third litter and the offspring in the third litter. These were two-way analyses, with intervals 1 and 2 as the main effects and high vs. low food as the levels of each effect. The qualities of the third litter were influenced by both interbrood intervals, indicating that the resources used for producing the litter were derived from both intervals. Specifically, higher food availability during either interval resulted in a significant increase in the number of offspring in litter 3, independent of the size of the mother. This result indicates that the number of offspring produced in a litter will be a function of both the immediate and the past environment. Lower food during either interval resulted in an increase in the number of days between the second and third litters, indicating that, if resource availability is low, the female may delay the initiation of the next litter, allowing her to acquire more resources. Resource availability during both intervals also influenced how resources were allocated to individual offspring. Females responded to low food during the first interbrood interval by producing heavier offspring in litter 3. This increase in mass was almost entirely attributable to an increase in fat reserves. Such a result could represent adaptive plasticity, if it can be demonstrated that maternal fitness increases through the production of heavier offspring in a low-food environment.fy >4*$Milton, Suzanne J. Wiegand, Thorsten 2001\UHow grazing turns rare seedling recruitment events to non-events in arid environmentst 60Breckle, Siegmar W. Veste, Maik Wucherer, Walter&Sustainable land use in desertsF "Heidelber, Berlin, New York7 Springer Verlag 197-207l& Mitsunaga, Takayuki Fuji, Koichi 1997~wThe effects of spatial and temporal environmental heterogeneities on persistence in a laboratory experimental community& Researches on Population Ecology392249-260JCAmong many stabilizing factors for community dynamics, spatial and temporal heterogeneities have been widely considered in recent years as two of the most important properties. However, the difference between the two types of heterogeneities have not been studied, except for Clark and Yoshimura (1993). We evaluated experimentally the effect of temporal and spatial heterogeneities on the persistence of a biological community. The experimental communities consisted of one parasitic wasp species, one bean weevil species, and two kinds of bean. Temporal and spatial heterogeneities of experimental communities were generated by kinds and timing of bean supply. Of all the experimental communities, the most persistent community was a temporally and spatially homogeneous community with Red Kidney bean as primary resource. Compared to spatially heterogeneous communities, temporally heterogeneous communities were more persistent. These results were easily explained by considering the attack rate of parasitic wasps and the difference between arithmetic and geometric means. In order to discuss the relative importance of environmental heterogeneity and the mode of biological interaction on community persistence, we have to measure the degree of environmental heterogeneity as the rate of change of the strength of interspecific interactions. 6/Montana, Carlos Cavagnaro, Bruno Briones, Oscar 1995\VSoil water by co-existing shrubs and grasses in the Southern Chihuahuan Desert, Mexico"Journal of Arid Environments311e 1-13 & Soil water use by shrubs and grasses of vegetation patches (vegetation arcs) occurring in two-phase mosaics of the Southern Chihuahuan Desert (Mexico) was investigated after an experimental irrigation equivalent to a 75 mm rainfall. Three shrubs (Flourensia cernua, Larrea tridentata and Prosopis glandulosa) and one grass (Hilaria mutica) were studied. Irrigation water did not percolate deeper than 40 cm. This soil layer contained more than 75% of the roots in all species, except P. glandulosa where a less developed, deeper root system was detected (but not quantified). Root distribution indicates that the water stored in the 0-40-cm soil layer after the experimental irrigation was available for the four species. However, predawn xylem water potential (XWP) of F. cernua and H. mutica were strongly influenced by soil water present in the 0-40-cm layer, whereas those of L. tridentata and P. glandulosa were not. Differences in predawn XWP between watered and unwatered individuals were greatest in F. cernua and H. mutica, and smallest in P. glandulosa. Changes in tissue osmotic potential (TOP) values as a consequence of watering were sharply marked in all species except P. glandulosa. H. mutica XWP approached zero for a few days in response to small rain events. The results indicate that adult individuals of grasses and shrubs are potential competitors for soil resources (to a variable degree according to the shrub species). Their co-existence in the arcs is probably favoured by a process of slow competitive displacement as long as the recruitment of new shrubs takes place mainly by colonization of the upslope fringe of the arcs where grass biomass is low. As development of the vegetation progresses in the colonization front and the grass canopy is almost closed, the chances of a shrub being suppressed by water competition diminishes in the following order: F. cernua, L. tridentata, P. glandulosa. A drastic reduction in grass biomass because of grazing would depress the competitive ability of the grasses and may preferentially facilitate the establishment of livestock dispersed species like P. glandulosa. A consequent shift to a more shrubby community seems unavoidable since the recovery of the grass strata will probably not suppress the newly-established shrubs tapping water from deep water sources.R :4Brown, James H. Valone, Thomas J. Curtin, Charles G. 1997NHReorganization of an arid ecosystem in response to recent climate changeVOProceedings of the National Academy of Sciences of the United States of America9418 9729-9733Natural ecosystems contain many individuals and species interacting with each other and with their abiotic environment. Such systems can be expected to exhibit complex dynamics in which small perturbations can be amplified to cause large changes. Here, we document the reorganization of an arid ecosystem that has occurred since the late 1970s. The density of woody shrubs increased 3-fold. Several previously common animal species went locally extinct, while other previously rare species increased. While these changes are symptomatic of desertification, they were not caused by livestock grazing or drought, the principal causes of historical desertification. The changes apparently were caused by a shift in regional climate: since 1977 winter precipitation throughout the region was substantially higher than average for this century. These changes illustrate the kinds of large, unexpected responses of complex natural ecosystems that can occur in response to both natural perturbations and human activities."Brown, Joel R. Archer, Steve 1999piShrub invasion of grassland: Recruitment is continuous and not regulated by herbaceous biomass or densityEcology807 2385-2396b Proliferation of woody plants in grasslands and savannas since the 1800s has been widely documented. In the southwestern United States, increased abundance of honey mesquite (Prosopis glandulosa var. glandulosa) has been attributed to heavy grazing by livestock. Here, we test the hypothesis that P. glandulosa invasion of grasslands requires, first, reductions in herbaceous biomass and density such as those that accompany livestock grazing and, second, episodes of high soil moisture availability. No combination of grass density (nonmanipulated or reduced 50%) or defoliation (none, moderate, heavy) significantly affected P. glandulosa seedling emergence within a watering regime (natural and supplemented) at our field site in semiarid southern Texas. Seedling emergence on plots receiving only natural rainfall was high (42%), despite the fact that precipitation was substantially below normal. Supplemental watering, to generate moisture levels approximating years of unusually high annual rainfall, increased emergence to 59%. Seedling survival after 2 yr was high (62-77%) and statistically comparable across the density, defoliation, and watering treatments. Net photosynthesis (An) of 1-yr-old seedlings was enhanced by supplemental watering, but reductions in grass density or biomass had little effect on seedling An or xylem water potential. Height, aboveground biomass, and leaf area were comparable among 1- and 2-yr-old seedings across all density, defoliation, and watering combinations. High seedling emergence and survival on unwatered plots, even during a "drought year," suggests that Prosopis recruitment is not contingent upon unusual or episodic rainfall. Reductions in biomass and density of herbaceous vegetation had no influence on seedling emergence, growth, or survival, suggesting that Prosopis invasion is minimally influenced by grass competition. Historic grazing at this site appears to have altered herbaceous composition and reduced above- and belowground biomass production below the threshold level required for competitive exclusion of woody vegetation. Such data suggest that rates and patterns of seed dispersal may be the primary determinants of P. glandulosa encroachment on present-day landscapes in semiarid regions. Minimizing livestock dispersal of seed (in the case of leguminous shrubs) and maintenance of an effective fire regime (through production of fine fuels) may be crucial for sustaining herbaceous composition and production in grazed systems prone to invasion by unpalatable woody plants. h tR482-488$://000165268000017e0*Jackson, R. B. Sperry, J. S. Dawson, T. E.ZTRoot water uptake and transport: using physiological processes in global predictionsTrends in Plant Sciencen|vhydraulic lift; plants; rhizosphere; aquaporins; systems; model; identification; architecture; conductance; vegetationPlant water loss, regulated by stomata and driven by atmospheric demand, cannot exceed the maximum steady-state supply through roots. Just as an electric circuit breaks when carrying excess current, the soil-plant continuum breaks if forced to transport water beyond its capacity. Exciting new molecular, biophysical and ecological research suggests that roots are the weakest link along this hydraulic flow path. We attempt here to predict rooting depth and water uptake using the hydraulic properties of plants and the soil, and also to suggest how new physiological tools might contribute to larger- scale studies of hydraulic lift, the water balance and biosphere-atmosphere interactions.Trends Plant Sci. 2000 Nov511373BF TRENDS PLANT SCIISI:000165268000017JDJeltsch, Florian Stephan, Thomas Wiegand, Thorsten Weber Gerhard, E. 2001ZSArid rangeland management supported by dynamic spatially explicit simulation models 60Breckle, Siegmar W. Veste, Maik Wucherer, Walter& Sustainable land use in deserts "Heidelber, Berlin, New Yorks Springer Verlago229-240  2953-2965s$://000171909200008 60Johansen, M. P. Hakonson, T. E. Breshears, D. D.piPost-fire runoff and erosion from rainfall simulation: contrasting forests with shrublands and grasslandseHydrological Processes erosion; sediment transport; runoff; fire; wildfire; hydrologic response; surficial processes ponderosa pine hillslope; sediment production; new-mexico; hydrologic characteristics; infiltration rates; surface runoff; soil-erosion; perspective; restoration; thresholdssRainfall simulations allow for controlled comparisons of runoff and erosion among ecosystems and land cover conditions. Runoff and erosion can increase greatly following fire, yet there are few rainfall simulation studies for post-fire plots, particularly after severe fire in semiarid forest. We conducted rainfall simulations shortly after a severe fire (Cerro Grande) in ponderosa pine forest near Los Alamos, New Mexico, USA, which completely burned organic ground cover and exposed unprotected soil. Measurements on burned plots showed 74% of mineral soil was exposed compared with an estimated 3% exposed prior to the fire. Most of the remaining 26% surface area was covered by easily moveable ash. Rainfall was applied at 60 mm h(-1) in three repeated tests over 2 days. Runoff from burned plots was about 45% of the total 120 mm of applied precipitation, but only 23% on the unburned plots. The most striking difference between the response of burned and unburned plots was the amount of sediment production; burned plots generated 25 times more sediment than unburned plots (76 kg ha(-1) and 3 kg ha(-1) respectively per millimetre of rain). Sediment yields were well correlated with percentage bare soil (r = 0.84). These sediment yields were more than an order of magnitude greater than nearly all comparable rainfall simulation studies conducted on burned plots in the USA, most of which have been in grasslands or shrublands. A synthesis of comparable studies suggests that an erosion threshold is reached as the amount of soil exposed by fire increases to 60- 70%. Our results provide sediment yield and runoff data from severely burned surfaces, a condition for which little rainfall simulation data exist. Further, our results contrast post-fire hydrologic responses in forests with those in grasslands and shrublands. These results can be applied to problems concerning post-fire erosion, flooding, contaminant transport, and development of associated remediation strategies. Copyright (C) 2001 John Wiley & Sons, Ltd.Hydrol. Process. 2001 Oct 301515487ZH HYDROL PROCESSISI:000171909200008373-392$://000165601400001w& Joseph, R. Ting, M. F. Kumar, P.d^Multiple-scale spatio-temporal variability of precipitation over the coterminous United States"Journal of Hydrometeorologytmoisture transport; multicomponent decomposition; nasa/dao reanalyses; rainfall fields; summer drought; north-america; 1988 drought; 1993 floods; pacific; anomaliesleThe spatio-temporal variability of precipitation over the United States using a 30-yr, gridded hourly precipitation dataset is studied. Orthogonal wavelet transform is applied to the time series at each grid box to capture the temporal scales of fluctuation at 17 different timescales ranging from 2 h to 15 yr. Rotated principal component analysis is then applied to the transformed series to identify spatial coherence of the temporal scales of fluctuations. The results indicate that the energy of the fluctuations shows an approximate power-law relationship with respect to scale in most regions. The spatial organization of the temporal variability shows coherence at distinct scales identified as the subdiurnal (2-16 h), synoptic (16 h-22 days), seasonal (42 days-1 yr), and climatic mode (15 yr). The synoptic scale explains the largest spatial variance of the fluctuations in precipitation and is spatially coherent; the subdiurnal mode is spatially less coherent. The seasonal mode is dominant over the Pacific Northwest, whereas the climatic mode has large amplitude only over California. When examining the winter and summer seasons separately, it is found that the winter precipitation fluctuation is more associated with synoptic scale; the summer fluctuation is associated with shorter timescales or the subdiurnal scale. Studies of extreme summer drought and hood events over the Midwest indicate that anomalously wet or dry years are manifestations of persistent anomalous wet or dry conditions across all temporal scales, with the maximum contribution for the wet events being affected by the synoptic-scale activities.J. Hydrometeorol. 2000 Oct15378TX J HYDROMETEOROLISI:000165601400001Effects of nutrient depletion on growth of Holcus lanatus L. and Festuca ovina L. and on the ability of their roots to absorb nitrogen at warm and cool temperaturesKachi, N. Rorison, I. H.New Phytologistt 115m3r531-538i 1990< F^PJMartens, Scott N. Breshears, David D. Meyer, Clifton W. Barnes, Fairley J. 1997piScales of above-ground and below-ground competition in a semi-arid woodland detected from spatial patternn$Journal of Vegetation Science48t5n655-664aSemi-arid woodlands are two-phase mosaics of canopy and intercanopy patches. We hypothesized that both aboveground competition (within canopy patches), and below-ground competition (between canopy patches), would be important structuring processes in these communities. We investigated the spatial pattern of trees in a Pinus edulis-Juniperus monosperma woodland in New Mexico using Ripley's K-function. We found strong aggregation of trees at scales of 2 to 4 m, which indicates the scale of canopy patches. Canopy patches were composed of individuals of both species. Crown centers of both species were always less aggregated than stem centers at scales less than canopy patch size, indicating morphological plasticity of competing crowns. In the smallest size classes of both species, aggregation was most intense, and occurred over a larger range of scales; aggregation decreased with increasing size as is consistent with density-dependent mortality from intraspecific competition. Within canopy patches, younger trees were associated with older trees of the other species. At scales larger than canopy patches, younger trees showed repulsion from older conspecifics, indicating below-ground competition. Hence, intraspecific competition was stronger than interspecific competition, probably because the species differ in rooting depth. Woodland dynamics depend on the scale and composition of canopy patches, aggregated seed deposition and facilitation, above- and below-ground competition, and temporal changes in the spatial scale of interactions. This woodland is intermediate in a grassland-forest continuum (a gradient of increasing woody canopy cover) and hence we expected, and were able to detect, the effects of both above- and below-ground competition.777-783$://000169094400008s4-Martens, S. N. Breshears, D. D. Barnes, F. J.e|Development of species dominance along an elevational gradient: Population dynamics of Pinus edulis and Juniperus monosperma.'International Journal of Plant Sciencesrcanonical correspondence analysis; pinon-juniper woodland; plant mortality northern new-mexico; spatial pattern; plant-communities; great- basin; grassland/forest continuum; environmental gradient; summer precipitation; drought tolerance; patagonian steppe; differential useWe evaluated species-environment relationships within pinon- juniper woodlands in northern New Mexico (United States) using canonical correspondence analysis (CCA). The first CCA axis was associated primarily with elevation. Our results showed separation between pinon and juniper along the elevation gradient, as expected: pinon is relatively more dominant at higher sites, whereas juniper is relatively more dominant at lower sites. To examine how this pattern of dominance might emerge with time, we plotted the position of centroids for three pinon and juniper size classes along the first CCA axis. We found that small pinons and junipers were distributed relatively uniformly across the gradient, whereas large pinons and junipers were strongly segregated along the gradient, with intermediate-sized pinons and junipers intermediate on the CCA axis between small and large. This produced a pattern of increased divergence between the two species that increased with size. We suggest that this pattern emerges as a result of differential mortality between the species rather than as a result of differences in seedling establishment along the gradient. These differences between the species could result from differences in resource use (i.e., physiology) and resource acquisition (i.e., rooting patterns relative to plant available water). We present a conceptual model of how differences between the species in resource acquisition increase with size (age). We suggest that differences in resource acquisition between species, which increase as individuals mature, may play a greater role in determining species dominance along resource gradients than has been previously appreciated.Int. J. Plant Sci. 2001 Jul 1624439CZ INT J PLANT SCIISI:0001690944000080*Martinez Meza, Ernesto Whitford, Walter G. 1996f_Stemflow, throughfall and channelization of stemflow by roots in three Chihuahuan desert shrubs"Journal of Arid Environments323r271-287cThree studies were conducted to examine precipitation-vegetation relationships in the multi-stemmed shrubs Larrea tridentata, Prosopis glandulosa and Flourensia cernua in a desert ecosystem. We measured stemflow and throughfall as affected by bulk precipitation, canopy architecture and stem-angles. Using fluorescent dye, we traced root channelization of stemflow water. Stepwise regression analysis showed that the best one-variable model for stemflow in L. tridentata was canopy volume, which accounted for 87% of variation. The best one-variable model for stemflow in P. glandulosa and F. cernua was canopy area, which accounted for 82% of variation in both species. Stemflow data from winter and summer months were statistically compared to determine the influence of leaves on stemflow generation in P. glandulosa and F. cernua. Stemflow amounts collected during winter months do not differ significantly from those of summer months demonstrating that in these winter deciduous species the absence of leaves during winter months does not affect generation of stemflow. Analysis of variance showed that the percentage of throughfall was different among species suggesting that variations in canopy characteristics could explain, in part, these interspecific throughfall differences. Both stem-angle and stem-length had a significant effect on stemflow generation in F. cernua and L. tridentata, whereas there was only a significant relationship between stem-angle and stemflow in P. glandulosa. Analysis of rhodamine-B dye distribution under shrubs indicated that root channels are preferential pathways for movement of stemflow water into soil, and that this water is potentially the source of soil moisture which allows shrubs to remain physiologically active under drought conditions.c t-segment model was combined with a soil water model to predict water uptake by roots of Agave deserti and Ferocactus acanthodes of four different lengths at 10 orientations from horizontal to vertical after various simulated rainfall events. 2. The soil wnhPlant, soil fauna and microbial responses to N-rich organic patches of contrasting temporal availabilityHAHodge, A. Stewart, J. Robinson, D. Griffiths, B. S. Fitter, A. H.t0*Soil Biology and Biochemistry. Oct., 1999;31111517-1530. FTXT: ScienceDirect (tm) http://www.sciencedirect.com/science?_ob=GatewayURL&_origin=SilverLinker&_urlversion=4&_method=citationSearch&_volkey=0038%2d0717%2331%231517%2311&_version=1&md5=20dccbadc4b9e0ce90f3be1581aa57a3 ScienceDirect (China) http://elsevier.lib.tsinghua.edu.cn/science?_ob=GatewayURL&_origin=SilverLinker&_urlversion=4&_method=citationSearch&_volkey=0038%2d0717%2331%231517%2311&_version=1&md5=20dccbadc4b9e0ce90f3be1581aa57a3 ScienceDirect (Taiwan) http://sdos.ejournal.ascc.net/science?_ob=GatewayURL&_origin=SilverLinker&_urlversion=4&_method=citationSearch&_volkey=0038%2d0717%2331%231517%2311&_version=1&md5=20dccbadc4b9e0ce90f3be1581aa57a3 1999 N HA simple (L-lysine) 15N/13C dual-labelled organic patch was added to soil microcosm units with or without Lolium perenne L. plants either as a single addition of 5 ml of 200 mM L-lysine ('patch' treatment) or as a series of 1 ml aliquots of 200 mM L-lysine added at 7 day intervals over 28 days ('pulse' treatment) thus both treatments supplied the same amount of nitrogen (N). Controls were added as 1 ml H2O over 28 days (control pulse treatment) or as a single addition of 5 ml H2O (control patch treatment) to planted tubes. Decomposition of the added L-lysine was rapid as shown by amounts of 13C detected in the soil atmosphere and were greatest from the planted L-lysine pulse treatment indicating the presence of plant roots was influencing decomposition of the pulse. Plant uptake of N, as 15N, from the added L-lysine was also rapid and detected in the shoots by day 4. However, the mean rate at which 15N appeared in the shoots did not differ between patch or pulse treatments. No 13C levels greater than background were detected in the plant material. Root production and mortality in the patch or pulse addition zone was measured in situ using minirhizotron tubes. Cumulative root births in the L-lysine patch treatment were greater than controls in the latter part of the experiment while instantaneous root births were greater in the L-lysine pulse treatment compared to all others at 21 d. Root death rate was faster in the L-lysine treatments than in the controls. Root and shoot dry weights at final harvest (35 d) were greater in the pulse treatments. Shoot, but not root, dry weights were also significantly greater in the L-Lysine treatments compared to controls. Total root and shoot nitrogen contents were greater in the L-lysine treatments than controls. Total N capture by the L. perenne plants from the added L-lysine was 57% (patch) and 61% (pulse) of the N added and did not differ significantly between treatments. Protozoan biomass measured at harvest was greater in the L-lysine treatments (planted and unplanted) than the planted controls. The physiological profile of the microbial community did not vary because of patch or pulse treatments although significant differences between planted and unplanted L-lysine treatments and planted and H2O controls occurred. The response of roots and microorganisms in relation to N capture is discussed.LSperry, J. S. Saliendra, N. Z. Pockman, W. T. Cochard, H. Cruiziat, P. Davis, S. D. Ewers, F. W. Tyree, M. T. jdNew evidence for large negative xylem pressures and their measurement by the pressure chamber method 1996F@Plant cell environ. Oxford, Blackwell Scientific Publishers. Apr194e427-436r Using Smart Source Parsingforest trees. angiosperms . gymnosperms . betula . xylem . press,%Snyder, Keirith A. Williams, David G. 2000d^Water sources used by riparian trees varies among stream types on the San Pedro River, Arizona*#Agricultural and Forest Meteorologyl 105 1-3227-2401d^Variation in the sources of water used by tree species has important ramifications for forest water balances. The fraction of tree transpiration water derived from the unsaturated soil zone and groundwater in a riparian forest was quantified for Populus fremontii, Salix gooddingii, and Prosopis velutina across a gradient of groundwater depth and streamflow regime on the San Pedro River in southeastern Arizona, US. The proportion of tree transpiration derived from different potential sources was determined using oxygen (delta18O) and hydrogen (deltaD) stable isotope analysis in conjunction with two- and three-compartment linear mixing models. Comparisons of delta18O and deltaD of tree xylem water with that of potential water sources indicated that Salix gooddingii did not take up water in the upper soil layers during the summer rainy period, but instead used only groundwater, even at an ephemeral stream site where depth to groundwater exceeded 4 m. Populus fremontii, a dominant 'phreatophyte' in these semi-arid riparian ecosystems, also used mainly groundwater, but at the ephemeral stream site during the summer rainy season this species derived between 26 and 33% of its transpiration water from upper soil layers. Similarly, at the ephemeral stream site during the summer rainy period, Prosopis velutina derived a greater fraction of its transpiration water from upper soil layers, than at a perennial stream site where groundwater depth was less than 2 m. Measurements of transpiration flux combined with stable isotope data revealed that Populus fremontii transpired a greater quantity of water from upper soil layers at the ephemeral stream site than at the perennial stream site. These results imply that transpiration from groundwater and unsaturated soil layers by riparian vegetation may depend on the interaction between site conditions and species assemblage. to almost 600%. Roots produced in response to the additions of water and water plus nitrogen lived longer than roots in the control treatments. Thus, additions of water and water plus nitrogen influenced both the proliferation of new roots and their longevity, with both proliferation and longevity related to the type and duration of resource supply. Results suggest that root longevity and mortality may be plastic in response to changes in soil resource availability, as is well known for root proliferation.F   4Hutchings, Michael J.s 1999RLClonal plants as cooperative systems: Benefits in heterogeneous environmentsPlant Species Biology141 1-10~All natural environments are spatially and temporally heterogeneous. Consequently, their ability to provide essential resources for the growth of plants is variable. Modular plant species produce repeated basic structures which, in the case of clonal species, are called ramets. Ramets belonging to the same clone are distributed throughout the environment in space and time, and therefore they may be located in sites which differ in resource-providing quality. The connections between ramets may allow resources to be shared, enabling the clone to behave as a cooperative system. As a result of such physiological integration, ramets can survive in conditions where there is lethal shortage of a resource because they are connected to, and supported by, ramets located in conditions where there is ample supply of the same resource. Physiological integration between connected ramets presents opportunities for heterogeneous environments to be exploited to an extent that is only just becoming apparent. As heterogeneity is ubiquitous in natural environments, it may be expected that plants, as relatively immobile organisms, will have evolved the capacity to cope with it by making appropriate localized morphological and/or physiological plastic responses. Recent studies suggest that such responses not only enable clonal species to cope with environmental heterogeneity, but that under some circumstances they can benefit more from environments which are heterogeneous rather than homogeneous, even when both types of environment contain the same amount of resources. Studies on Glechoma hederacea (Lamiaceae) that illustrate this phenomenon are described. IPCC 1996Climate Change 1995. Contribution of Working group I to the Second Assessment Report of the Intergovernmental Panel on Climate Change.  Cambridge\ Cambridge University Press& Ives, Anthony R. Eric D. Klopfer 1997D=Spatial variation in abundance created by stochastic temporal}Ecology178 1907-1913p@Campanula americanaCaretta carettademographic loop analysis vs. summed elasticities; Dipsacus sylvestriselasticity analysis; graph theory; life-cycle graphs; life history; loop analysis; population growth rate; projection matrices.   Q d A demographic analysis of the life-cycle graph can be used to quantify the separate contributions of different life-history types to the population growth rate. Loop analysis has been proposed (van Groenendael et al. 1994 ) as the appropriate method for partitioning the elasticity matrix to determine these contributions. However, in the analysis of complex demographic models it is difficult to derive the loops by simple inspection of the life-cycle graph. I show how graph theory can be used to describe a general and systematic procedure for deriving the loops from the structure of the life-cycle graph. I demonstrate that the concept of nullity (from graph theory) can be applied in this context to correctly determine the number of loops for any graph. Using examples from Campanula americana, Dipsacus sylvestris, and Caretta caretta, I illustrate the relationship of the loops to biologically relevant life-history contrasts. This relationship is crucial for the application of loop analysis to life-history evolution for the purpose of partitioning the separate effects on the population growth rate among different life-history components.t     6 ; K Jackson, L. E. Roy, J. 1986Growth Patterns of Mediterranean Annual and Perennial Grasses Under Simulated Rainfall Regimes of Southern France and California Usa*#Acta Oecologica Oecologia Plantarum7-2-191-212a  Phenology, growth and biomass allocation were studied in three annual grasses (with French and Californian populations of each species) and four perennial grasses (two French and two Californian) to suggest reasons for the different roles of annual and perennial grasses under the mediterranean-type climates of France and California. We tested the hypothesis that the earlier and longer summer drought in California may be a factor in preventing the successional replacement of the non-annuals by native perennials. The annuals (Avena barbata, Bromus mollis and B. madritensis) had similar phenologies characterized by the completion of their life cycle before the date of the onset of summer drought typical for southern France. The reproductive phenology of the perennials (Dactylis glomerata, Bromus erectus, Stipa pulchra and Poa scabrella) was delayed one month compared to the annuals. Due to higher allocation to leaves and lower sensitivity to winter temperatures, annuals had higher growth rates; they were an order of magnitude larger through the winter and by June annual plants were twice as large as the perennials. In autumn and winter, annuals allocated approximately 50% of their biomass to leaves and 30% to roots, while the perennials allocated 30 and 50% to leaves and roots, respectively. The biomass allocated to sexual reproduction was approximately 60% in the annuals and 40% in the perennials. The switch from vegetative to reproductive growth was rapid and sudden in the annuals and more graded in the perennials. The annuals had similar growth and reproductive output when grown under the simulated summer drought regimes of southern France and California, but perennials had fewer leaves and tillers under the earlier drought (Californian) regime. S. pulchra had less reproductive output, and all perennials (except P. scabrella which did not break its summer dormancy) had reduced autumn regrowth when the length of the summer drought increased. Californian populations of the annuals had earlier spring reproductive phenologies, but did not grow larger or have different reproductive outputs that the French populations when subjected to the Californian rainfall regime. Since production and yield in the perennials were increased by shorter summer drought, different rainfall patterns in southern France and California may partially explain the contrasting roles of perennial grasses in the two regions. Annual grasses appear to be phenologically adapted to set seed before summer drought begins, pre-adapting them to the earlier drought in California.o(z (<6Lipson, David A. Schmidt, Steven K. Monson, Russell K. 1999b\Links between microbial population dynamics and nitrogen availability in an alpine ecosystemEcology805y 1623-16313 P IPast studies of plant-microbe interactions in the alpine nitrogen cycle have revealed a seasonal separation of N use, with plants absorbing N primarily during the summer months and microbes immobilizing N primarily during the autumn months. On the basis of these studies, it has been concluded that competition for N between plants and microbes is minimized along this seasonal gradient. In this study, we examined more deeply the links between microbial population dynamics and plant N availability in an alpine dry meadow. We conducted a year-round field study and performed experiments on isolated soil microorganisms. Based on previous work in this ecosystem, we hypothesized that microbial biomass would decline before the plant growing season and would release N that would become available to plants. Microbial biomass was highest when soils were cold, in autumn, winter, and early spring. During this time, N was immobilized in microbial biomass. After snow melt in spring, microbial biomassdecreased. A peak in the soil protein concentration was seen at this time, followed by peaks in soil amino acid and ammonium concentrations in late June. Soil protease rates were initially high after snow melt, decreased to below detection limits by midsummer, and partially recovered by late summer. Proteolytic activity in soil was saturated early in the growing season and became protein limited later in the summer. We concluded that the key event controlling N availability to alpine plants occurs after snow melt, when protein is released from the winter microbial biomass. This protein pulse provides substrate for soil proteases, which supply plants with amino acids during the growing season. On average, microbial biomass was lower in the summer than at other times, although the biomass fluctuated widely during the summer. Within the summer months, maximum numbers of amino-acid-degrading microorganisms and the maximum amount of microbial biomass coincided with the peak in soil amino acids, when plants are most active. All bacterial strains isolated from this summer community had the ability to grow rapidly on low concentrations of amino acids and to degrade protein. This explains the previously observed result that the soil microbial biomass can compete strongly with plants for organic N, despite the seasonal offset of maximum plant and microbial N uptake.l243-249$://000165620100010n@:Liu, X. Y. Lindemann, W. C. Whitford, W. G. Steiner, R. L.ZTMicrobial diversity and activity of disturbed soil in the northern Chihuahuan Desert$Biology and Fertility of Soilsmicroorganisms; diversity; activity; grazing; stress simulated rainfall; litter quantities; communities; microarthropods; respiration; patterns; dynamics; biomass; carbon; rhizospheresVOThe effects of intense grazing, seasonal drought, and fire on soil microbial diversity (substrate utilization) and activity in a northern Chihuahuan Desert grassland were measured in summer 1997, winter 1998, and spring 1998. Intense livestock grazing was initiated in winter 1995, burning occurred in August 1994, and drought stresses were imposed from October 1994 to June 1997. Microbial diversity was inferred from the cal-bon substrate utilization patterns in both gram (+) and gram (-) Biolog plates. Microbial activity was estimated by the activity of selected enzymes. Neither microbial diversity nor activity was affected by grazing. The interaction of intense grazing and stress sub-treatments only occurred in spring for one set of diversity measurements. The maximum microbial diversity and activity occurred in the winter-drought-stress sub-plots in summer and spring. Burning reduced microbial diversity and most enzyme activities as compared to the control in summer and spring. Microbial diversity was also lower in summer-drought-stress sub-plots than in thr: control in summer and spring. Microbial diversity was highest in summer, intermediate in winter, and lowest in spring. Microbial activity was generally higher in summer and lower in winter. it was concluded that substrate availability was the most important factor affecting the diversity and activity of soil microorganisms within a season. Soil moisture was not the factor causing differences in microbial diversity and activity among the stress treatments, but it was a predictor for some microbial responses under a particular stress.Biol. Fertil. Soils 2000 Nov323379BB BIOL FERT SOILSISI:0001656201000104.Ludwig, J. A. Whitford, W. G. Cornelius, J. M. 1989~xEffects of Water Nitrogen and Sulfur Amendments On Cover Density and Size of Chihuahuan Desert New Mexico Usa Ephemerals"Journal of Arid Environments161s 35-42,%The generality of the water-limited-nitrogen-regulated plant growth hypothesis for desert ecosystems was tested by applying water, nitrogen and sulfur amendments in a complete factorial design to three Chihuahuan Desert ecosystems. Water was added every 2 weeks at a rate to about double the long-term annual precipitation average of 225 mm and nitrogen and sulfur were added once in May 1983 (10 g/m2). Living-canopy area of all herbaceous plants was measured monthly (from April 1983 to December 1984). Since plants can respond to water and nutrients either by establishing more, but smaller, plants or by growing fewer, but larger, plants, density and plant size were measured (once, in mid-April 1984). Only C3 winter-ephemerals significantly changed in cover, density or plant size in response to the amendments; no significant responses were observed in C4 summer-ephermerals or perennial grasses and forbs. Species that significantly increased in density did not significantly increase in size and vice versa. Water tended to limit the number (richness) and density of species, whereas nitrogen regulated plant growth (size). Sulfur generally reduced soil pH and species density. This study demonstrated that these general plant responses to increased water and nutrients were highly species and site specific.p \ZTSchuster, William S. F. Sandquist, Darren R. Phillips, Susan L. Ehleringer, James R. 1994hbHigh levels of genetic variation in populations of four dominant aridland plant species in Arizona"Journal of Arid Environments272 159-167dNGIntrapopulation genetic variation in four dominant, perennial plant species from the deserts of southwestern North America was assessed and compared using isozyme techniques. In each of two localities, a warm desert and a cold desert environment, one population of the most common long-lived perennial was compared with a nearby population of a dominant, comparatively short-lived perennial. The warm desert species were Encelia farinosa and Larrea tridentata while Gutierrezia microcephala and Coleogyne ramosissima were examined at the cold desert site. All samples were electrophoresed and stained for 18 enzyme systems. Mean values for these four species were 0.173 for gene diversity, 66.9% for polymorphism, and 2.02 for number of alleles per locus. These values are significantly greater than published means from a recent survey of the plant isozyme literature, and are comparable to or higher than levels of genetic variation in other widespread plant species. Differences in level of variation among the species were not significant, thus failing to indicate any relationship between life expectancy and genetic variation. These results demonstrate that aridland plant populations can harbor high levels of genetic variation, and suggest that environmental heterogeneity may be important in the development and maintenance of this diversity.t.(Schwinning, Susanne Ehleringer, James R. 2001RLWater use trade-offs and optimal adaptations to pulse-driven arid ecosystemsJournal of Ecology893t464-480 $1 We introduce a hydraulic soil-plant model with water uptake from two soil layers; one a pulse-dominated shallow soil layer, the other a deeper soil layer with continuous, but generally less than saturated soil moisture. Water uptake is linked to photosynthetic carbon assimilation through a photosynthesis model for C3 plants. 2 A genetic algorithm is used to identify character suites that maximize photosynthetic carbon gain for plants that experience a particular soil moisture pattern. The character suites include allocation fraction to stem, leaves and shallow root, stem capacitance and stem water storage capacity, maximal leaf conductance and sensitivity of leaf conductance to plant water potential, and a critical soil water potential at which shallow roots cease to transfer water. 3 We find that if pulse water is a more important water source than deeper soil water in the environment, optimal phenotypes lean towards adaptations that maximize pulse water use (small root:shoot ratio, predominantly shallow root system, high leaf conductance with high stomatal sensitivity to plant water status). If deeper soil water is more important, phenotypes lean towards adaptations that maximize deeper soil water use (large root:shoot ratio, predominantly deep root system, lower leaf conductance with low stomatal sensitivity). Stem succulence is adaptive only when deeper soil water is unavailable. 4 From among the continuum of derived phenotypes, four phenotypes are selected that resemble the character suites of winter annuals, drought-deciduous perennials, evergreen perennials and stem succulents. Under common conditions, these phenotypes reproduce many of the responses to drought and water pulse observed in their respective life-form counterparts. The comparison also highlights the differences in plant life-form sensitivity to summer and winter drought conditions. 5 Based on these results, we discuss the possible role of annual precipitation patterns in shaping plant adaptations and determining the plant composition of arid and semi-arid environments.i>8Schwinning, S. Davis, K. Richardson, L. Ehleringer, J.R. 2002xrDeuterium enriched irrigation indicates different forms of rain use in shrub/grass species of the Colorado Plateau Oecologia 130345-355 We contrasted the seasonal use of simulated large rain events (24 mm) by three native species of the arid Colorado Plateau: the perennial grass Hilaria jamesii and two shrubs Artemesia filifolia and Coleogyne ramosissima. Deuterium-enriched water was used to distinguish shallow "pulse" water from water in deeper soil layers that were unaffected by the water input. We also measured the leaf gas exchange rates of watered and unwatered control plants for 5 days after the rain event. H. jamesii had twice the pulse water proportion in its xylem than the two shrubs in spring (approx. 70% vs 35%). In summer, the pulse water proportions of all species were around 70%. The increase in the relative pulse water uptake of the two shrubs was caused primarily by a reduction in the rate of water uptake from deeper sources, consistent with the decrease in the availability of stored winter water. Rain increased the rates of gas exchange in C. ramosissima in both seasons, in H. jamesii only in summer and had no significant effect on A. filifolia. In H. jamesii, summer rain also increased water use efficiency. This suggests three principle mechanisms for rainwater use: (1) immediate increase in gas exchange via stomatal opening (C. ramisissima), (2) immediate increase in water use efficiency through restoration of the photosynthetic apparatus (H. jamesii) and (3) conservation of deeper soil water, potentially extending photosynthetic activity into later drought periods (A. filifolia). On a ground-area basis, A. filifolia was by far the largest consumer of spring and summer rain, due to its greater ground cover, while rain use by H. jamesii was negligible. We hypothesize that a population's fraction of the total community Leaf Area Index, more than species identity, determines which species takes up most of the spring and summer precipitation and we discuss this idea in the context of Walter and Stadelmann's (1974, In: Brown JW Jr (ed) Desert biology. Academic Press, New York, pp 213-310) water partitioning hypothesis.                "   C M     f p v 0<5Lane, Diana R. Coffin, Debra P. Lauenroth, William K. 2000LEChanges in grassland canopy structure across a precipitation gradient$Journal of Vegetation Science113359-368& In temperate grasslands, the relative importance of above-ground competition for light compared to below-ground competition for water and nutrients is hypothesized to increase with increasing precipitation. Thus, competition for light is likely to exert an increasing influence on canopy structure and species composition as precipitation increases. We quantified canopy structure, light availability and changes in species composition at seven sites across the central grassland region of the United States to determine how these properties change across a precipitation gradient. Across the region, there was a disproportionate increase in leaf area and canopy height with increasing precipitation, indicating that plants become taller and leafier across the gradient. Leaf area index increased by a factor of 12 across the gradient while above-ground net primary productivity increased by a factor of only 5.5. As precipitation increased, there was decreased light availability at the soil surface, increased seasonal variability in light transmission, increased biomass and leaf area at higher canopy layers and an increased proportion of tallstatured species. These observed changes in canopy structure support the prediction that competition for light increases in importance with increasing precipitation.F@Lange, Otto L. Belnap, Jayne Reichenberger, Hans Meyer, Angelika 1997Photosynthesis of green algal soil crust lichens farm arid lands in southern Utah, USA: Role of water content on light and temperature responses of CO-2 exchangee Flora Jena 192r1/ 1-15 Biotic soil crusts are a worldwide phenomenon in arid and semi-arid landscapes. Metabolic activity of the poikilohydric organisms found in these crusts is dominated by quick and drastic changes in moisture availability and long periods of drought. Under controlled conditions, we studied the role of water content on photosynthetic and respiratory CO-2 exchange of three green algal soil crust lichens from a desert site in southern Utah (USA): Diploschistes diacapsis (ACH.) LUMBSCH, Psora cerebriformis W. WEBER, and Squamarina lentigera (WEBER) POELT. Photosynthetic metabolism is activated by extremely small amounts of moisture; lower compensation values for net photosynthesis (NP) are reached between 0.05 and 0.27 mm of precipitation equivalent. Thus, the lichens can use very low degrees of hydration for carbon gain. Maximal NP occurs between 0.39 and 0.94 mm precipitation equivalent, and area-related rates equal 2.6-5.2 mu-nol CO-2 m-2s-1. All three tested species show 'sun plant' features, including high light requirements for CO-2 exchange compensation and for NP saturation. Diploschistes diacapsis maintains high rates of NP at full water saturation. In contrast, suprasaturated thalli of the other two species show a strong depression in NP which can be removed or reduced by increased external CO-2 concentration. Consequently, this depression is most probably caused by increased thallus diffusive resistances due to pathway blockage by water. This depression will greatly limit carbon gain of these species in the field after heavy rain. It occurs at all temperatures of ecological relevance and also under conditions of low light. However, maximum water holding capacity of P. cerebriformis and S. lentigera is higher than that of D. diacapsis. This could mean that periods of hydration favorable for metabolic activity for those two species last longer than those of D. diacapsis. This might compensate for their lower rates of NP during suprasaturation. Thus, two different strategies might have developed for lichen existence in the specific and extreme and soil crust habitat. Data about habitat conditions for the different lichen species are needed in order to test this hypothesis and to allow interpretation and prediction of performance of these soil crust lichens in nature. Lange, O.L. 2001NHPhotosynthesis of soil crust biota as dependent on environmental factorsB;Biological soil crusts: Structure, function, and managementw "Heidelberg,Berlin, New YorkY Springer Verlag 217-240.B://000081369600004i Lhomme, J. P.tnhFormulation of root water uptake in a multi-layer soil-plant model: does van den Honert's equation hold?*#Hydrology and Earth System SciencesHPIresistance; field; transport; flow; transpiration; conductivity; moisturetThe withdrawal of water from soil by vegetation, which in steady state conditions is equivalent to the transpiration rate, can be written in terms of R-ater potential in the form of an Ohm's law analogy, known as van den Honert's equation. The difference between an effective soil water potential and the bulk canopy water potential is divided by an effective soil-plant resistance. This equation is commonly used, but little is known about the precise definition of its parameters. The issue of this paper is to bridge the gap between the bulk approach and a multi-layer description of soil-plant water transfer by interpreting the bulk parameters in terms of the characteristics of the multi-layer approach. Water flow through an elementary path within the soil or the root is assumed to follow an Ohm's law analogy, and the soil and root characterisies are allowed to vary with depth. Starting from the basic equations of the multi-layer approach, it is proved that the total rate of transpiration can also be expressed in the form of an Ohm's law analogy. This means that van den Honert's equation holds at canopy scale, insofar as the assumptions made on the physics of root water uptake hold. In the bulk formulation derived, the effective soil-plane resistance appears as a combination of the elementary resistances making up the multi-layer model; and the effective soil water potential is a weighted mean of the water potentials in each soil layer, the weighting system involving the complete set of elementary resistances. Simpler representations of soil- plant interaction leading to Ohm's law type formulations are also examined: a simplified multi-layer model, in which xylem (root axial) resistance is neglected, and a bulk approach, in which soil-root interaction is represented by only one layer. Numerical simulations performed in different standard conditions show that these simpler representations do not provide accurate estimates of the transpiration rate, when compared to the values obtained by the complete algorithm.Hydrol. Earth Syst. Sci. 1998 Marc2a1 "215EU HYDROL EARTH SYST SCIEISI:000081369600004u("Li, Tao Xiao, Honglang Li, Xinrong 2001`YModeling the effects of crust on rain infiltration in vegetated sand dunes in arid desertu(!Arid Land Research and Management151s 41-48Shapotou Desert Research Station is located at the southeast edge of Tengger Desert in China. Shapotou sand dunes have been vegetated to stabilize them. The vegetated dunes have been completely covered by a natural crust. Rain infiltration was reduced by 36% to 74% on the crusted site while there was no reduction of rain infiltration on the sandy site. The infiltration rate was positively correlated with total rainfall and negatively correlated with rain intensity. Runoff on the crusted site provided water for vegetation growth in the hollows between sand dunes. The crust significantly impeded increased soil moisture. To increase soil moisture in the surface 40 cm layer, rainfall had to be above 8.5 mm on the slopes of sand dunes on the crust site, and increased with rain intensity. In the Shapotou area, the amount of rainfall that vegetation on slopes of sand dunes could utilize, was reduced to about 40 mm per year by the crust, although the mean annual precipitation was approximately 180 mm. It is expected that vegetation cover will continuously degrade in the future. To maintain vegetative cover adequate to stabilize the active sand dune, it was necessary to break part of the crust at slope of sand dune.neral soil and corresponding site factors (mean annual rainfall and temperature, latitude, altitude and soil pH). The delta15N of whole soil was related to all of the site variables (including foliar delta15N) except altitude and, when regressed on latitude and rainfall, provided the best model of these data, accounting for 49% of the variation in whole soil delta15N. As single linear regressions, site-averaged foliar delta15N was more strongly related to rainfall than was whole soil delta15N. A smaller data set showed similar, negative correlations between whole soil delta15N, site-averaged foliar delta15N and soil moisture variations during a single growing season. The negative correlation between water availability (measured here by rainfall and temperature) and soil or plant delta15N fails at the landscape scale, where wet spots are delta15N-enriched relative to their drier surroundings. Here we present global and seasonal data, postulate a proximate mechanism for the overall relationship between water availability and ecosystem delta15N and, newly, a mechanism accounting for the highly delta15N-depleted values found in the foliage and soils of many wet/cold ecosystems. These hypotheses are complemented by documentation of the present gaps in knowledge, suggesting lines of research which will provide new insights into terrestrial N-cycling. Our conclusions are consistent with those of Austin and Vitousek (1998) that foliar (and soil) delta15N appear to be related to the residence time of whole ecosystem N.yL8 @o \kngPregitzer, Kurt S. Zak, Donald R. Curtis, Peter S. Kubiske, Mark E. Teeri, James A. Vogel, Christoph S. 1995@:Atmospheric CO-2, soil nitrogen and turnover of fine rootsNew Phytologist 1294579-585In most natural ecosystems a significant portion of carbon fixed through photosynthesis is allocated to the production and maintenance of fine roots, the ephemeral portion of the root system that absorbs growth-limiting moisture and nutrients. In turn, senescence of fine roots can be the greatest source of C input to forest soils. Consequently, important questions in ecology entail the extent to which increasing atmospheric CO-2 may alter the allocation of carbon to, and demography of, fine roots. Using microvideo and image analysis technology, we demonstrate that elevated atmospheric CO-2 increases the rates of both fine root production and mortality. Rates of root mortality also increased substantially as soil nitrogen availability increased, regardless of CO-2 concentration. Nitrogen greatly influenced the proportional allocation of carbon to leaves vs. fine roots. The amount of available nitrogen in the soil appears to be the most important factor regulating fine root demography in Populus trees.od^Pregitzer, Kurt S. Laskowski, Michele J. Burton, Andrew J. Lessard, Veronica C. Zak, Donald R. 1998RKVariation in sugar maple root respiration with root diameter and soil depthTree Physiology1810665-670nf`Root respiration may account for as much as 60% of total soil respiration. Therefore, factors that regulate the metabolic activity of roots and associated microbes are an important component of terrestrial carbon budgets. Root systems are often sampled by diameter and depth classes to enable researchers to process samples in a systematic and timely fashion. We recently discovered that small, lateral roots at the distal end of the root system have much greater tissue N concentrations than larger roots, and this led to the hypothesis that the smallest roots have significantly higher rates of respiration than larger roots. This study was designed to determine roots in the soil profile. We examined relationships among root respiration rates and N concentration in four diameter classes from three soil depths in two sugar maple (Acer saccharum Marsh.) forests in Michigan. Root respiration declined as root diameter increased and was lower at deeper soil depths than at the soil surface. Surface roots (0-10 cm depth) respired at rates up to 40% greater than deeper roots, and respiration rates for roots < 0.5 mm in diameter were 2.4 to 3.4 times higher than those for roots in larger diameter classes. Root N concentration explained 70% of the observed variation in respiration across sites and size and depth classes. Differences in respiration among root diameter classes and soil depths appeared to be consistent with hypothesized effects of variation in root function on metabolic activity. Among roots, very fine roots in zones of high nutrient availability had the highest respiration rates. Large roots and roots from depths of low nutrient availability had low respiration rates consistent with structural and transport functions rather than with active nutrient uptake and assimilation. These results suggest that broadly defined root classes, e.g., fine roots are equivalent to all roots < 2.0 mm in diameter, do not accurately reflect the functional categories typically is associated with fine roots. Tissue N concentration or N content (mass X concentration N) may be a better indicator of root function than root diameter.&Price, Mary V. Joyner, Jamie W.e 1997VOWhat resources are available to desert granivores: Seed rain or soil seed bank?Ecology783764-773Patterns of resource availability mold many ecological processes, but we know little about the availability of resources to consumers in nature, even for well-studied systems such as the granivorous animals of North American deserts. What we do know about seed resources in deserts is based primarily on seeds extracted from soil samples, but this might present a distorted view of resource availability if animals mostly harvest newly produced seeds before they enter the soil seed bank. In order to assess how large the distortion might be, we simultaneously monitored the seed bank and "seed rain" over a 19-mo period in the eastern Mojave Desert of California. The seed bank averaged apprxeq 106 000 seeds/m-2 and 38 g/m-2, much higher than values reported for other North American desert sites. This corresponds roughly to the seed production of a single year, since daily seed rain averaged 262 seeds/ml and 0.26 g/m-2. However, input from the seed rain did not accumulate in the soil. Instead, the seed bank decreased by a daily average of 114 seeds/m-2 and 0.007 g/m-2 during our study. This suggests that virtually all seeds germinate, die, or are harvested by granivores soon after being dispersed. Large seeds comprised a greater fraction of the seed rain than of the seed bank, suggesting that such seeds are differentially depleted, probably by granivores, before they enter the soil. Because seed drop was seasonal, temporal variation comprised a significant component of among-sample variance in the seed rain. Temporal variance in the seed bank was much smaller, presumably because granivores harvested most of the seed rain. Conversely, spatial variance was a significant component for the seed bank, but not the seed rain, perhaps as a result of spatial patterns of seed harvest or seed caching by granivores. By virtue of these variance patterns, as well as other attributes, seeds in the soil present different challenges to granivores than do newly produced seeds. Our understanding of desert granivore foraging and community ecology, and of granivore-seed interactions, depends critically on choosing the appropriate measure of seed availability to granivores.6/Prince, S. D. De Colstoun, E. B. Kravitz, L. L. 1998^XEvidence from rain-use efficiencies does not indicate extensive Sahelian desertificationGlobal Change Biology44359-374^WDesertification is regarded as one of the major global environmental problems of the 20th century and the African Sahel is often quoted as the most seriously affected region. Previous attempts to map the occurrence and severity of desertification in the Sahel have been unsatisfactory, mainly because of the lack of any readily measured, objective indicators. We explore here the properties of the ratio of net primary production (NPP) to precipitation - the rain-use efficiency (RUE) - calculated from remotely sensed vegetation indices and rain gauge data. Negative deviations from the normal range of RUE values are shown to be an indicator of desertification. Observations of NPP of the entire Sahel were possible using satellite platforms for the period 1982-90, including the 1984 drought. The results suggest that NPP was remarkably resilient, a fact that was reflected in only little variation in the RUE during the period of study. Thus, in much of the region, NPP seems to be in step with rainfall, recovering rapidly following drought and not supporting the fears of widespread, subcontinental scale desertification taking place in the 9-year period that is studied. In fact the results show a small but systematic increase in RUE for the Sahel as a whole from 1982 to 1990, although some areas contained within the region did have persistently low values.tt that was reflected in only little variation in the RUE during the period of study. Thus, in much of the region, NPP seems to be in step with rainfall, recovering rapidly following drought and not supporting the fears of widespread, subcontinental scale desertification taking place in the 9-year period that is studied. In fact the results show a small but systematic increase in RUE for the Sahel as a whole from 1982 to 1990, although some areas contained within the region did have persistently low values.tTned the effects of vertebrate granivores and folivores on winter and summer plant communities in the Chihuahuan Desert by selectively excluding different vertebratef`Resource competition in a variable environment: Phytoplankton growing according to Monod's model Grover, J. P.lAmerican Naturalist 13668771-789o 1990Theoretical models of interspecific competition often assume equilibrium in population and resource dynamics, an assumption that is often criticized. Departures from equilibrium are hypothesized to reduce interspecific competition and facilitate coexistence. When competition between two algae for one resource is modeled using Monod equations, the species that wins at equilibrium does not usually win when resources are supplied as periodic pulse, if the second competitor has the higher maximal growth rate. For model parameters typical of nitrogen and phosphorus limitation, this selection for species with high maximal growth rates is strong. A review of laboratory studies of algal competition is largely inconclusive concerning this prediction. Theory suggests, however, two reasons to doubt that selection for species with high maximal growth rates occurs by this mechanism in nature. First, when variability in resource supply occurs as sinusoidal fluctuations, rather than as pulses, selection for species with high maximal growth rates is weak. Second, when intracellular storage of resources occurs, selection for species with high maximal growth rates is also weak. Under either of these conditions, it is common for algae that are superior competitors at equilibrium to be superior competitors under nonequilibrium conditions also. Under all conditions tested, coexistence of two competitors on one resource is less common than competitive exclusion. Lack of equilibrium may, however, be a likely explanation for species diversity in the more complicated ecosystems with more than one resource, or more than one trophic level, that are nearly universal in nature.eC& Groeneveld, D. P. Crowley, D. E. 1988D>Root System Response to Flooding in Three Desert Shrub SpeciesFunctional Ecology24491-498\UThe responses of root systems to flooding were examined in Chrysothamnus nauseosus ssp. viridulus (Greene) Hall & Clem., Atriplex torreyi (Wats.) Wats. and Sarcobatus vermiculatus (Hook) Torr. These species inhabit shallow ground water habits on the valley floors of the arid western Great Basin of the United States. S. vermiculatus failed to survive 6 months of continuous flooding. A. torreyi and C. n. viridulus survived flooding with restructured root systems that differed markedly from those of adjacent non-flooded plants with respeft to depth, lateral branching, degree of suberization and presence of lysigenous aerenchyma within the root cortices. Such morphological and anatomical changes occur in the root systems of flood-tolerant wetland plants and are remarkable in these two species which both have xeric canopy morphology and relatively strong drought tolerance. Primary roots of each of the three species from a non-flooded location and from containerized specimens grown under relatively poor aeration were examined microscopically for comparison with flooded roots. Aerenchyma developed in roots fo all three species grown under container aeration but was absent in specimens from the non-flooded site. Therefore, failure of S. vermiculatus to survive flooding may not be due to the lack of aerenchyma development but simply to lower tolerance.lZVCui, Muyi Nobel Park, S. 1992pjNutrient status, water uptake and gas exchange for three desert succulents infected with mycorrhizal fungiNew Phytologist 1224643-649&Mycorrhizal infection and its consequences were assessed for Agave deserti Engelm., Ferocactus acanthodes (Lem.) Britton & Rose, and Opuntia ficus-indica (L.) Miller. For A. deserti and F. acanthodes in the field, mycorrhizal infection varied from 2 to 11%; the higher infection in May than in March 1991 was probably associated with improved soil water conditions. Using field-collected fungi, mycorrhizal infection for glasshouse-grown A. deserti, F. acanthodes and O. ficus-indica varied from 8 to 64%, with the highest infection occurring for lateral roots of A. deserti. Five months after inoculation, root P was significantly higher (P lt 0.01) for inoculated A. deserti and shoot P and Zn were significantly higher (P lt 0.02) for inoculated A. deserti and F. acanthodes compared with uninoculated plants. Root hydraulic conductivity (L-p) of lateral roots of A. deserti was 24% higher for inoculated than for uninoculated plants (P lt 0.1). The increase in L-p was caused by a higher radial conductivity, which could be due to increased root cell membrane permeability and more hyphal entry points. Dairy net CO-2 uptake was 19% higher for inoculated than uninoculated A. deserti. Mycorrhizal infection apparently enhances water and nutrient uptake in dry environments for these three desert succulents.\UWater budgets and root hydraulic conductivity of opuntias shifted to low temperatureslCui, Muyi Nobel Park, S. 1994.'International Journal of Plant Sciencesl 155e2s167-172  Using Smart Source ParsingCladodes (stem segments) of platyopuntias generally lose water as ambient temperatures decrease, even when the plants are in wet soil. Two weeks after shifting plants from day/night air temperatures of 30 degree C/20 degree C to 10 degree C/1 degree C, cladode thickness decreased 15% for Opuntia ficus-indica and 25% for Opuntia polyacantha. During this period, daily transpirational water loss gradually decreased, but root water uptake from the soil immediately decreased about 90%. As the temperature was decreased from 30 degree C to 0 degree C, the root hydraulic conductivity (L-p) for excised root segments also decreased about 90%, with larger fractional decreases occurring at lower temperatures. The large decrease in L-p at the lower temperatures was chiefly responsible for the accompanying cladode desiccation. Such net loss of cladode water is apparently essential for certain cacti to survive seasonally cold periods in the field.$Cui, Muyi Caldwell Martyn, M.  1996Facilitation of plant phosphate acquisition by arbuscular mycorrhizas from enriched soil patches. II. Hyphae exploiting root-free soilNew PhytologistY 13383 461-467E,%Effects of arbuscular mycorrhizas (AM) on plant exploitation of soil nutrient heterogeneity were studied with nonmycorrhizal and mycorrhizal Agropyron desertorum (Fisch. ex Link) Schult. in two-compartment containers. A central cylindrical plant compartment was separated from an outer hyphal compartment by two layers of stainless steel screen with a 2 mm air gap between the screen layers. Patchy or uniform nitrate (NO-3-) and phosphate (P) distribution patterns were created in the outer compartment. Only AM hyphae could cross the double-screen barrier to access those nutrients. Mycorrhizal plants acquired significantly more labelled P in both the patchy- and the uniform-nutrient treatments than did non-mycorrhizal plants. Mycelia in root-free soil delivered similar amounts of P from the more distant rich patches to mycorrhizal plants as from the uniform and more proximate labelling. The uptake of a more mobile and abundant element, nitrate, was not affected significantly by either mycorrhizal infection or by nutrient distribution patterns in the root-free soil. Despite a lower root: shoot mass ratio, mycorrhizal plants had significantly greater shoot phosphorus concentration than did nonmycorrhizal plants. There was no significant difference in P uptake or phosphorus concentration between the two nutrient distribution patterns for mycorrhizal plants, indicating that AM hyphae can explore the root-free soil for available P and transport it to host plants equally well when P was distributed in either patchy or uniform patterns in the root-free soil. $Cui, Muyi Caldwell, Martyn M. 1997rlA large ephemeral release of nitrogen upon wetting of dry soil and corresponding root responses in the fieldPlant and Soil 191.2a291-299eTo assess changes in soil nutrients, root growth and mycrorrhizal infection in response to rain events, a water pulse was applied to a very dry soil. Wetting of a dry soil in the Great Basin of the Western United States led to a striking pulse of available soil nitrate in a field plot, but available phosphate. was not affected. This is the first field demonstration of this phenomenon in the Great Basin as far as we are aware. This pulse was only apparent for a few days, probably due to microbial immobilization of the nitrogen. Root ammonium uptake capacity increased within one day of the water pulse, but new root growth was not apparent until 3 days after the water pulse. Thus, to capture this ephemeral release of nitrouen, enhanced uptake capacity of existing roots was probably more important than development of new roots. Mycorrhizal infection was not affected by the water pulse treatments. However, since the water pulse only affected nitrogen availability and mycorrhizae are generally most effective in facilitating acquisition of less mobile nutrients such as phosphate, mycorrhizae likely do not play an important role in taking advantage of this opportunity provided by the pulse of water.c  <5Dettinger, M. D. Cayan, D. R. Diaz, H. F. Meko, D. M. 1998f`North-South Precipitation Patterns in Western North America on Interannual-to-Decadal TimescalesJournal of Climate1112 3095-3111r 4 .The overall amount of percipitation deposited along the West Coast and western cordillera of North America from 25 degree to 55 degree N varies from year to year, and superimposed on this domain-average variability are varying north-south contrasts on timescales from at least interannual to interdecadal. In order to better understand the north-south precipitation contrasts, their interannual and decadal variations are studied in terms of how much they affect overall precipitation amounts and how they are related to large-scale climatic patterns. Spatial empirical orthogonal functions (EOFs) and spatial moments (domain average, central latitude, and latitudinal spread) of zonally averaged percipitation anomalies along the westernmost parts of North America are analyzed, and each is correlated with global sea level pressure (SLP) and sea surface temperature series, on interannual (defined here as 3-7 yr) and decadal (>7 yr) timescales. The interannual band considered here corresponds to timescales that are particularly strong in tropical climate variations and thus is expected to contain much precipitation variability that is related to El Nino-Southern Oscillation; the decadal scale is defined so as to capture the whole range of long-term climatic variations affecting western North America. Zonal EOFs of the interannual and decadal filtered versions of the zonal-precipitation series are remarkably similar. At both timescales, two leading EOFs describe 1) a north-south seesaw of precipitation pivoting near 40 degree N and 2) variations in precipitation near 40 degree N, respectively. The amount of overall precipitation variability is only about 10% of the mean and is largely determined by precipitation variations around 40 degree -45 degree N and most consistently influenced by nearby circulation patterns; in this sense, domain-average precipitation is closely related to the second EOF. The central latitude and latitudinal spread of precipitation distributions are strongly influenced by precipitation variations in the southern parts of western North America and are closely related to the first EOF. Central latitude of precipitation moves south (north) with tropical warming (cooling) in association with midlatitude western Pacific SLP variations, on both interannual and decadal timescales. Regional patterns and zonal averages of precipitation-sensitive tree-ring series are used to corroborate these patterns and to extend them into the past and appear to share much long- and short-term information with the instrumentally based zonal precipitation EOFs and moments.LEDevitt, D. A. Piorkowski, J. M. Smith, S. D. Cleverly, J. R. Sala, A. 1997xrPlant water relations of Tamarix ramosissima in response to the imposition and alleviation of soil moisture stress"Journal of Arid Environments363h527-540a The effect of an extended drydown on the water relations of Tamarix ramosissima (saltcedar), a desert phreatophyte, was investigated along the floodplain of the Virgin River (southern Nevada) during a hot dry summer period. Seedlings of Tamarix were grown in lysimeters (120 cm depth, 51 cm radius) positioned along a gradient from the desert's edge to the interior of the stand and monitored for growth and evapotranspiration over a 2-year period prior to this study. Water tables were maintained at approximately 60 cm during the evapo-transpiration study by applying weekly irrigation directly to the water table via piezometers. At the end of the 2-year period, three lysimeters were selected for a drydown experiment (desert's edge, river's edge, open stand). On 7 July 1995 all irrigation was terminated for a 29-day period. After the drydown period was over, irrigation was applied for an 18-day period to re-establish water table depths. Sapflow (transpiration) was monitored during the entire 47-day period (drydown, wetup) with stem flow gauges, soil moisture with time domain reflectometry, and plant water relations with a steady state porometer and pressure chamber. Results indicated that sapflow decreased significantly as water tables and stored soil water declined. Daily sapflow totals on a leaf area basis were higher for the plant growing along the river's edge, with midday hourly values significantly higher when a water table was present. On day 26, sapflow reached zero in the open stand lysimeter. As sapflow approached zero, a significant decrease in the time in which daily sapflow stopped occurred, rather than a delayed sapflow start time. Upon application of water on day 29, all three plants growing in the lysimeters responded by increasing sapflow within a 24-h period. Relative stomatal conductance (p = 0.001), leaf xylem water potential (p = 0.001) and sapflow (p = 0.001) measured during both the drydown and wetup stages were all linearly correlated with relative soil water in storage, indicating that relative soil water in storage could decline by as much as 90% and that upon alleviation of the soil moisture stress no hysteresis would be observed within these parameters. Such results indicate that Tamarix can be subjected to significant soil water deficits and still respond rapidly to the presence of water. We conclude that significantly longer drydown periods without plant contact with a water table would be required to cause catastrophic cavitation in Tamarix.  F("Weltzin, Jake F. McPherson, Guy R. 2000\UImplications of precipitation redistribution for shifts in temperate savanna ecotonesEcology817 1902-1913 In contrast to documented increases in woody plant dominance of former savannas and grasslands of North America, ecotones between oak (Quercus L.) woodlands and semi-desert grasslands of the southwestern United States and northwestern Mexico have been relatively stable over the past several centuries. Soil resource partitioning, wherein shallow-rooted grasses use summer precipitation and deep-rooted woody plants use winter precipitation, may have contributed to the stable coexistence of grasses and trees that form savannas at this ecotone. Thus, predicted changes in regional precipitation patterns and soil moisture caused by anthropogenic trace gas emissions have the potential to alter interactions between woody plants and grasses with potential ramifications for their relative abundance and distribution. We used a field experiment to investigate the response of the dominant woodland and savanna tree Quercus emoryi to simulated potential scenarios of precipitation redistribution within the context of shifts in the woodland-grassland ecotone. Experimental soil volumes isolated from ambient precipitation and soil moisture were hand-watered between July 1994 and October 1996. Control plots received mean annual precipitation (602 mm), whereas treated plots received all possible combinations of 50% additions and reductions to mean summer and winter precipitation (n = 4). Increases in summer precipitation increased seedling emergence and recruitment rates as much as threefold, whereas emergence and recruitment were independent of altered winter precipitation regimes. Seedling survival, size, growth, and biomass allocation were largely independent of shifts in seasonal precipitation regimes. We conclude that Q. emoryi recruitment and subsequent abundance and distribution would more likely be affected by changes in summer precipitation than changes in winter precipitation. For example, increases in summer precipitation that increase rates of seedling emergence and establishment may facilitate downslope shifts in the woodland-grassland ecotone. Similarly, this mechanism may explain downslope shifts in this ecotone that occurred coincident with particularly high summer precipitation during the "Medieval Warm" period, 645-1295 yr BP. Further, whereas soil moisture resource partitioning between adult Q. emoryi and coexisting grasses may contribute to savanna maintenance, soil resource partitioning does not occur within the first three growing seasons after Q. emoryi germination. Results illustrate the importance of consideration of the regeneration niche, which is often overlooked when predicting the response of woody plants to impending climate change. ngWest, Neil E. Stark, John M. Johnson, Dale W. Abrams, Mary M. Wight, J. Ross Heggem, Daniel Peck, Susan 1994lfEffects of climatic change on the edaphic features of arid and semiarid lands of western North America,%Arid Soil Research and Rehabilitationo8i4r307-351ungA group of specialists was asked by the Environmental Protection Agency to use their judgment as to which soil variables would be most impacted by five scenarios of climatic change in deserts of North America that could occur over the next 40 years. The following soil characteristics were evaluated in terms of their potential for change: physical, chemical, and biological crusts; the vesicular layer; soil organic matter; organic C and N content; the C/N ratio; carbonate pool; inorganic N, P, and S; salinity levels; micro-element content; microbial community composition; free-living microbial N fixation; denitrification; ammonia volatilization; salinization rates; water infiltration; evaporation; lateral flow and leaching; wind and water erosion; and litter decomposition. The Delphi approach was used to reach consensus on expected trends. Computer modeling was used to integrate and project interactive changes. We expect physical and chemical crusting, vesicularity, ammonium volatilization, soil erosion, and salt accumulation to increase and microphytic crusts to decrease under all scenarios of climatic change. Both soil organic C and N will decline, especially under increased temperatures, whereas the C/N ratio will decline to its lowest range of possible values. Both free and symbiotic N fixation should decline unless there is a shift to greater absolute precipitation during summers. Only slight changes in soil P, S, and trace element contents are expected under any of the five scenarios of climatic change. Production of litter will change relatively little, hut its chemical quality will decrease and nutrient cycling will he accelerated when the vegetation shifts from perennials to annuals. Use of the Century Model showed that soil organic matter is more sensitive to temperature changes than precipitation. Indicators of desert ecosystem "health" that we identified are relatively low albedo, patchiness of plant cover and interspace (trend depends on context), changes in drainage patterns and microrelief biological crusting, and ratios of microbial biomass C to total organic C. Need for further research is outlined.otly marked vegetation transects in Big Bend National Park, Texas, USA were monitored to follow temporal dynamics of desert grassland communities on a variety of landforms and soil types over a 26-yr period after the removal of domestic livestock. Historic record.'Williams, David G. Ehleringer, James R. 2000b[Intra- and interspecific variation for summer precipitation use in pinyon-juniper woodlandsEcological Monographs704517-537-In the arid southwest of North America, winter precipitation penetrates to deep soil layers, whereas summer "monsoon" precipitation generally wets only surface layers. Use of these spatially separated water sources was determined for three dominant tree species of the pinyon-juniper ecosystem at six sites along a gradient of increasing summer precipitation in Utah and Arizona. Mean summer precipitation ranged from 79 to 286 mm, or from 18% to 60% of the annual total across the gradient. We predicted that, along this summer rainfall gradient, populations of dominant tree species would exhibit a clinal off-on response for use of water from upper soil layers, responding at particular threshold levels of summer precipitation input. This prediction was largely supported by our observations of tree water source use over a two-year period and from irrigation experiments. Hydrogen and oxygen stable isotope ratios (deltaD and delta18O) of tree xylem water were compared to that of precipitation, groundwater, and deep and shallow soil water to distinguish among possible tree water sources. deltaD-delta18O relationships and seasonal xylem water potential changes revealed that trees of this ecosystem used a mixture of soil water and recent precipitation, but not groundwater. During the monsoon period, a large proportion of xylem water in Pinus edulis and Juniperus osteosperma was from monsoon precipitation, but use of this precipitation declined sharply with decreasing summer rain input at sites near the regional monsoon boundary in Utah. Quercus gambelii at most sites along the gradient used only deep soil water even following substantial inputs of summer rain. Populations of Quercus at sites with the highest average summer precipitation input, however, predominantly used water in upper soil layers from recent summer rain events. Soil temperature correlated with patterns of summer precipitation use across the gradient; high soil temperatures north of the monsoon boundary may have inhibited surface root activity for some or all of the three tree species. Irrigation experiments with deuterium-labeled water revealed that Quercus gambelii in northern Arizona and southern Utah did not use water from surface layers. In contrast, Juniperus osteosperma at these sites responded significantly to the irrigations: between 37% and 41% of xylem water originated from irrigations that wetted only the top 30 cm of soil. Responses by Pinus edulis to these irrigations were variable; uptake of labeled water by this species was greater in September at the end of the summer than during the hot midsummer period. Inactivity of Pinus roots in midsummer supports the hypothesis that root activity in this species is sensitive to soil temperature. Seasonal patterns of leaf gas exchange and plant water potential corresponded to the seasonality of rainfall at different sites. However, no correlation between a species' ability to use summer rainfall and its tolerance to water deficits at the leaf level was found. Midday stomatal conductance (gs) for Pinus needles approached zero at predawn water potentials near -2 MPa, whereas gs in Quercus and Juniperus declined to zero at -2.8 and -3.7 MPa, respectively. The relationship between photosynthesis (A) and gs was similar among the three species, although Quercus maintained higher overall rates of gas exchange and tended to operate higher on the A/gs curve than the two conifers. At sites in eastern Arizona where Quercus fully used moisture from summer rains, leaf gas exchange characteristics were similar to those of Pinus and Juniperus.a+0$Sperry, J. S. Saliendra, N. Z. 1994PJIntra-and inter-plant variation in xylem cavitation in Betula occidentalis Plant Cell and Environment1711 1233-1241A modified version of a method that uses positive air pressures to determine the complete cavitation response of a single axis is presented. Application of the method to Betula occidentalis Hook. gave a cavitation response indistinguishable from that obtained by dehydration, thus verifying the technique and providing additional evidence that cavitation under tension occurs by air entry through interconduit pits. Incidentally, this also verified pressure-bomb estimates of xylem tension and confirmed the existence of large (i.e. gt 0.4 MPa) tensions in xylem, which have been questioned in recent pressure-probe studies. The air injection method was used to investigate variation within and amongst individuals of B. occidentalis. Within an individual, the average cavitation tension increased from 0.66 +- 0.27 MPa in roots (3.9 to 10.7 mm diameter), to 1.17 +- 0.10 MPa in trunks (12 to 16 mm diameter), to 1.36 +- 0.04 MPa in twigs (3.9 to 5 mm diameter). Cavitation tension was negatively correlated with the hydraulically weighted mean of the vessel diameter, and was negatively correlated with the conductance of the xylem per xylem area. Native cavitation was within the range predicted from the measured cavitation response and in situ maximum xylem tensions: roots were significantly cavitated compared with minimal cavitation in trunks and twigs. Leaf turgor pressure declined to zero at the xylem tensions predicted to initiate cavitation in petiole xylem (1.5 MPa). Amongst individuals within B. occidentalis, average cavitation tension in the main axis varied from 0.90 to 1.90 MPa and showed no correlation with vessel diameter. The main axes of juveniles (2-3 years old) had significantly narrower vessel diameters than those of adults, but there was no difference in the average cavitation tension. However, juvenile xylem retained hydraulic conductance to a much higher xylem tension (3.25 MPa) than did adult xylem (2.25 MPa), which could facilitate drought survival during establishment.NHSperry, John S. Nichols, Kirk L. Sullivan, June E. M. Eastlack, Sonda E. 1994nhXylem embolism in ring-porous, diffuse-porous, and coniferous trees of northern Utah and interior AlaskaEcology756i 1736-1752aXylem embolism was measured in nine tree species for one or more years. Species were ring-porous (Quercus sp.), diffuse-porous (Alnus, Betula, Populus spp.) or coniferous (Picea, Larix, Abies spp.). Intraspecific (Populus tremuloides) and intrageneric (Betula, Alnus) comparisons were made between sites in northern Utah and interior Alaska. Most embolism, gt 90% in some dicot species, occurred in winter. Within sites, dicot trees embolized more than conifers. Between sites, Alaskan dicot trees embolized less than their Utah counterparts. Differences were explained by vulnerability to embolism caused by freeze-thaw cycles. Most conifers were entirely resistant, whereas dicot trees were vulnerable. Less embolism in Alaskan dicot trees was associated with fewer freeze-thaw events in Alaska vs. Utah. Vulnerability was positively correlated with conduit volume and hydraulic conductance per unit xylem area (k-s). Tracheids were superior to vessels in avoiding freeze-thaw-induced embolism, and had lower k-s. At the other extreme, ring-porous xylem had the highest k-s but lost gt 90% of hydraulic conductance after a single freeze-thaw event. Vulnerability to water-stress-induced cavitation was not correlated with conduit volume or k-s. Dicot species either reversed winter embolism by refilling vessels with positive root pressures during spring (Betula, Alnus spp.), or tolerated it and relied on new xylem production to restore hydraulic conductance (Quercus sp.). Conifers reversed embolism by refilling tracheids in the absence of positive pressure. Populus species behaved inconsistently, showing some reversal one year but none the next. Even without embolism reversal, Populus species had hydraulic conductances per unit leaf area equal to other diffuse-porous species.r+ ;&Kalin, Arroyo Mary T. Castor, Carmen Marticorena, Clodomiro Munoz, Melica Cavieres, Lohengrin Matthei, Oscar Squeo, Francisco A. Grosjean, Martin Rodriguez, Roberto 1998}The flora of Llullaillaco National Park located in the transitional winter-summer rainfall area of the Northern Chilean AndesiGayana Botanicae552 93-110. The vascular plant flora of Llullaillaco National Park, situated mostly above 3500 m elevation in the high Andes of the II Region of Chile (24degree30'S - 25degree10'S; 68degree30'W - 69degree15'W) in the transitional zone between the summer and winter rainfall regimes, is given, along with analysis of endemism, presence of distributional limits and species-accumulation tendencies. The known flora stands at 91 species and subspecies (90 species), in 58 genera and 26 families, with an additional 3 species likely to occur in the park. Thirteen (14%) of taxa are endemic to the Chilean Andes, with two entirely restricted to the park. Outstanding numbers of taxa (35 in total: 38%) reach their northern, southern or north-south distributional limits within or very close to the park boundary. Knowledge of the park's flora has accumulated steadily since R. A. Philippi's landmark expedition to the Atacama desert in 1860. By 1925 over half of the known taxa had been collected, and by 1975 over 2/3 of the specieswere known. The species-accumulation curve for the park predicts that a few more species are likely to be discovered. Llullaillaco National Park plays an important role in protecting marginal populations of many high Andean species of subtropical and mediterranean affinity. The composition and characteristics of the flora indicate that the park constitutes a critical area for understanding climatic evolution in the northern Chilean Andes.611-623$://A1997WQ77800011u:4Katul, G. Todd, P. Pataki, D. Kabala, Z. J. Oren, R.zsSoil water depletion by oak trees and the influence of root water uptake on the moisture content spatial statisticspWater Resources ResearchPJtime domain reflectometry; hydraulic conductivity; variability; tdr; modelThe space-time statistical structure of soil water uptake by oak trees was investigated in a 3.1-m-diameter closed top chamber using a three-dimensional measurement grid of soil moisture and pressure, and measurements of tree transpiration. Using the time domain reflectometery (TDR) measured moisture content, resistance block measured soil water pressure, and a compact constant head permeameter measured saturated hydraulic conductivity, the soil hydraulic properties for the chamber were first estimated. Then, dimensionless statistical measures that utilize the soil water pressure were proposed and used to assess the relative importance of lateral to vertical flow. On the basis of the measured statistical properties of the soil- water pressure, it was found that the vertical flow is at least an order of magnitude larger than the lateral flow, and thus a one-dimensional flow approximation to continuity was utilized. Using continuity and a first-order Taylor series expansion of the Buckingham-Darcy vertical flux about the spatial mean moisture content state, an approximate relation for the time variation of the spatial mean moisture content was derived and tested with the TDR measurements, Despite a large spatial coefficient of variation in the TDR measured moisture content (which was also shown to be comparable to reported values from larger-scale field experiments), good agreement between mean moisture content predictions and measurements were found for two separate drying cycles. The approximate Taylor series flux expansion was utilized for deriving an analogous relation for the time variation of the spatial moisture content variance. The resultant variance budget was used to assess the role of root water uptake on the spatial variability of moisture content. It was found that the root uptake component, which resulted from a covariance between the root water uptake and moisture content spatial perturbations, is comparable to the contribution from soil hydraulic properties and soil water redistribution, One of the main findings in this study is that root water uptake is central to the moisture content spatial variance dissipation especially for dry soil moisture conditions. These results were further investigated using Monte-Carlo simulations.Water Resour. Res. 1997 Apr334WQ778 WATER RESOUR RESISI:A1997WQ778000112,Kemp, P. R. Cornelius, J. M. Reynolds, J. F. 1992JDA Simple Model For Predicting Soil Temperatures in Desert Ecosystems Soil Science 1534280-287:3We present a model for predicting soil temperatures in arid regions using current above surface conditions (temperature or temperature and solar radiation) and the annual sinusoidal behavior of temperature in the deeper soil profile. The model is based on the of Parton (Parton, W. J. 1984, Soil Sci. 139:93-101) and provides considerable temperature detail with a daily time step: maximum, minimum, and mean temperature at any depth; hourly temperature detail if desired. The error in predicted temperatures is about the same as those in models that require soil temperature inputs from at least one depth. This model, however, can be used for predicting soil temperatures in arid sites for which there are no soil tempeatures data or for predicting soil temperatures under future climates or environmental conditions.d8= Abbott1997 Abrams19944 Adiku20006 Adler1998 Agnew1997 Aguado1998 Aguiar1994 Aguiar1996 Aguiar19988e Aguiar1999Aguilera19988< Aguirre1999( Alder19933 Alder1996@ Ali1999 Allen1985 Allen1998 Allen1998a Allen Craig1998O Alm1991S Alm1991Q Alm1992R Alm1992Y Alm1993Anderson1996Anderson1997e Anghera1996 Anthoni1999< Aravena1999T Archer1990N Archer1995(O Archer1995V Archer19988R Archer199919 Armesto1988 Austin1998 Austin19999 Austin1999 Austin2000 B.19779Bachelet1996r; Baker1995k Barber20010^ Barnes19979 Barnes19979 Barnes19999 Barnes20010E Bartolome1988 Bassirirad1992(  Bassirirad1992 BassiriRad1999D Bauer1996 Bauer1998 Bazzaz1988 Bazzaz1990 Bazzaz1991 Bazzaz1991 Bazzaz19922 Bazzaz19977 Beeskow1991 Belnap1995 Belnap19971 Belnap1999( Belnap2001a Belnap2001 Belnap2001 Belnap2001^ Beltran1998[ Beltran2000 Benner1988| Berndtsson1994( Bertiller1991 Bertiller2000 Betancourt1999 Betancourt2000h Bignell David2000 Bilbrough1997 Bilbrough2001I Bingham1988 Black1991 Black1992 Black1993; Blake1995 Bliss1995Boisvert2001 Bond20012 Bonham1996Bonham Charles1995 Boo1994 Boo1996C Borghetti1995F Borghetti1998Bouskila2001 Bouten20000V Boutton1998 Bowers1987 Bowers1997 Bowman1987 Bowman19979 Bowman2001?Braddock2000 Breshears1994^ Breshears1997 Breshears1997 Breshears1997 Breshears1998 Breshears1998 Breshears1998  Breshears1999  Breshears1999  Breshears2001  Breshears2001[Breshears David1994\Breshears David1996aBreshears David1998y Briones1995z Briones1996{ Briones1998 Briske1998y Brooks1997T Brown1990 Brown1990@ Brown1993B Brown1995 Brown1997R Brown1999 Brown2001E Brown James1996h Brown Valerie2000 Bruen2001 Brunell1999h Brussaard2000H Buchard1999DBuchmann1996yBuchmann1997zBuchmann1997 w Burgess1988 Burgess1990 Burke1996! Burke1997 Burke1997q Burke1998 Burke1998_ Burke1999 Burke1999 Burke2000 Burke Ingrid1999o Burton19989 Caesar19955Caldwell19755Caldwell1977Caldwell1985Caldwell1987Caldwell19888Caldwell1989Caldwell19898Caldwell1993Caldwell1994Caldwell1996Caldwell1997Caldwell1997Caldwell19977Caldwell1998Caldwell2000Caldwell2001Caldwell2001Caldwell2002Caldwell Martyn1992 Caldwell Martyn1992Caldwell Martyn1996&Campbell1989Campbell199776Campbell1998DCanadell1996{Canadell1996Canadell1996Canadell2000H Canny1999 Carlson1995k Carpenter2001 Carrera2000; Castor1998oy Cavagnaro1995QCavelier1992RCavelier1992;Cavieres19989 Cayan1992 Cayan1994 Cayan1998 Cayan1998 Cayan1999 Cayan1999| Chen1994n Chen1996a0 Chen1997 Chesson1985 Chesson1988 Chesson1989 Chesson1990 Chesson1991 Chesson1993 Chesson1994 Chesson1995 Chesson1997 Chesson2000 Chesson2001F Cinnirella1998Cioccale1999k Clark2001 Clark2001cCleverly19979Clothier1997 Cody1986 Cody1986 Cody1989 Cody1991l Coffin19949 Coffin19969 Coffin2000 Cohen1970@ Cohen1999" Colasanti1993 Colello2000 Coleman1992 Coleman1997h Coleman David2000 Collin1998k Collins2001!Comstock19909Comstock1992Comstock199292Comstock199556Comstock1998! Cook19901 Cook2000a Cornelius1989+ Cornelius19921 Cornelius1994Coronato19911Coughlan1996, Cowling19941 Cowling1994C Crowley1988P Cui1992V Cui1992W Cui1992X Cui1992Z Cui1994 Cui1996 Cui1996 Cui1997 Cui1997 Cui1998 Cunningham1996 Curtin1997h Curtis19939k Curtis19959 D'Odorico1999 D'Odorico2000 D'Odorico2000 D'Odorico2000hDangerfield John20000 Davenport1998\Davenport David1996% Davis2000 Davis2002! Dawson19900: Dawson1993 Dawson20000 Dawson2001L De Colstoun1998 De Jong2001DDe Paoli1996 de Rosnay2001h de Ruiter2000(De Soyza1996de Soyza19999de Soyza19996 Deangelis1994 Delatorre1992 Derner1998 Dettinger1998 Dettinger1998c Devitt1997 Diaz1998 Diaz19988 Dickinson2000lDickmann1996 Dingle19878Dirmeyer20011 Distel1994( Distel1996B Dobrenz1989k Dobson20011 Dodd1998! Donovan1990" Donovan1991~ Donovan1992 Donovan1993* Donovan1993. Donovan1994 Donovan1994 Donovan1994 Donovan2001} Dougherty1996 Duke2000 Duke20015 Dyer2001i(Eastlack1993+Eastlack1994 Ebeling1996? Edwards1994? Edwards199496? Edwards199496? Edwards199496? Edwards199496? Edwards199496? Edwards199496? Edwards1994? Edwards199496? Edwards199496? Edwards1994? Edwards199496? Edwards1994 Edwards199496? Edwards199496? Edwards199496? Edwards1994? Edwards199496? Edwards199496? Edwards199496? Edwards19946NMLKf`A two-dimensional model for water uptake by desert succulents: Implications of root distribution"Hunt, E. R. Jr Nobel, P. S.Annals of Botany595559-570 1987F@Water uptake by Agave deserti and Ferocactus acanthodes was predicted using a two-dimensional simulation model in which the soil around a plant was divided into a series of layers and concentric cylindrical shells. Root lengths in 0.05 m thick soil layers were determined for both species in the field, where mean root depths were only 0.11 m for A. deserti and 0.10 m for F. acanthodes. For a year with average precipitation (159 mm), 42 per cent of the annual precipitation could be taken up by A. deserti and 25 per cent by F. acanthodes. Predicted water uptake by both species was greater from the upper soil layers (above 0.15 m) for average and dry years, but was greater from the deeper layers for a wet year. The actual root distribution for both species led to more water uptake than when all of the roots were in a single layer. The large number of days per year when the soil temperatures exceeded 57 degree C (the temperature for 50 per cent inhibition of uptake of a vital stain by root cells) may exclude roots from the 0.00-0.05 m soil layer, even though water uptake when all roots were located there was predicted to be maximal. Therefore, the observed root distribution of A. deserti and F. acanthodes may be limited near the soil surface by high temperatures and at maximum depths by water availability for all but wet years.ZTAllometric root/shoot relationships and predicted water uptake for desert succulents"Hunt, E. R. Jr Nobel, P. S.Annals of Botany595a571-578 1987Root morphology, shoot morphology, and water uptake for Agave deserti and Ferocactus acanthodes of various sizes were studied using allometric relationships (y=ax-b) and a previously developed water uptake model. Shoot surface area increased with shoot volume with an exponent b of 0.75 for both species. Root length and the ground area explored by the roots increased with shoot surface area with b's of 0.72 for A. deserti and 0.92 for F. acanthodes. Various sized individuals had about the same ratio of root length to explored ground area, with higher values occurring for A. deserti. Predicted water uptake averaged over the explored ground area was approximately constant over a 10-5-fold range in shoot surface area, suggesting that shoot size confers no intraspecific competitive advantage for water uptake. For the root lengths per explored ground area observed in the field, water uptake was predicted to be 85 per cent of maximal; water uptake could be increased by the production of more rain roots. When differences in shoot volume were accounted for by allometry, small plants had relatively less shoot surface area and relatively more root length per shoot volume than did large plants, which may be important for the water relations of seedling establishment.\VWater costs and water revenues for established and rain-induced roots of Agave deserti2+Hunt, E. R. Jr Zakir, N. J. D. Nobel, P. S.oFunctional Ecology172125-130e 1987 1 Production of new roots leads to increased water uptake and increased respiration. Water uptake can be considered as the revenue from root production, and root respiration divided by the water-use-efficiency as the water cost. Using an analogy to economics, the optimal root mass occurs when the marginal cost of root production equals the marginal revenue of root production. 2 Agave deserti Engelm. has two types of roots, established and rain-induced; the latter were shed three times as the soil dried during the year considered. Per unit dry mass, respiration required for growth was 90-fold higher and respiration required for maintaining living tissue was 40-fold higher for rain roots compared with established roots. Conductance to water flow per unit dry mass was 11-fold higher for rain roots compared with established roots. 3 Previous excavations showed that A. deserti can have an established-root length of 44 +- 8 m (mean +- SD) and a rain-root length of 13 +- 3 m. Optimal root lengths predicted for this plant were 51 m of established roots and 31 m of rain roots, which were within 1SD of the measured length for established roots but greater than 6 SD for rain roots. Thus, water may be the currency for the production of established roots but not for rain roots.ZSNon-steady-state water flow for three desert perennials with different capacitances."Hunt, E. R. Jr Nobel, P. S.S,&Australian Journal of Plant Physiology144-363-376e 1987Non-steady-state water flow through plants was modelled using an electric circuit analog incorporating capacitance (change in water volume per unit change in water potential). Predictions of leaf water potential agreed with measurements for a C-4 grass with a small capacitance. Hilaria rigida, and for a C-3 shrub with an intermediate capacitance, Encelia farinosa. Predictions differed from measured stem water potentials for a stem succulent having crassulacean acid metabolism and a large capacitance, Ferocactus acanthodes, presumably because of daily variations in osmotic pressure. As capacitance increased, minimum shoot water potential and the maximum water uptake rate by the roots lagged further behind maximum transpiration rate. Predicted daily water uptake by roots was equal to daily water loss for H. rigida and E. farinosa, but not for F. acanthodes for which capacitance effects were particularly important. Because tissue volumes were large, water flow for F. acanthodes would be expected to reach the steady state only if conditions were constant for about 2 days, hence steady-state flow is not expected in the field. For all three species, capacitance was largely determined by total water volume rather than by the bulk elastic modulus..'Huston, Michael A. Deangelis, Donald L.r 1994VOCompetition and coexistence: The effects of resource transport and supply ratesAmerican Naturalist 1446954-977tClassical resource competition theory can be generalized to apply to a variety of specific resource types and specific supply media (e.g., soil, water, or air). We develop a general model that relaxes the assumptions that (1) resources and organisms are sufficiently mixed that all organisms experience the same resource concentration and (2) the organisms themselves regulate the resource concentration of their shared environment. These assumptions are shown to apply to a limited subset of conditions in which the resource input rate is low and the resource transport rate in the environment is high. Under such conditions, the coexistence criteria of our general model converge with those of classical resource competition models. Such conditions may be met in some aquatic environments, but under other conditions, in which resource transport rates may be low or input fluxes high, the general model makes predictions that differ radically from those of the classical models. Specifically, our model predicts that, instead of a 1:1 ratio between limiting resources and locally coexisting species, a large number of species can coexist on a single limiting resource under steady-state conditions. Shifts from limitation by one type of resource to limitation by another type can dramatically alter the nature and intensity of competitive interactions. This phenomenon is proposed as the explanation for the ubiquitous unimodel curve of autotroph diversity along productivity gradients.%\v$Cui, Muyi Caldwell, Martyn M.S 1998Nitrate and phosphate uptake by Agropyron desertorum and Artemisia tridentata from soil patches with balanced and unbalanced nitrate and phosphate supply New Phytologist 139823267-272mTo test the ability of plants to integrate small-scale imbalances in soil nitrate and phosphate patches, plant growth and acquisition of nitrate and phosphate were measured for the perennial grass Agropyron desertorum (Fisch. ex Link) Schult. and the shrub Artemisia tridentata Nutt. ssp. vaseyana (Rydb.) Beetle in soil where the principal supply of nitrate and phosphate came from two enriched patches. The soil was calcareous loamy-skeletal Typic Haploxerolls. These patches were applied in two treatments: either nitrate and phosphate were applied in both patches (balanced treatment) or one patch contained only nitrate and the other only phosphate (unbalanced treatment). The same total quantity of nutrients was applied in both treatments and these included 15N and 32P tracers. The plants were in large pots in open field conditions. There were no significant differences in total biomass production and nitrogen concentration between the two treatments, indicating that both species had the physiological ability to integrate soil nutrient resources. Artemisia was able to acquire more phosphate in the unbalanced treatment, probably due to the high local solution phosphate concentration. Generally Artemisia acquired more N and P than did Agropyron.>8D'Odorico, P Ridolfi, L Porporato, A Rodriguez-Iturbe, I 2000XQPreferential states of seasonal soil moisture: The impact of climate fluctuationsaWater Resources Research36 2209-2219c231-240$://000072673700017eB://000166336200011  Horton, J. L. Clark, J. L.hbWater table decline alters growth and survival of Salix gooddingii and Tamarix chinensis seedlings$Forest Ecology and ManagementiSalix gooddingii; Tamarix chinensis; seedling establishment; water table decline; riparian ecology riparian vegetation; fremont cottonwood; southern alberta; river; establishment; streamflow; arizona; poplar; recruitment; floodplainIn the western United States, many native riparian forests are declining downstream from dams and diversions, primarily due to a lack of successful regeneration of native species. Conceptual models have been described that incorporate stream Row characteristics that promote successful regeneration of western riparian Populus seedlings. However, these models need to be calibrated for additional species and specific stream characteristics. In this study, we examined the growth and survival of two dominant Sonoran desert riparian tree species, a native Salix gooddingii, and an exotic, Tamarix chinensis, to simulated water table declines at rates of 0, 1, 2, and 4 cm/day. Salix seedlings performed best in the 0 cm/day treatment in which the soil was saturated and showed decreasing survival and growth with increasing rates of water table decline. Root-to-shoot ratios were high for Salix, but root elongation rates were low with an emphasis on lateral root development. This root growth pattern may be an adaptation to habitat where Salix typically grows, near the stream channel when the water table is shallow but the likelihood of Rood scour is high. Lateral root development may help to protect Salix seedlings from late season flood scour. Tamarix seedlings showed consistently high survival across all treatments and had the greatest growth in the 0 and 1 cm/day treatments. Tamarix seedlings had greater root elongation rates in response to water table decline relative to the control, with the greatest elongation in the 1 cm/day treatment. Greater root elongation rates and higher drought tolerance allowed Tamarix seedlings to persist in dry soils where Salix seedlings died. Differences in growth and survival of these two species in response to water table decline will need to be incorporated into management practices to promote successful native riparian tree regeneration. (C) 2001 Elsevier Science B.V. All rights reserved.For. Ecol. Manage. 2001 Jan 15 140 2-3391CH FOREST ECOL MANAGEISI:0001663362000112v$Y(X<WNobel, Park S. Cui, Muyi 1992Shrinkage of attached roots of Opuntia ficus indica in response to lowered water potentials: Predicted consequences for water uptake or loss to soilAnnals of Botany706485-491Attached 2-month-old roots of the succulent plant, Opuntia ficus-indica, shrank 0.4% radially during periods of maximal transpiration under wet conditions. In contrast, reversible decreases in diameter of nearly 20% occurred for these roots as their ambient water potential (psi) in the vapour phase decreased from -0.01 to -10 MPa over 8 d, the changes being slightly more rapid at 40 degree C than at 10 degree C. Such substantial diameter changes became progressively less with root age, from a 43% decrease in diameter at 3 weeks to a 6% decrease at 12 months. Root shrinkage was slight when psi was decreased from -0.01 to -0.3 MPa, the latter psi being similar to the root water potential. As psi was further decreased from -0.3 to -10 MPa, water movement out of cortical cells caused considerable root shrinkage. The root hydraulic conductivity (L-P) decreased only 30 to 60% for a change in psi from -0.01 to -10 MPa compared with a decrease of over 10-6-fold for the soil hydraulic conductivity over this range. The overall conductivity of the soil, the root-soil air gap, and the root was predicted to be dominated by L-P for psi-soil above -0.3 MPa. As simulated psi-soil decreased below -0.3 MPa, the root-soil air gap initially became the primary limiter of water loss from the roots. Below -5 MPa for 1-month-old roots and below -2 MPa for 12-month-old roots, the soil became the main limiter of water loss. Thus, water uptake from wet soils apparently was mainly controlled by root properties. Water loss to drying soils was controlled by the development of a root-soil air gap around shrinking roots during the initial phase of soil drying and by the reduction of the soil hydraulic conductivity at even lower psi-soil.lNobel, Park S. Cui, Muyi 1992~xPrediction and measurement of gap water vapor conductance for roots located concentrically and eccentrically in air gapsPlant and Soil 1452a157-166`ZWater movement between a root and the soil depends on the hydraulic conductances of the soil, the root, and the intervening root-soil air gap (L-gap) created as roots shrink during soil drying. To measure L-gap, segments of young cylindrical roots of Agave deserti about 3 mm in diameter were placed concentrically or eccentrically within tubes of moistened filter paper at a known water potential. As the width of the air gap between the filter paper and a concentrically located root was made smaller, measured L-gap increased less than did predicted L-gap based on isothermal conditions. For gaps of the size expected in the soil during water loss from roots (e.g., 10% of the root radius), the underprediction was about 70% and was primarily caused by a lowering of the root surface temperature accompanying water evaporation. As a root segment was eccentrically moved toward the filter paper, the measured L-gap increased. For the most eccentric case of touching the filter paper, the measured L-gap was 2.4-fold greater than for the concentric case, compared with an infinite L-gap predicted if the water potential were constant around the root surface. When a root touched soil with a water potential of -1.0 MPa, L-gap estimated using a graphical method increased about 2.3-fold and the overall conductance of the root-soil system increased by 31% compared with the concentric case. For markedly eccentric locations of roots in air gaps, L-gap, which can be the principal conductance initially limiting water loss from roots in a drying soil, can be about 60% of the value predicted for the concentric isothermal case.Nobel, P. S. Alm, D. M. 1993|Root orientation vs water uptake simulated for monocotyledonous and dicotyledonous desert succulents by a root-segment modelFunctional Ecology75600-6090F@1. A root-segment model was combined with a soil water model to predict water uptake by roots of Agave deserti and Ferocactus acanthodes of four different lengths at 10 orientations from horizontal to vertical after various simulated rainfall events. 2. The soil was divided into 15 strata with specific water potentials (psi-soil) and temperatures; each root segment had specific values of radial hydraulic conductivity from the root surface to the xylem (L-R) and of axial hydraulic conductivity along the xylem (K-h). For roots of the monocotyledonous A. deserti, L-R decreased with distance from the root tip up to 80 cm and K-h became constant and maximal after 40 cm, favouring water uptake from the distal (younger) portions. For roots of the dicotyledonous F. acanthodes, L-R increased slightly and K-h increased greatly away from the root tip, favouring water uptake from the proximal (older) portions. 3. As simulated rainfalls were increased from 10 to 30 to 50 mm, the duration and the depth at which psi-soil was above the shoot water potential of -0.5 MPa increased. Horizontal roots of A. deserti 60 cm in length took up more water than those oriented more vertically, although differences became less as the rainfall amount increased. 4. Compared with A. deserti, roots of F. acanthodes had maximal simulated water uptake at steeper orientations, reflecting the different variations of L-R and K-h along the root. Also, water uptake at essentially all orientations was highest for the shortest (20 cm long) and hence youngest root of A. deserti and the longest (80 cm long) and hence oldest root of F. acanthodes. 5. The root-segment model, which took into account temperature effects on L-R and Kh as well as variations in psi-soil along a root, can be used to predict water uptake for root systems under realistic conditions in the field. Nobel, P. S. 1994>7Root-soil responses to water pulses in dry environments $Caldwell, M. M. Pearcy, R. W.ngExploitation of environmental hetrogeneity by plants: ecophysiological processes above and below ground  San Diego Academic Press Inc285-304t.'Novoplansky, Ariel Goldberg, Deborah E.0 2001^XEffects of water pulsing on individual performance and competitive hierarchies in plants$Journal of Vegetation Science122h199-208u82In a glass house experiment, we investigated the effect of both the frequency of water pulses and the total amount of water supplied on individual performance in the absence and presence of neighbors. We used monocultures and all combinations of pairs of seedlings of three species of perennial grasses, characteristic of different points along a soil moisture gradient within a semi-arid grassland in New Mexico, USA. In the absence of neighbors, higher total water or more frequent (but smaller) pulses significantly increased growth of all three species. The species with the fastest intrinsic growth rate, and from the most productive habitat, exhibited the largest increase in absolute and relative growth in response to higher total water quantity. Competitive effects were highly significant overall and there were significant hierarchies of competitive ability. Under frequent pulses, the fast-growing species from the most productive environment was the best competitor in terms of both ability to suppress other plants and ability to tolerate the presence of neighbors. However, under infrequent pulses, the slowest growing species from the least productive environment became a much stronger competitor, again in terms of both suppression and tolerance of neighbors. While differences in total water availability had greater effects than differences in pulsing regime on individual plant performance in the absence of competition, pulsing regime had much stronger effects on relative competitive abilities and thus may be more likely to influence field distribution patterns.] rid landscapes, the ratio of herbaceous to woody plant biomass is a major determinant of ecosystem properties. This ratio depends to a large extent on the amount and spatial distribution of soil moisture that is available to plants, and these variables, in turn, are determined primarily by clima`YBreshears, David D. Myers, Orrin B. Johnson, Susan R. Meyer, Clifton W. Martens, Scott N. 1997~Differential use of spatially heterogeneous soil moisture by two semiarid woody species: Pinus edulis and Juniperus monospermaJournal of Ecology853289-299 1. Soil moisture in semiarid woodlands varies both vertically with depth and horizontally between canopy patches beneath woody plants and the intercanopy patches that separate them, such that shallow soil layers in intercanopy locations are wettest, yet few studies have considered both dimensions of spatial variability in testing for acquisition of resources by plants. 2. Three hypotheses were tested relative to the use of shallow water in intercanopy locations by two coexisting semiarid-woodland tree species, Pinus edulis (a pinon) and Juniperus monosperma (a juniper): (i) both P. edulis and J. monosperma can use shallow water from intercanopy locations, (ii) J. monosperma is able to obtain more shallow water from intercanopy locations than P. edulis, and (iii) the spatial arrangement of the trees influences the amount of water they obtain. Soil moisture and plant water potential (i.e. plant water stress) were measured before and after the addition of water to shallow depths (0-30 cm) of intercanopy locations for trees of both species in two spatial arrangements: isolated and paired with a contiguous tree of the other species. 3. Both species responded to the addition of shallow water in intercanopy locations, as measured by plant water potential. The response of J. monosperma was significantly greater than that of P. edulis, as measured by depletion of shallow soil moisture in intercanopy locations and by change in plant water potential per unit change in soil water potential (the difference was not detectable on the basis of plant water potential alone); in addition, the amount of depletion was correlated with basal area of J. monosperma but not of P. edulis. The responses were not influenced by spatial arrangement (isolated vs. paired with a contiguous tree of the other species). 4. The results of this study are consistent with differences in the relative abundances of the two species across locations, suggesting that species differences in ability to use shallow water in intercanopy locations is important in structuring semiarid woodlands. Further, the results suggest that current theoretical concepts for semiarid ecosystems, which ignore either vertical or horizontal variability in soil moisture, may be inadequate for predicting changes in the ratio of woody to herbaceous plant biomass, particularly for plant communities with co-dominant woody species that differ in ability to acquire spatially heterogeneous resources.351-357$://A1995RC92700019o$Shein, E. V. Pachepsky, Y. A.1D>Influence of Root Density on the Critical Soil-Water PotentialPlant and Soilroot density; root system structure; root water uptake; soil texture; soil water potential plant-roots; model; simulation; extraction; transport; fieldtEstimation of root water uptake in crops is important for making many other agricultural predictions. This estimation often involves two assumptions: (1) that a critical soil water potential exists which is constant for a given combination of soil and crop and which does not depend on root length density, and (2) that the local root water uptake at given soil water potential is proportional to root length density. Recent results of both mathematical modeling and computer tomography show that these assumptions may not be valid when the soil water potential is averaged over a volume of soil containing roots. We tested these assumptions for plants with distinctly different root systems. Root water uptake rates and the critical soil water potential values were determined in several adjacent soil layers for horse bean (Vicia faba) and oat (Avena sativa) grown in lysimeters, and for field-grown cotton (Gossypium L.), maize (Zea mays) and alfalfa (Medicago sativa L.) crops. Root water uptake was calculated from the water balance of each layer in lysimeters. Water uptake rate was proportional to root length density at high soil water potentials, for both horse bean and oat plants, but root water uptake did not depend on root density for horse bean at potentials lower than -25 kPa. We observed a linear dependency of a critical soil water potential on the logarithm of root length density for all plants studied. Soil texture modified the critical water potential values, but not the linearity of the relationship. Plant Soil 1995 Aprn 171h2lRC927 PLANT SOILISI:A1995RC92700019 from the subsoil to the 30 cm topsoil during the night. Snakeweed seedlings were transplanted in buried pots allowing the deeper roots to grow into the subsoil 30 cm below the surface. Soil water content inside and outside of the pot was measured seasonally and diurnally with time domain reflectometry technique (TDR). An increase in water content was detected in the pot after the plant was covered for 3 h by an opaque plastic bag during the day, suggesting hydraulic lift from deeper depths and exudation of water into the drier topsoil. Root exudation was also observed on native range sites dominated by snakeweed. Water efflux in the pot was 271 g per plant per night. which was equivalent to 15.3% of the extrapolated, porometer-derived whole-plant daily transpiration. Hydraulic lift observed in Gutierrezia improved water uptake during the day when evaporative demand is high and less water is available in the topsoil. We concluded that hydraulic lift might help snakeweed to alleviate the effect of water stress.z 465-478$://000082510000005w$Breshears, D. D. Barnes, F. J.Interrelationships between plant functional types and soil moisture heterogeneity for semiarid landscapes within the grassland/forest continuum: a unified conceptual modeltLandscape Ecologymnhcatastrophe theory; desertification; grassland/forest continuum; savanna; shrubland; woodland; soil moisture heterogeneity; plant community composition; plant functional types; Walter's two-layer hypothesis pinyon-juniper woodlands; water-balance; vegetation systems; patagonian steppe; spatial pattern; rooting depth; arid savanna; new-mexico; dynamics; grass  In semiarid landscapes, the ratio of herbaceous to woody plant biomass is a major determinant of ecosystem properties. This ratio depends to a large extent on the amount and spatial distribution of soil moisture that is available to plants, and these variables, in turn, are determined primarily by climate and land use. Current conceptual models for determining the ratio of herbaceous to woody plant biomass in semiarid plant communities are based either on differences in soil moisture with depth (vertical heterogeneity) from one site to another (Walter's two-layer model) or on differences in soil moisture between canopy and intercanopy patches at the same site (horizontal heterogeneity) that result from disturbances associated with land use (Schlesinger et al.'s model of desertification). We developed a model that unifies these two perspectives by relaxing two assumptions of Walter's two-layer model. First, our model recognizes that soil moisture varies horizontally between canopy and intercanopy patches, not only due to land-use disturbance, a general assumption of the Schlesinger et al. model, but also due to the physical nature of the canopy itself. Second, while retaining the general assumption of Walter that woody plants obtain moisture from deeper soil layers than do herbaceous plants, our model recognizes the existence of two types of woody plants: those that extract a substantial proportion of their moisture from deeper layers and those that extract mainly from shallower layers. By modifying the two-layer hypothesis to include four soil compartments and distinguishing between shallow- and deeper-rooted woody species, our model integrates three key concepts in semiarid ecology: (1) the proportion of woody cover increases as moisture in the deeper soil layers increases (Walter's two-layer hypothesis for coexistence of herbaceous and woody plants); (2) land use practices that cause a reduction in herbaceous vegetation and compaction of intercanopy soils lead to a long-term increase in the proportion of woody plants (Schlesinger et al.'s concept, or more generally, that at a given site multiple variations in the proportions of herbaceous and woody plant biomass are possible); and (3) changes in the ratios of herbaceous to woody plant biomass exhibit complex behavior (changes can happen quickly and are not directly reversible without intensive management). This integration of concepts results because rather than assuming a simple, one-way dependence of plant functional types on soil moisture heterogeneity, our model assumes an interdependence between the two: soil moisture heterogeneity constrains the composition of the plant community, which in turn modifies soil moisture heterogeneity. The four-compartment model that we propose enables, for the first time, an integrated picture of both dimensions of soil moisture heterogeneity - horizontal and vertical - and of the interdependence between soil moisture heterogeneity and the proportions of the plant functional types that make up a given plant community. This unified conceptual model can be applied to provide insight into the individual and the combined effects of climate and land use on semiarid plant communities within the grassland/forest continuum, which vary in the proportions of canopy and intercanopy patches. Landsc. Ecol. 1999 Oct145234XQ LANDSCAPE ECOLISI:00008251000000560Briones, Oscar Montana, Carlos Ezcurra, Exequiel 1996|vCompetition between three Chihuahuan desert species: Evidence from plant size-distance relations and root distribution$Journal of Vegetation Science 7a3453-460Using size-distance data we tested the intensity and importance of competition between Hilaria mutica (a tussock grass), Larrea tridentata (a microphyllous shrub) and Opuntia rastrera (a succulent) in the Chihuahuan desert. We also compared the vertical and horizontal distribution of roots to assess the potential degree of overlap in the use of soil resources. The relationships between sizes and distances of nearest-neighbour plants suggested that intraspecific competition is generally more important than interspecific competition. However, evidence of stronger inter than intraspecific competition was found in some cases. Species combinations showing significant interspecific competition involved always Opuntia, whereas Larrea and Hilaria did not influence each other. The analysis of the symmetry of competition showed that Opuntia was adversely affected by the presence of Hilaria or Larrea. Although differences were found in the distribution of roots, the results of the size-distance study support the idea that, (particularly) Opuntia, below-ground niche differentiation is not sufficiently important to totally avoid the negative effects of plant competition."Chesson, Peter Huntly, Nancy 1997^WThe roles of harsh and fluctuating conditions in the dynamics of ecological communitiesAmerican Naturalisty 150f5519-553i& Harsh conditions (e.g., mortality and stress) reduce population growth rates directly; secondarily, they may reduce the intensity of interactions between organisms. Near-exclusive focus on the secondary effect of these forms of harshness has led ecologists to believe that they reduce the importance of ecological interactions, such as competition, and favor coexistence of even ecologically very similar species. By examining both the costs and the benefits, we show that harshness alone does not lessen the importance of species interactions or limit their rote in community structure. Species coexistence requires niche differences, and harshness does not in itself make coexistence more likely. Fluctuations in environmental conditions (e.g., disturbance, seasonal change, and weather variation) also have been regarded as decreasing species interactions and favoring coexistence, but we argue that coexistence can only be favored when fluctuations create spatial or temporal niche opportunities. We argue that important diversity-promoting roles for harsh and fluctuating conditions depend on deviations from the assumptions of additive effects and linear dependencies most commonly found in ecological models. Such considerations imply strong roles for species interactions in the diversity of a community. 343-+$://000166011500015 Chesson, P.4.Mechanisms of maintenance of species diversity.(Annual Review of Ecology and SystematicsTNcoexistence; competition; predation; niche; spatial and temporal variation forest architecture hypothesis; predator-mediated coexistence; randomly varying environments; ess germination strategies; tropical rain-forests; variable environment; ecological communities; interspecific competition; apparent competition; limiting similarity{The focus of most ideas on diversity maintenance is species coexistence, which may be stable or unstable, Stable coexistence can be quantified by the long-term rates at which community members recover from low density. Quantification shows that coexistence mechanisms function in two major ways: They may be (a) equalizing because they tend to minimize average fitness differences between species, or (b) stabilizing because they tend to increase negative intraspecific interactions relative to negative interspecific interactions. Stabilizing mechanisms are essential for species coexistence and include traditional mechanisms such as resource partitioning and frequency-dependent predation, as well as mechanisms that depend on fluctuations in population densities and environmental factors in space and time. Equalizing mechanisms contribute to stable coexistence because they reduce large average fitness inequalities which might negate the effects of stabilizing mechanisms. Models of unstable coexitence, in which species diversity slowly decays over time, have focused almost exclusively on equalizing mechanisms. These models would be more robust if they also included stabilizing mechanisms, which arise in many and varied ways but need not be adequate for full stability of a system. Models of unstable coexistence invite a broader view of diversity maintenance incorporating species turnover.Annu. Rev. Ecol. Syst. 200031385NX ANNU REV ECOL SYSTISI:000166011500015(!Chesson, P Pacala, S Neuhauser, Cl 20014.Environmental niches and ecosystem functioning $Kinzig, A Pacala, S Tilman, Da2+The Functional Consequences of Biodiversity Princeton, New JerseyJ Princeton University Press213-245H 35-47$://000086051900005.Cioccale, M. A.sVOClimatic fluctuations in the Central Region of Argentina in the last 1000 yearsnQuaternary InternationalSeveral lines of evidence concur to explain the climatic fluctuations that occurred in the central region of Argentina during the last millennium. The investigation was advanced in two ways: on the one hand, a geographic model was elaborated; and on the other, a temporal sequence for various climatic situations was developed. During the last 1000 yr, two significant events related to global changes occurred: the Medieval Warm Period (MWP) and the Little Ice Age (LIA). The Medieval Warm Period was characterized by a humid and warm climate in the plains and recession of the Andean glaciers. In contrast, during the Little Ice Age the plains had temperate, semi-arid to arid climates, and Andean glaciers advanced. In the western region, the fluvial-lacustrine systems were more extensive during cold events (LIA) and contracted during warm events (MWP). In contrast, in the eastern region the fluvial- lacustrine systems showed a diminution during cold events and increased their extent during warm episodes. During the LIA, the occurrence of two cold pulses separated by an intermediate period has been established. The first cold pulse extended from the beginning of the XV century to the end of the XVI century; the second cold pulse (the main one) began at the beginning of the XVIII century and lasted until the beginning of the XIX century. Both cold pulses can be related to the Sporer and Maunder Minimums respectively. These climatic changes modified the landforms, influenced the vegetation distribution and were one of the main factors for control of human activities during the last 1000 yr. (C) 2000 Elsevier Science Ltd and INQUA. All rights reserved. Quat. Int. 199962296XK QUATERN INTISI:000086051900005mperature in intercanopy patches was greater than in canopy patches between May and September, by as much as 10degreeC, while soil temperatures in intercanopy patches were lower than in canopy patches during colder parts of the day in the fall and winter months. Equations for soil drying rates for sandy loam soil samples were determined in laboratory experiments over a range of temperatures and soil water contents. Drying rates were disproportionately greater at high soil moisture and high soil temperature. Intercanopy patches were predicted to dry-more than canopy patches for days in April through September by as much as 2% volumetric soil water content per day. The difference between patches was amplified at lower soil water contents when expressed as soil water potential, which more directly determines plant-available water. Our results quantify the effects of woody plants on the microclimate with respect to soil temperature and evaporation, which in turn affect herbaceous and woody plants by modifying factors such as germination, the potential for facilitation, and the amount of plant-available water. 1010-10170$://000077495000013>7Breshears, D. D. Nyhan, J. W. Heil, C. E. Wilcox, B. P.oEffects of woody plants on microclimate in a semiarid woodland: Soil temperature and evaporation in canopy and intercanopy patches.'International Journal of Plant SciencesaInt. J. Plant Sci. 1998 Nov 1596A148FU INT J PLANT SCI2ISI:000077495000013,gJe }Genetic and maternal influences on life history plasticity in response to photoperiod by milkweed bugs (Oncopeltus fasciatus) Groeters, F. R. Dingle, H.American Naturalist 129 3o332-346t 1987 Using two geographically adjacent, temperate-zone populations of the milkweed bug Oncopeltus fasciatus, we employed a split-family experimental design to estimate genotype-environment interaction (g-e) and across-environment genetic correlations for two female life history characters, age at first reproduction and fecundity over the first 5 days of reproduction. For each of a number of families, some sisters were reared in a short-day environment (LD 11:13, 23 degree C) and others in a long-day environment (LD 14:10, 23 degree C); the environments were chosen to simulate the temporal environmental heterogeneity experienced by lineages of temperate-zone bugs. Bugs normally exhibit a plastic response to such heterogeneity by delaying the onset of reproduction in short days relative to that in long days, the delay providing time for migration to occur. In one population, there is significant variation caused by g-e for the age at first reproduction, indicating the existence of genetic variance for plasticity in this character. This result suggests that there is potential for the evolution of phenotypic plasticity in the timing of reproduction. In each population, however, there is a positive genetic correlation between the timing of reproduction in the long- and short-day environments, implying that the evolution of life history in one environment can influence evolution in the alternative environment. In particular, the potential rate of evolution toward a joint optimum of early reproduction in long days and delayed reproduction in short days will be less than if the across-environment correlation were zero or negative. During the attainment of equilibrium at the joint optimum, selection can prevent optimization of the age at first reproduction and maintain genetic variation for this character within environments. However, a maternal effect that prevents long delays in the onset of reproduction from occurring in two successive generations of bugs provides a means for lessening the influence of selection in the alternative environments. This maternal effect also ensures that a spring generation of bugs does not exhibit an inappropriate diapause. For fecundity, one population has a positive across-environment genetic correlation, indicating that performance generalizes across environments for this fitness component. In the other population, primarily because of the performance of one sibship, a negative across-environment correlation implies that performance is specialized. The populations also differ in life history. The basis for such microgeographic differentiation, the first recorded for migratory temperate-zone populations of this species, may be sampling, differential migration, or adapation. More work is necessary to distinguish among these hypotheses. Consistent with previous studies, we found high levels of genetic variation for life history characters within populations and, in general, a lack of genetic correlation between the age at first reproduction and fecundity.RKSeedling growth and root morphology of plants with different life-historiesf.(Gross, K. L. Maruca, D. Pregitzer, K. S.New Phytologistw 120.4E535-542r 1992B;Differences among plant species in morphology and patterns of growth are assumed to influence their ability to acquire resources and, consequently, their competitive ability. Despite the acknowledged importance of below-ground resources for plant growth, our knowledge of species differences in root morphology of non-agricultural plants is limited. Comparisons of root morphology, growth rate and topology of seedlings of 12 herbaceous plant species that occur in early to mid-successional fields revealed significant differences among species that were largely related to life history. Annuals grew faster and produced longer and more branched roots than did biennials and perennials. Only among the annuals was there a positive correlation between seed mass and root growth. The grasses allocated proportionately more biomass to roots than the dicots, but did not differ in root length or branching pattern. As seedlings, all 12 species exhibited a herringbone topology; although after 10 d there were significant differences in topology between annuals and perennials indicating that the annuals had begun to develop a more dichotomously branched root system. The possible effects of early differences in root morphology among life-history types in acquiring soil resources in heterogeneous and successional environments are discussed. <5Gross, Katherine L. Peters, Andrew Pregitzer, Kurt S. 1993d^Fine root growth and demographic responses to nutrient patches in four old-field plant species Oecologia951 61-64rkProliferation of roots in a nutrient patch can occur either as a result of an increase in root length (morphological response) or by a change in root birth or death rates (demographic responses). In this study we attempted to distinguish between these two mechanisms of response to nutrient patches and to compare the responses of four old-field plant species (two annuals, two perennials). For all four species combined, there were significant increases in root numbers and root length in fertilized patches. Root proliferation in fertilized patches was largely due to increased birth (= branching) rates of new roots. However, there was also a significant increase in root death rates in the fertilized patches which reduced the magnitude of the increase in net root numbers. Plots for individual species suggested they differed in the magnitude and timing of root proliferation in fertilized patches due to differences in root birth and death rates. However, because of the limited sample size in this study, there was only a marginally significant difference among species in root birth rates, and no difference in death rates. Further studies are currently under-way to better quantify species differences in the demographic mechanism, as well as magnitude, of response to nutrient patches and if this would affect the ability to exploit small-scale heterogeneity in soil resources.a\VDynamics of competition in a variable environment: Experiments with two diatom species Grover, J. P.iEcology 692A408-417  1988~The hypothesis that environmental variability promotes phytoplankton species diversity was invesitgated experimentally using phosphorus-limited continuous cultures. Variability was introduced as a series of phosphorus pulses delivered at 8-d intervals. The growth of Synedra sp. and Fragilaria crotonensis was examined in cultures of natural phytoplankton, in cultures containing both species but not other phytoplankton, and in monocutures. In cultures inoculated with comparable densities of both species, Synedra was competitively dominant to Fragilaria in both constant and varying cultures, but the rate of competitive exclusion was slower in varying cultures. Synedra was able to invade both constant and varying monocultures of Fragilaria when injected into those monocultures, but Fragilaria could invade monocultures of Synedra. In monocultures, the average yield of Fragilaria was increased by variability, but that of Synedra was not. During pulses, Fragilaria had a higher uptake rate per cell than Synedra, but the two species had similar specific uptake rates. Previous studies (Tilman 1981) suggest that Synedra is a better steady-state competitor for phosphorus than Fragilaria. Although Fragilaria may be better able to exploit phosphorus pulses than Synedra, this advantage is apparently too weak to prevent the dominance of Synedra under the regime of pulsed phosphorus supply used here.f ^}Do woody plants operate near the point of catastrophic xylem dysfunction caused by dynamic water stress? Answers from a model Tyree, M. T. Sperry, J. S.Plant Physiology883a574-580t 1988ztWe discuss the relationship between the dynamically changing tension gradients required to move water rapidly through the xylem conduits of plants and the proportion of conduits lost through embolism as a result of water tension. We consider the implications of this relationship to the water relations of trees. We have compiled quantitative data on the water relations, hydraulic architecture and vulnerability of embolism of four widely different species: Rhizophora mangle, Cassipourea, elliptica, Acer saccharum, and Thuja occidentalis. Using these data, we modeled the dynamics of water flow and xylem blockage for these species. The model is specifically focused on the conditions required to generate 'runaway embolism', whereby the blockage of xylem conduits through embolism leads to reduced hydraulic conductance causing increased tension in the remaining vessels and generating more tension in a vicious circle. The model predicted that all species operate near the point of catastrophic xylem failure due to dynamic water stress. The model supports Zimmermann's plant segmentation hypothesis. Zimmermann suggested that plants are designed hydraulically to sacrifice highly vulnerable minor branches and thus improve the water balance of remaining parts. The model results are discussed in terms of the morphology, hydraulic architecture, eco-phsiology, and evolution of woody plants. Tyree, M. T. Sperry, J. S. 198981Vulnerability of xylem to cavitation and embolismD>Annual Review of Plant Physiology and Plant Molecular Biology40 19-38ZSplant anatomy. xylem . cavities in trees. water stress. freezing . stress response.t@9Valentini, R. Mugnozza, G. E. Scarascia Ehleringer, J. R. 1992b[Hydrogen and carbon isotope ratios of selected species of a Mediterranean macchia ecosystemFunctional Ecology66627-631Various woody species belonging to the mediterranean macchia ecosystem have been studied in respect of their strategies of water resources acquisition and water-use efficiencies, using stable isotopes analysis. Evergreen species seem to depend more on rain-water utilization than the deciduous ones, which utilize ground-water almost exclusively. This pattern is paralleled by the distribution of delta-13 C values which show a greater water-use efficiency for the evergreen species vs the deciduous ones.RLHolocene vegetation of the Hornaday Mountains of northwestern Sonora, MexicoD>Van Devender, T. R. Burgess, T. L. Felger, R. S. Turner, R. M.>7Proceedings of the San Diego Society of Natural History.2h 1-19 1990The 94 taxa of plants identified from 10 packrat (Neotoma sp.) middens from 240 to 260 m elevation in the granitic Hornaday Mountains provide an excellent history of vegetation and climate for the last 10,000 yr B.P. in the Pinacate Region of northwestern Sonora, Mexico. Rocky slopes in this arid area (ca. 120 mm/yr precipitation) in the Lower Colorado River subdivision of the Sonoran Desert have supported desert scrub dominated by Encelia farinosa (brittlebush), Larrea divaricata (creosotebush), and Carnegiea gigantea (saguaro) throughout the Holocene, Community composition changed continuously on both long (10,000 yrs)and shorter time scales. Ephedra nevadensis (Mormon tea), codominant with Encelia farinosa, Larrea divaricata, and Carnegiea gigantea in early Holocene (8910 to 10,000 yr B.P.) samples, and rare Juniperus cf. californica (California juniper) imply a continuation of the cool summers and primarily winter rainfall of the late Wisconsin glacial. Middle Holocene (4430 to 8660 yr B.P.) samples contained trees and shrubs such as Acacia greggii (catclaw acacia), Cercidium floridum (blue paloverde), and Prosopis velutina (velvet mesquite) that are now restricted to relatively mesic washes, implying summer rainfall greater and freezes more frequent than today. Late Holocene (1720 to 2320 yr B.P.) samples dominated by Encelia farinosa and Olneya tesota (ironwood) with C. microphyllum (foothills paloverde) are similar to present Sonoran desert vegetation and imply a relatively modern biseasonal rainfall regime with the warmest winters of the Holocene. In the last 1700 years, O. tesota disappeared as Ambrosia deltoidea (triangleleaf bursage), Bursera microphylla (elephant tree), Jatropha cuneata (cuneate limberbush) and L. divaricata appeared or increased at the sites. The modern vegetation of the Hornaday Mountains is as sparse and the climate is as hot and dry as at any time in the last 10,000 years. The variability in community composition through the Holocene was similar in magnitude to the modern variability among different midden sites and appears to reflect the highly variable climate. Responses of individual species to climatic fluctuations on scales from years to millennia may prevent desert scrub communities from ever reaching equilibrium. Disjunct populations of Artemisia ludoviciana (white sage), Berberis haematocarpa (barberry), Opuntia chlorotica (silver dollar cactus), Rhus aromatica (skunk bush), Salvia mohavensis (Mohave sage), and Stipa speciosa (desert needlegrass) above 650 to 800 m elevation on the north side of Pinacate Peak are relicts from ice age woodland expansion into the Sonoran Desert. In the late Wisconsin and early Holocene, Pinus monophylla (singleleaf pinyon), several species of Juniperus (junipers), and Yucca brevifolia (Joshua tree) were widespread down to 460 m elevation in the Tinajas Altas Mountains of southwestern Arizona. An 18,700 yr B.P. radiocarbon date on twigs of Larrea divaricata documents its regional presence at 330 m in a full-glacial juniper woodland dominated by J. californica and Y. brevifolia. Creosotebush desert scrub samples with abundant J. californica have been found as low as 240 m in the Butler Mountains just to the west. Ice age desert scrub dominated by Larrea divaricata without woodland plants was apparently restricted to below 250-300 m in the Lower Colorado River valley and the Gran Desierto. A lowering of sea level of about 100 m during the Wisconsin exposed additional lowlands around the head of the Gulf of California. Mohave Desert plants such as Coleogyne ramosissima (blackbrush), Salvia mohavensis, Y. brevifolia and Y. whipplei (Whipple yucca) expanded their ranges southward and to lower elevations. Dominants of arid central Baha California communities such as Fouquieria columnaris (boojum tree) and Pachycormus discolor (elephant tree) were conspicuously absent from low-elevation Lower Colorado River Valley and Gran Desierto midden assemblages. Arid desert scrub communities have probably been in these Sonoran Desert lowlands throughout the Pleis ocene.Hd Lauenroth1997 Lauenroth1997r Lauenroth1998q Lauenroth1998 Lauenroth1998 Lauenroth1998 Lauenroth1998 Lauenroth1998 Lauenroth1998_ Lauenroth1999 Lauenroth1999 Lauenroth2000 Lauenroth2000 Lauenroth2001Lauenroth William19991 Laurie19944h Lavelle2000 Lavorel1995 Law1999K Le Houerou1984I Le Houerou1988 Leary1995 Leavitt1999 Leffler2001 Leffler2002 Lenz20010o Lessard1998 Levins1979 Lhomme1998 Li2001 Li20010 Li2001 Lilly2001 Lin1996 Lindemann2000 Link19911 Linton20011 Lipson1999 Liu2000= Livingston19979DLo Gullo1996k Lodge2001Loescher1992S Lopez1991D Loreti19966i Loreti20012 Ludwig1989 Ludwig19959@ Lux1999@ Luxova1999 M.20002 Mabry1997  MacDonald1999C Magnani1995F Magnani1998? Mahoney1998 Mandujano2001 Manwaring1996 Marion19877^ Martens1997 Martens1997  Martens2001; Marticorena1998< Martinez Meza1996e Maruca1992; Matthei1998*Mausberg19931 Mayor1994 Mayor1996H McCully1999G McCully1999 McGraw1996 McIvor1995q McMichael1993p McMichael1993 McMichael1993r McMichael1994 McMichael1995s McMichael1996t McMichael1998 McPherson1997 McPherson1999 McPherson2000 Meko19988 Meyer1997^ Meyer1997 Meyer1997 Miao1990 Miao1991 Miao1991, Midgley1994 Midgley Guy2000V Midwood1998v Milchunas1987G Milchunas1995 Milchunas1998 Milchunas2001Milchunas Daniel1999 Milton2001aMilton Suzanne2000 Minchin1987 Mitsunaga1997~ Modise19961 Moen19949[Moen Ron1994? Moncrieff1994 Monson19999y Montana1995z Montana1996{ Montana1998 Montaa2001 Mooney19921D Mooney19966{ Mooney19961 Mooney19961 Mooney19981 Moore1977,Moorhead1991fMugnozza19922; Munoz1998 Murphy199891 Mustart1994 Musters2000 Myers1997A Nardini1999 Neilson2000 Neuhauser2001l Nguyen19969+ Nichols1994M Nicholson1994 Nilsen19901K Nobel1987L Nobel1987M Nobel1987N Nobel1987O Nobel1991S Nobel1991P Nobel1992Q Nobel1992R Nobel1992W Nobel1992X Nobel1992Y Nobel1993v Nobel1994V Nobel Park1992Z Nobel Park1994 Novoplansky19972 Novoplansky20013 Novoplansky2001 Nowak1999 Nowak2000 Noy-Meir1973WNoy-Meir1985XNoy-Meir1998 Nungesser1994[Nungesser Martha1994 Nyhan19984 O'Leary1995D Oesterheld1996(i Oesterheld2001 Ojha1996 Ojima1996 Ojima Dennis1995 Ojima Dennis19969<Olivares1999c<Olivares1999c5Olsvig Whittaker2001 Or20020 Oren19979 Oren1998 Oren20000 Orr2001 Ostfeld2000 Owen2001Ozier-Lafontaine2000 Pacala20011 Pachepsky19950 Pachepsky1997 Palma1992 Palma1998 Pandey1999( Panetta1988w Parton1988t Parton19929 Parton19979 Parton20000\ Paruelo1995 Paruelo1995 Paruelo1996 Paruelo1996^ Paruelo1998q Paruelo1998 Paruelo1998_ Paruelo1999[ Paruelo2000 Paruelo2000k Pascual2001 Passioura1996 Pataki1997d Paul19929 Peck19949 Pelaez1994( Pelaez1996 Pendall1999 Peng1996ag Peters1993rPhillips19919Phillips1992Phillips19929Phillips19922Phillips19933Phillips19944Phillips1995Phillips1996Phillips1998Phillips2001k Pielke20011 Piorkowski1995(c Piorkowski1997 Pitman20011A Pitt19999k Pizer2001) Pockman19934 Pockman19953 Pockman19965 Pockman1997 Pockman2000< Pollastri1999 Porporato1999 Porporato2000 Porporato2000 Porporato2000 Porporato2001 Porporato2001 Porporato2001 Porporato2001 Prasse2001d Pregitzer1992e Pregitzer1992g Pregitzer1993h Pregitzer1993i Pregitzer1993j Pregitzer1993k Pregitzer1995l Pregitzer1996m Pregitzer1996n Pregitzer1996o Pregitzer1998 Price1997 Primack1991 Primack1991L Prince1998(k Pringle2001 Pyke19981 Quade2000 Rai1996 Rajagopalan2000hRandlett19933 Raven1999 Ray2000 Rech20000 Redmond1999 Reed1996 Reichenberger1997  Reid1999k Reid20010,Reynolds1991+Reynolds199221Reynolds19944(Reynolds19960Reynolds1997Reynolds19999Reynolds1999 Reynolds20000Reynolds2000 Reznick1993 Rice19971 Rice1997193 Rice19971 Rice19971 Rice19971 Rice19971 Rice1997193 Rice199711993 Rice19971 Rice199711993 Rice19971 Rice19971 fl<5Dickmann, Donald I. Nguyen, Phu V. Pregitzer, Kurt S.l 1996Effects of irrigation and coppicing on above-ground growth, physiology, and fine-root dynamics of two field-grown hybrid poplar clones$Forest Ecology and Management80 1-3h163-174rTo determine the effects of irrigation and coppicing on above and below-ground growth dynamics, a plantation of Populus times euramericana cv. 'Eugenei' and Populus tristis times Populus balsamifera cv. 'Tristis 1' was established in May 1984 on a level site in East Lansing, MI, USA. Supplemental water in the form of drip irrigation was applied to half the trees beginning the first growing season. All trees were cut down in March 1988 and the stumps allowed to coppice. Pre- and post-coppice height and diameter growth of 'Eugenei' always exceeded that of 'Tristis', and the former clone showed a positive response to irrigation, whereas the latter did not. The greater growth of 'Eugenei' was primarily due to its full use of the growing season. Post-coppice rates of photosynthesis were not affected by irrigation in either clone, but stomatal conductances were reduced in non-irrigated trees. Analysis of microvideo images taken in minirhizotron tubes buried in the soil close to the trees showed that 'Tristis' produced a greater length and number of fine roots in the top 30 cm of soil than 'Eugenei', regardless of treatment. Irrigated trees consistently produced more fine roots at 0-30 cm soil depth than non-irrigated trees only in 'Eugenei', but both clones showed greater fine-root production in non-irrigated trees at 30-100 cm. Both clones also showed substantial fine-root production in the spring immediately following coppicing, with no evidence of a shock-induced dieback of roots. The root systems of these two poplar genotypes apparently contain sufficient carbon and nitrogen reserves to fuel a spring flush of fine-root growth, even though the tops have been severed during the dormant season.F?Distel, R. A. Pelaez, D. V. Boo, R. M. Mayor, M. D. Elia, O. R. 1996Growth of Prosopis caldenia seedlings in the field as related to grazing history of the site and in a greenhouse as related to different levels of competition from Stipa tenuis"Journal of Arid Environments323n251-257oIn temperate semi-arid rangelands of Argentina grazing by domestic livestock is apparently linked with the conversion of open grasslands to dense shrublands. Prosopsis caldenia and Stipa tenuis are dominant species of the shrub and grass life-forms, respectively. Research was conducted to determine the growth responses of P. caldenia seedlings to grazing history of the site and competition from S. tenuis, at different levels of water availability. The Prosopsis seedlings grew better (p lt 0.05) in long-term protected than in long-term grazed sites, and at low rather than high levels of competition from S. tenuis. Both overgrazing and high levels of grass competition may limit the establishment of P. caldenia seedlings in grasslands, particularly under conditions of water shortage.d0*Dodd, M. B. Lauenroth, W. K. Welker, J. M. 1998piDifferential water resource use by herbaceous and woody plant life-forms in a shortgrass steppe community Oecologia 1174504-512oWe conducted a study to test the predictions of Walter's two-layer model in the shortgrass steppe of northeastern Colorado. The model suggests that grasses and woody plants use water resources from different layers of the soil profile. Four plant removal treatments were applied in the spring of 1996 within a plant community codominated by Atriplex canescens (a C4 shrub) and Bouteloua gracilis (a C4 grass). During the subsequent growing season, soil water content was monitored to a depth of 180 cm. In addition, stem and leaf tissue of Atriplex, Bouteloua and the streamside tree Populus sargentii were collected monthly during the growing seasons of 1995 and 1996 for analysis of the delta18O value of plant stem water (for comparison with potential water sources) and the delta13C value of leaves (as an indicator of plant water status). Selective removal of shrubs did not significantly increase water storage at any depth in the measured soil profile. Selective removal of the herbaceous understory (mainly grasses) increased water storage in the top 60 cm of the soil. Some of this water gradually percolated to lower layers, where it was utilized by the shrubs. Based on stem water delta18O values, grasses were exclusively using spring and summer rain extracted from the uppermost soil layers. In contrast, trees were exclusively using groundwater, and, the consistent delta13C values of tree leaves over the course of the summer indicated no seasonal changes in gas exchange and therefore minimal water stress in this life-form. Based on anecdotal rooting-depth information and initial measurements of stem water delta18O, shrubs may have also had access to groundwater. However, their overall delta18O values indicated that they mainly used water from spring and summer precipitation events, extracted from subsurface soil layers. These findings indicate that the diversity of life-forms found in this shortgrass steppe community may be a function of the spatial partitioning of soil water resources, and their differential use by grasses, shrubs, and trees. Consequently, our findings support the two-layer model in a broad sense, but indicate a relatively flexible strategy of water acquisition by shrubs.t :4Wan, Changgui Sosebee, Ronald E. McMichael, Bobby L. 1998RKWater relations and root growth of two populations of Gutierrezia sarothrae,%Environmental and Experimental Botany 391t 11-206We hypothesise that genotypic differences in transpiration and root growth in the southern and northern populations of Gutierrezia sarothrae are driven by growing season vapour pressure deficit (VPD) and that ecotypic differentiations are linked to corresponding variations in tissue and leaf water relations. Seedlings from an Idaho (ID) and a Texas (TX) seed source were grown either in an open nursery (full sunlight) or under shade. There were no population differences in transpiration, root growth and tissue water relation parameters when the plants were grown under the shade. However, significant population differences were observed in the plants grown in the open where VPD was substantially higher. Transpiration in the TX population increased twice as rapidly as the ID population in response to rising potential evapotranspiration (PET). In addition, the TX plants grew longer and larger lateral roots than the ID plants when both populations were grown in the open. Moreover, the TX plants had more elastic cell walls (lower bulk cell elastic modulus, E) and were able to maintain leaf turgor at lower relative water content than the ID plants when grown in the open. Similar changes in tissue water relation parameters were observed in the New Mexico (NM) and the ID seedlings subjected to soil water deficit. Cell elastic modulus (E), relative water content at zero turgor (RWC0) and apoplastic water fraction (Wa) were significantly lower in the NM population than in the ID population in response to soil water deficit, suggesting a higher turgor maintenance ability in the NM population. RWC0 was highly correlated with E in the NM (r = 0.92, P < 0.0001) and the TX (r = 0.91, P < 0.05) plants, but poorly correlated with osmotic potential at full turgor (II100), indicating significance of cell wall elasticity in turgor maintenance. Rigid cell walls and poor turgor maintenance ability in the ID plants subjected to drought may lead to restricted growth, which reflected hydraulic limitation of plant root systems. Higher water use efficiency (WUE) and higher apoplastic water fraction in the ID population reflected conservative water use at the leaf and tissue levels, which was consistent with the water use pattern at the canopy level. RKWand, Stephanie J. E. Esler, Karen J. Rundel, Philip W. Sherwin, Heather W. 1999A preliminary study of the responsiveness to seasonal atmospheric and rainfall patterns of wash woodland species in the arid Richtersveld Plant Ecology- 142 1-2e149-160ab[Seasonal carbon and water relations were compared among seven tree or shrub wash woodland species in the winter rainfall desert of the Richtersveld National Park, South Africa. Plants were generally aseasonal with respect to gas exchange, but responsive to rainfall events with respect to water relations and phenology. Relatively narrow annual ranges in potential evapotranspiration due to the maritime influence could explain why these plants respond more to fluctuations in water acquisition potential than to evaporative demand. Two species were summer-deciduous, but one of them (Ozoroa concolor) responded to aseasonal summer rainfall by leafing out and flowering. These two species had high shoot xylem water potentials when in leaf. All other species were sclerophyllous evergreens with low water potentials, particularly the shallow-rooted shrub Zygophyllum prismatocarpum, and Boscia albitrunca which may have a different rooting pattern to the other phreatophytes. The latter specieswas also unique due to its high leaf nitrogen contents, photosynthetic rates and stomatal conductances, despite very low leaf water potentials. Leaf stable carbon isotope composition (13deltaC) varied between species (-22 to -27permill), but was lower than the mean for arid regions worldwide. The values indicated moderately high levels of water use efficiency, but a less conservative strategy in two species, including Boscia albitrunca. The affinities of these species to summer rainfall biomes, their apparent decline in the western arid regions in recent geological history following aridification, and their absence southwards in the winter rainfall regions, suggest that these wash species rely on sporadic summer rainfall events to some extent. They may be at risk if predicted increases in temperature and changes in rainfall patterns alter their effective moisture availability.ix6 2457-2470w$://000171211300002m>7Vrugt, J. A. van Wijk, M. T. Hopmans, J. W. Simunek, J. ZTOne-, two-, and three-dimensional root water uptake functions for transient modelingWater Resources Researchrainfall-runoff models; soil-water; hydraulic conductivity; unsaturated flow; solute transport; extraction; irrigation; equation; growth; plantsAlthough solutions of multidimensional transient water flow can be obtained by numerical modeling, their application may be limited as root water uptake is generally considered to be one- or two-dimensional only. This is especially the case for trees. The first objective of this paper is to test the suitability of a three-dimensional root water uptake model for the simultaneous simulation of transient soil water flow around an almond tree. The soil hydraulic and root water uptake parameters were optimized by minimizing the residuals between measured and simulated water content data. Water content was measured in a three-dimensional grid around a sprinkler- irrigated almond tree for a 16 day period, following irrigation. A second objective was to compare the performance and results of the three-dimensional flow model with one- and two-dimensional root water uptake models. For this purpose, measured water contents were aggregated in the x and y direction in the one-dimensional case and in the radial direction for the two-dimensional uptake model. For the estimation of root water uptake model parameters a genetic algorithm was used to estimate the approximate global minimum of the parameter space, whereas final parameters were determined using the Simplex optimization algorithm. With the optimized root water uptake parameters, simulated and measured water contents during the 16-day period were in excellent agreement for all root water uptake models. Most significantly, the spatial variation in flux density below the rooting zone decreased when reducing multidimensional root water uptake to fewer dimensions, thereby justifying the proposed multidimensional approach.Water Resour. Res. 2001 Oct3710476DQ WATER RESOUR RESISI:000171211300002153-167$://A1996UQ56000014 $Walker, B. H. Langridge, J. L.\VModelling plant and soil water dynamics in semi-arid ecosystems with limited site dataEcological Modelling Ecol. Model. 1996 Jun.87 1-3aUQ560 ECOL MODELISI:A1996UQ56000014356-373$://000170394100003 2+Walker, J. P. Willgoose, G. R. Kalma, J. D. One-dimensional soil moisture profile retrieval by assimilation of near-surface measurements: A simplified soil moisture model and field application"Journal of Hydrometeorologyo>7water; hydrology; parameterization; vegetation; terraind The Kalman filter assimilation technique is applied to a simplified soil moisture model for retrieval of the soil moisture profile from near-surface soil moisture measurements. First, the simplified soil moisture model is developed, based on an approximation to the Buckingham-Darcy equation. This model is then used in a 12-month one-dimensional field application, with updating at 1-, 5-, 10-, and 20- day intervals. The data used are for the Nerrigundah field site, New South Wales, Australia. This study has identified (i) the importance of knowing the depth over which the near-surface soil moisture measurements are representative (i.e., observation depth), (ii) soil porosity and residual soil moisture content as the most important soil parameters for correct retrieval of the soil moisture profile, (iii) the importance of a soil moisture model that represents the dominant soil physical processes correctly, and (iv) an appropriate forecasting model as far more important than the temporal resolution of near-surface soil moisture measurements. Although the soil moisture model developed here is a good approximation to the Richards equation, it requires a root water uptake term or calibration to an extreme drying event to model extremely dry periods at the field site correctly.iJ. Hydrometeorol.  20012e4n461ZQ J HYDROMETEOROLsISI:000170394100003 H. Walter E. Stadelmann. 1974<6A new approach to the water relations of desert plants R. BrownDesert Biology Volume 2 New York Academic Press2213-310ois and g were more affected by the removal of four deep roots 30 cm below the surface than by the removal of four shallow roots in the top 30 cm of soil. The soil water extraction patterns in Gutierrezia plants grown in 60-cm soil columns were determined for the upper 30 cm and 30 to 60 cm of soil. Water extraction was significantly greater (P lt .05) from the wetter, lower section of the soil column than from the drier, upper column. Higher photosynthesis (138%, P lt .05), transpiration (154%, P lt .01), stomatal conductance (223%, P lt .01), and canopy development (340%, P lt .05) were observed in plants with deep roots growing beyond the pots than well-watered, pot-grown plants without deep roots. This was related to higher hydraulic conductance, which in turn appeared enhanced by the greater ratio of deep root length to total root length. Stomatal conductance, photosynthesis, and canopy development in Gutierrezia may be regulated by hydraulic capacity, which is determined largely by deep root development. $Jackson, R. B. Caldwell, M. M. 1994jdBelowground competition in the sagebrush-steppe: Experimental evidence and conclusions from modeling American Journal of Botany816 36\VJackson, R. B. Canadell, J. Ehleringer, J. R. Mooney, H. A. Sala, O. E. Schulze, E. D. 1996D>A global analysis of root distributions for terrestrial biomes Oecologia 1083a389-411o Understanding and predicting ecosystem functioning (e.g., carbon and water fluxes) and the role of soils in carbon storage requires an accurate assessment of plant rooting distributions. Here, in a comprehensive literature synthesis, we analyze rooting patterns for terrestrial biomes and compare distributions for various plant functional groups. We compiled a database of 250 root studies, subdividing suitable results into 11 biomes, and fitted the depth coefficient beta to the data for each biome (Gale and Grigal 1987). beta is a simple numerical index of rooting distribution based on the asymptotic equation Y= 1-beta-d, where d = depth and Y = the proportion of roots from the surface to depth d. High values of beta correspond to a greater proportion of roots with depth. Tundra, boreal forest, and temperate grasslands showed the shallowest rooting profiles (beta = 0.913, 0.943, and 0.943, respectively), with 80-90% of roots in the top 30 cm of soil; deserts and temperate coniferous forests showed the deepest profiles (beta = 0.975 and 0.976, respectively) and had only 50% of their roots in the upper 30 cm. Standing root biomass varied by over an order of magnitude across biomes, from approximately 0.2 to 5 kg m-2. Tropical evergreen forests had the highest root biomass (5 kg m-2), but other forest biomes and sclerophyllous shrublands were of similar magnitude. Root biomass for croplands, deserts, tundra and grasslands was below 1.5 kg m-2. Root/shoot (R/S) ratios were highest for tundra, grasslands, and cold deserts (ranging from 4 to 7); forest ecosystems and croplands had the lowest R/S ratios (approximately 0.1 to 0.5). Comparing data across biomes for plant functional groups, grasses had 44% of their roots in the top 10 cm of soil (beta = 0.952), while shrubs had only 21% in the same depth increment (beta = 0.978). The rooting distribution of all temperate and tropical trees was beta = 0.970 with 26% of roots in the top 10 cm and 60% in the top 30 cm. Overall, the globally averaged root distribution for all ecosystems was beta = 0.966 (r-2 = 0.89) with approximately 30%, 50%, and 75% of roots in the top 10 cm, 20 cm, and 40 cm, respectively. We discuss the merits and possible shortcomings of our analysis in the context of root biomass and root functioning.Ty J{ *#Briones, O. Montana, C. Ezcurra, E. 1998ZTCompetition intensity as a function of resource availability in a semiarid ecosystem Oecologia 1163365-372 V OTwo field experiments were conducted using three dominant perennial species of the Chihuahuan Desert: Hilaria mutica (a tussock grass), Larrea tridentata (a microphyllous shrub) and Opuntia rastrera (a flat-stemmed succulent cactus). Two hypotheses concerning competition in arid plant communities were tested. (1) Marked resource partitioning with no interspecific competition could be expected since the three species belong to different life-forms, and that plant growth in deserts is basically limited by harsh environmental conditions. (2) Alternatively, resource scarcity (particularly water) will result in strong plant competition. In a 1-year removal experiment, water status and plant growth of the three species were monitored in twelve 10 m X 10 m plots randomized in three blocks and assigned to the following treatments: (a) removal of all species, except H. mutica; (b) removal of all species, except L. tridentata; (c) removal of all species, except O. rastrera, and (d) control without any manipulation. In a watering experiment, under two neighbourhood conditions (growing isolated or in associations of plants of the three species in plots of 20 m2), the water status of the three species and the growth of H. mutica and L. tridentata were studied for 32 days after an irrigation equivalent to 30 mm of rain, similar to a strong storm event at the site. In the removal experiment, where plants were free to capture water, no evidence of competition was observed. However, during the watering experiment, in which water was forced into the soil, competitive effects were observed. Associated individuals of L. tridentata had lower xylem water potentials and osmotic potentials (OPs) and produced shorter twigs and less leaves and nodes. Although less pronounced, neighbours also had a negative effect on the OP in O. rastrera. According to these results, the intensity of the interspecific competition for water seems to depend on the level of resource availability in the soil. Thus, the validity of the two hypotheses tested in this study also depends on the level of resources. Competition could be absent or very low in years of low precipitation, as in the year of this study (173 mm against a 25-year average of 264 mm). However, when soil water availability is high, e.g. following heavy rain, the negative interactions between species could be more intense.vPJBrooks, J. Renee Flanagan, Lawrence B. Buchmann, Nina Ehleringer, James R. 1997TNCarbon isotope composition of boreal plants: Functional grouping of life forms Oecologia 1103301-311i j cWe tested the hypothesis that life forms (trees, shrubs, forbs, and mosses; deciduous or evergreen) can be used to group plants with similar physiological characteristics. Carbon isotope ratios (delta-13C) and carbon isotope discrimination (A) were used as functional characteristics because delta-13C and DELTA integrate information about CO-2 and water fluxes, and so are useful in global change and scaling studies. We examined delta-13C values of the dominant species in three boreal forest ecosystems: wet Picea mariana stands, mesic Populus tremuloides stands, and dry Pinus banksiana stands. Life form groups explained a significant fraction of the variation in leaf carbon isotope composition; seven life-form categories explained 50% of the variation in delta-13C and 42% of the variation in DELTA and 52% of the variance not due to intraspecific genetic differences (n=335). The life forms were ranked in the following order based on their values: evergreen trees lt deciduous trees=evergreen and deciduous shrubs=evergreen forbs lt deciduous forbs=mosses. This ranking of the life forms differed between deciduous (Populus) and evergreen (Pinus and Picea) ecosystems. Furthermore, life forms in the Populus ecosystem had higher discrimination values than life forms in the dry Pinus ecosystem; the Picea ecosystem had intermediate DELTA values. These correlations between DELTA and life form were related to differences in plant stature and leaf longevity. Shorter plants had lower A values than taller plants, resulting from reduced light intensity at lower levels in the forest. After height differences were accounted for, deciduous leaves had higher discrimination values than evergreen leaves, indicating that deciduous leaves maintained higher ratios of intracellular to ambient CO-2 (c-i/c-a) than did evergreen leaves in a similar environment within these boreal ecosystems. We found the same pattern of carbon isotope discrimination in a year with above-average precipitation as in a year with below-average precipitation, indicating that environmental fluctuations did not affect the ranking of life forms. Furthermore, plants from sites near the northern and southern boundaries of the boreal forest had similar patterns of discrimination. We concluded that life forms are robust indicators of functional groups that are related to carbon and water fluxes within boreal ecosystems.Brown, J. R. Archer, S. 1990tnWater Relations of a Perennial Grass and Seedling Versus Adult Woody Plants in a Subtropical Savanna Texas Usa Oikos573366-3742Over 52 .+-. 16% (Mean .+-. SE) of the seeds of the arborescent legume Prosopis glandulosa var. glandulosa germinated within two weeks of dissemination in plots dominated by a perennial grass (Chloris cucullata) in July 1984 and 63 .+-. 7% of those germinating survived through September 1985. Our observations spanned a period of normal temperatures and below-normal precipitation, suggesting the apparent increased abundance of Prosopis on this site in recent times has probably not been episodic with regard to moisture. Over 60% of the herbaceous root biomass occurred in the upper 30 cm of soil. In contrast, tap roots of Prosopis seedlings had penetrated beyond 40 cm within 4 months of germination and their mean proportion of total biomass belowground increased from 0.27 .+-. 0.09 in May to 0.52 .+-. 0.15 in August. Net photosynthesis (Pn) and conductance (g) of Chloris were closely coupled to fluctuations in moisture in the upper soil horizons (< 30 cm), whereas Pn and g of one-year old P. glandulosa seedlings were correlated with soil moisture at depths between 30 and 90 cm. Among mature Prosopis plants, Pn and g varied as a function of soil moisture at depths > 90 cm. We conclude that rapid development of roots of Prosopis seedlings during their first year of growth apparently enhanced survival by enabling them to access soil moisture beyond the zone effectively utilized by grasses. On sites with a history of grazing, competition for water from herbaceous vegetation may not play a significant role in limiting establishment of P. glandulosa seedlings or the growth of mature plants in most years. Partitioning of soil moisture between P. glandulosa and grasses, achieved early in the life cycle of Prosopis, helps explain how this woody plant can successfully establish in grasslands.1 ,>7Cowling, R. M. Esler, K. J. Midgley, G. F. Honig, M. A." 1994f_Plant functional diversity, species diversity and climate in arid and semi-arid southern Africa"Journal of Arid Environments272141-1586This paper analyses data on growth form (functional) diversity, species richness and climate from nine Nama-Karoo/Desert sites (predominantly summer rain) and 10 Succulent Karoo sites (predominantly winter rain) along comparable gradients of total rainfall in arid and semi-arid southern Africa. Sites from the latter region were richer in total species and succulent species, but had lower growth form diversity. Contrary to the predictions of niche theory, the more heterogeneous rocky habitats did not have significantly more species or higher growth form diversity than adjacent loamy flats at the community level (25 m-2 plot). However, significantly more species were recorded in rocky than flat habitats at the site scale (10,000 m-2), probably reflecting the longer resource axes and higher internal beta diversity in the former habitat. Species richness showed a significant positive relationship with growth form diversity. This relationship was much tighter for the Nama-Karoo/Desert than Succulent Karoo sites indicating a higher incidence of functional redundancy (coexistence of species with similar growth forms) in the latter region. An index of climatic heterogeneity (reflecting the diversity of conditions suitable for plant growth) and measures of rainfall evenness throughout the year, emerged as the strongest predictors of species richness and growth form diversity. These results are consistent with predictions of classical niche theory. Sites with a high percentage winter rain as well as those with evenly distributed rain (with a predictable winter component) had the highest cover of succulents and Mesembryanthema. In Succulent Karoo communities functional redundancy is associated with a high number of relatively short-lived leaf succulent shrub species, belonging largely to the Mesembryanthema. However, owing to their differential response to rare but catastrophic drought-induced mortality, the diversity within this functional type ensures long-term stability of community-level growth form mix or functional diversity.^WSpecies diversity; functional diversity and functional redundancy in fynbos communities4>8Cowling, R. M. Mustart, P. J. Laurie, H. Richards, M. B. 1994& South African Journal of Science906i333-337h Using Smart Source Parsing~"b\Ecophysiological differences among juvenile and reproductive plants of several woody species& Donovan, L. A. Ehleringer, J. R. Oecologia 864A594-597. 1991Photosynthetic and water relations characteristics of small juvenile and large reproductive plants were investigated during one growing season for four woody species native to Red Butte Canyon, Utah, USA: Acer negundo, Artemisia tridentata, Chrysothamnus nauseosus, and Salix exigua. For all species, juvenile plants differed from reproductive plants in at least one of the following characters: water potential, stomatal conductance, photosynthetic rate, or water-use efficiency. Late in the growing season, mortality occurred within juvenile plants (apparently due to a lack of water), but not within reproductive plants. The observed differences between juvenile and reproductive classes are discussed in terms of environment, development, and mortality selection.y^WContrasting water-use patterns among size and life-history classes of a semi-arid shrube& Donovan, L. A. Ehleringer, J. R.Functional Ecology64482-488o 19920)Ecophysiological characteristics of plants from different size and life-history classes were investigated in a field population of Chrysothamnus nauseosus. Juvenile plants ( gt 1-year-old but pre-reproductive) had higher rates of photosynthesis, stomatal conductance and transpiration than reproductive adults, even though pre-dawn xylem pressure potentials of juveniles were slightly lower. Juveniles were also less water-use efficient than adults based on instantaneous gas exchange (photosynthesis/transpiration) and carbon isotope discrimination (DELTA). A comparison of DELTA-values indicated a gradient of water-use efficiency that was correlated with size: Seedlings lt Juveniles = Small Adults lt Large Adults. Reproductive status did not account for any variation independent of size. Small establishing plants may experience short-term environmental conditions and long-term selective pressures different from those of larger reproductive plants. The pattern of smaller plants having higher rates of gas exchange and less efficient use of water in the absence of higher pre-dawn xylem pressure potentials suggests a developmental progression in ecophysiological parameters with increasing size, in addition to the environmentally induced variation that has been documented for these characters for many species..*<6Donovan, Lisa A. Mausberg, James Ehleringer, James, R. 1993<6Seedling size and survival for Chrysothamnus nauseosusGreat Basin Naturalist533237-245p$Seedling size and survival in relation to summer drought were examined for Chrysothamnus nauseosus growing under field and greenhouse conditions. In the field, summer survival rates were less than 2% annually for the three years monitored. The effect of initial seedling height on subsequent survival was examined in both the field and greenhouse by grouping seedlings into live and dead categories on each census date and comparing the initial heights for seedlings in these categories. For a majority of the census dates, the initial height of surviving seedlings was greater than the initial height of those that subsequently died (significant differences ranged from 1 to 8 mm), indicating that seedlings that were taller at the initiation of the drought period had a higher probability of survival. In the greenhouse, taller seedlings had greater shoot and root biomass and rooting depth. Seedlings that are larger (i.e., taller and have greater aboveground biomass) in late spring appear to have a higher probability of surviving the summer drought due to greater rooting depth and hence increased access to moisture in deeper soil layers. Seed availability and safe sites for germination were probably not limiting since large numbers of seedlings successfully germinated in a patchy pattern during the study period. Seedling size and probability of survival were not related to either seedling density or the distance to nearest seedling neighbor. Survival through summer drought appears to be the main limitations to seedling recruitment in this population.& Donovan, L. A. Ehleringer, J. R. 1994TMWater stress and use of summer precipitation in a Great Basin shrub communityaFunctional Ecology8 32289-297c1. Seasonal patterns of water stress (determined from predawn xylem pressure potentials) and relative use of summer precipitation (determined from hydrogen isotope composition) varied intraspecifically and interspecifically in a Great Basin shrub community. 2. The development of water stress during the dry season from June to July was positively correlated with the use of summer precipitation in August. The smallest plants generally developed the greatest water stress and took up the most summer precipitation, presumably due to being more dependent on shallow roots. 3. Among mature shrubs examined, moisture from summer precipitation was taken up by Chrysothamnus viscidiflorus and Artemisia tridentata, but not by C. nauseosus, Juniperus osteosperma and Tetradymia canescens. 4. For shrub species that are deeply rooted at maturity, such as C. nauseosus, the quantity and timing of available soil moisture is different for small establishing plants as compared to large reproductively mature plants.g} DL,%Donovan, Lisa A. Ehleringer, James R. 1994nhPotential for selection on plants for water-use efficiency as estimated by carbon isotope discrimination American Journal of Botany817927-935Water-use efficiency is thought to be related to plant performance and natural selection for plants in arid habitats, based on a general expectation that increased water-use efficiency is associated with decreased carbon gain and biomass accumulation. Using leaf carbon isotope discrimination (DELTA) to determine integrated water-use efficiency, we estimated genetic variance for, and examined the relationships among DELTA biomass, and gas exchange characters for full-sibling families of the woody shrub, Chrysothamnus nauseosus, grown from seed collected at Tintic, Utah. In both well-watered greenhouse and common garden experiments, and water-limited common garden experiments, there were significant family differences for DELTA, biomass, and morphological characters, indicating a potential for genetic change in response to selection. However, estimates of broadsense heritabilities for DELTA were low, indicating that the rate of change in response to selection would be relatively slow. This was consistent with the large amount of phenotypic plasticity observed for DELTA as it differed with water treatment and year in the garden experiment. Phenotypically, aboveground biomass and DELTA were negatively correlated within the well-watered treatments (i.e., more water-use efficient plants were larger), not correlated within the water-limited treatment, and positively correlated for combined well-watered and water-limited garden treatments, suggesting that variation in both photosynthetic capacity and stomatal limitation contribute to the variation in DELTA. In contrast to the phenotypic correlations, genetic correlations for biomass and DELTA were consistently negative within each treatment, and selection for higher water-use efficiency through low DELTA for C. nauseosus plants in this population would tend to shift populations toward larger plants. For C. nauseosus, increased water-use efficiency is not necessarily associated with decreased carbon gain.,%Donovan, Lisa A. Ehleringer, James R.o 1994ngCarbon isotope discrimination, water-use efficiency, growth and mortality in a natural shrub populationn Oecologia 1003347-354voIn order to scale up from the ecophysiological characters of individual plants to population-level questions, we need to determine if character patterns in natural populations are stable through time, and if the characters are related to growth and survival. We investigated these questions in a 3-year study for one character, integrated water-use efficiency (WUE) as estimated by carbon isotope discrimination (DELTA) in a population of the Great Basin shrub, Chrysothamnus nauseosus. WUE was a conservative character for a given plant within and across seasons, and a previously documented difference between two size classes (represented by juveniles and adults) was maintained-, smaller juveniles had a lower WUE than larger adults. The lower WUE of juveniles was often accompanied by higher rates of photosynthesis and stomatal conductance as compared to adults even though juveniles generally had more negative xylem pressure potentials. Although many discussions of the role of WUE in natural populations have been based on the expectation that higher WUE (lower DELTA) is generally associated with less growth, we found no such relationship for juvenile plants in this population (i.e. DELTA was not positively correlated with height increase). In addition, juvenile plant mortality was not correlated with DELTA. Although there were stable patterns of WUE for plants in this population, the positive correlation between WUE and size, and the lack of a negative correlation between WUE and height growth. make it unlikely that the WUE of an individual plant will be related in a simple manner to its growth and survival in the population.g328-335$://000172418100002x2,Donovan, L. A. Linton, M. J. Richards, J. H.|vPredawn plant water potential does not necessarily equilibrate with soil water potential under well-watered conditions Oecologialapoplastic solutes; desert shrubs; chaparral shrubs; halophytes; leaf water potential shrub larrea-tridentata; stomatal conductance; gas-exchange; picea-abies; sonoran desert; drying soil; root; resistance; vegetation; drought Predawn leaf water potential (Psi (w)) and xylem pressure potential (Psi (p)) are expected to be in equilibrium with the soil water potential (soil Psi (w)) around roots of well- watered plants. We surveyed 21 plant species (desert, chaparral, and coastal salt marsh species, as well as two temperate tree and two crop species) for departures from this expectation and for two potential mechanisms explaining the departures. We measured soil Psi (w), leaf Psi (w), and xylem Psi (p) in the glasshouse under well-watered conditions that eliminated soil moisture heterogeneity and ensured good soil- root hydraulic continuity. Most species failed to equilibrate fully with soil Psi (w), depending on whether leaf Psi (w) or xylem Psi (p) was used as the measure of predawn plant water potential. The contribution of nighttime transpiration to predawn disequilibrium was assessed by comparing plants with bagged canopies (enclosed overnight in plastic bags to eliminate transpiration) to plants with unbagged canopies. Nighttime transpiration significantly reduced predawn xylem Psi (p) for 16 of 21 species and the magnitude of this contribution to predawn disequilibrium was large (0.50-0.87 MPa) in four woody species: Atriplex confertifolia, Batis maritima, Larrea tridentata, and Sarcobatus vermiculatus. The contribution of nighttime transpiration to predawn disequilibrium was not more prevalent in mesic compared with xeric or desert phreatophytic compared with non-phreatophytic species. Even with bagging that eliminated nighttime transpiration, plants did not necessarily equilibrate with soil Psi (w). Plant xylem Psi (p) or leaf Psi (w) were significantly more negative than soil Psi (w) for 15 of 15 species where soil Psi (w) was measured. Predawn disequilibrium based on leaf Psi (w) was of large magnitude (0.50-2.34 MPa) for seven of those 15 species, predominately halophytes and Larrea tridentata. A portion of the discrepancy between leaf and soil Psi (w) is consistent with the putative mechanism of high concentrations of leaf apoplastic solutes as previously modeled for a halophyte, but an additional portion remains unexplained. Predawn leaf Psi (w) and xylem Psi (p) may not reflect soil Psi (w), particularly for woody plants and halophytes, even under well-watered conditions. Oecologia 2001 Nov 1293496VQ OECOLOGIAISI:0001724181000024.Dougherty, R. L. Lauenroth, W. K. Singh, J. S. 1996tmResponse of a grassland cactus to frequency and size of rainfall events in a North American shortgrass steppeiJournal of Ecology842c177-183a1 Field experiments were conducted to define the types of rainfall that most effectively contribute to the water resource of a grassland cactus, Opuntia polyacantha in a shortgrass steppe. 2 The study indicated that O. polyacantha is capable of responding to rain events as small as 2.5 cm although a 10-mm event has considerably more value than a 2.5-mm or a 5-mm event. 3 Water content of the cladodes was closely related to recent rainfall, and shoot growth was greatly influenced by cladode water content. 4 Results indicate that this cactus utilizes small rainfall events (2.5-5 mm), and that its success in the short term depends more on the frequency of usable rain than on rain amount.l. Evans, R. D. Belnap, J. 1999VOLong-term consequences of disturbance on nitrogen dynamics in an arid ecosystemrEcology 801o150-160uVPAnthropogenic activity is causing dramatic changes in the nitrogen (N) cycle in many ecosystems. Most research has focused on the increase in N input caused by atmospheric deposition and invasion of N-fixing species, and on their effects on resource availability and species composition. However, in contrast to many ecosystems experiencing large increases in N input, many arid ecosystems are experiencing loss of nutrients due to land-use change. An important component of many arid ecosystems on a worldwide basis is the microbiotic crust, a biological soil crust composed of lichens, cyanobacteria, mosses, and algae. Nitrogen fixation by lichens and cyanobacteria comprising the crust is the primary source of N input in many of these ecosystems. We quantified the long-term consequences of surface disturbance in an arid ecosystem on the Colorado Plateau by comparing pristine sites with those of known disturbance history. Disturbance caused an increase in the abundance of cyanobacteria and a decrease in lichens within the microbiotic crust. Carbon isotope composition (delta13C) of the crust reflects this shift in species composition; values for disturbed sites were 4.5permill higher than undisturbed sites. Nitrogen isotope composition (delta15N) of the microbiotic crust was 1.5-2.2permill higher for disturbed sites, probably resulting from relatively greater gaseous N loss from the crust. Historic disturbance has caused a long-term decrease in rates of N fixation by the microbiotic crust; nitrogenase activity in pristine sites was 250% greater than sites intermittently disturbed 30 yr ago. The decrease in N input from fixation and continued gaseous N loss has caused a 25-75% decrease in soil N content. Altering relative rates of N input and loss, coupled with input of N from microbiotic crusts with relatively higher delta15N, has caused an increase in soil and plant delta15N at disturbed sites. This decrease in soil N caused by disturbance will likely cause changes in species composition similar to those observed in ecosystems that have been disrupted by excess N input from atmospheric deposition.<5R. D. Evans J. Belnap F. Garcia-Pichel S. L. Phillips 2001<6Global change and the future of biological soil crusts "Belnap, Jayne Lange, Otto L.B;Biological soil crusts: Structure, function, and management "Heidelberg,Berlin, New York Springer Verlag417-430 2797-2809 $://000172410600005lFeddes, R. A. Hoff, H. Bruen, M. Dawson, T. de Rosnay, P. Dirmeyer, O. Jackson, R. B. Kabat, P. Kleidon, A. Lilly, A. Pitman, A. J.D=Modeling root water uptake in hydrological and climate modelsf6/Bulletin of the American Meteorological Society land-surface schemes; amazonian deforestation; atmospheric models; hydraulic lift; global-scale; vegetation; soil; transpiration; sensitivity; simulationi*#From 30 September to 2 October 1999 a workshop was held in Gif- sur-Yvette, France. with the central objective to develop a research strategy for the next 3-5 years. aiming at a systematic description of root functioning, rooting depth, and root distribution for modeling root water uptake from local and regional to global scales. The goal was to link more closely the weather prediction and climate and hydrological models with ecological and plant physiological information in order to improve the understanding of the impact that root functioning has on the hydrological cycle at various scales. The major outcome of the workshop was a number of recommendations, detailed at the end of this paper, on root water uptake parameterization and modeling and on collection of root and soil hydraulic data. Bull. Amer. Meteorol. Soc. 2001 Dec8212"496RT BULL AMER METEOROL SOCISI:000172410600005& Fernandez, O. A. Caldwell, M. M. 1975d]Phenology and dynamics of root growth of three cool semi-desert shrubs under field conditionsJournal of Ecology63703-714f snakeweed ecotype 11-17$://A1993LW77500002o0*Wan, C. G. Sosebee, R. E. McMichael, B. L.yDoes Hydraulic Lift Exist in Shallow-Rooted Species - a Quantitative Examination with a Half-Shrub Gutierrezia- SarothraeePlant and Soildesert xerophyte; root exudation; semiarid grassland time-domain reflectometry; soil-water content; electromagnetic determination; desert succulents; moisture; system^XHydraulic lift occurs in some deep-rooted shrub and herbaceous species. In this process, water taken up by deep roots from the moist subsoil is delivered to the drier topsoil where it is later reabsorbed by shallow roots. However, little is known about the existence of hydraulic lift in shallow-rooted xeric species. The objectives of this study were 1) to ascertain whether hydraulic lift exists in Gutierrezia sarothrae (broom snakeweed), a widespread North American desert species with a shallow root system, grown in pot and field conditions and 2) if it does, how much water can be transferred from the subsoil to the 30 cm topsoil during the night. Snakeweed seedlings were transplanted in buried pots allowing the deeper roots to grow into the subsoil 30 cm below the surface. Soil water content inside and outside of the pot was measured seasonally and diurnally with time domain reflectometry technique (TDR). An increase in water content was detected in the pot after the plant was covered for 3 h by an opaque plastic bag during the day, suggesting hydraulic lift from deeper depths and exudation of water into the drier topsoil. Root exudation was also observed on native range sites dominated by snakeweed. Water efflux in the pot was 271 g per plant per night. which was equivalent to 15.3% of the extrapolated, porometer-derived whole-plant daily transpiration. Hydraulic lift observed in Gutierrezia improved water uptake during the day when evaporative demand is high and less water is available in the topsoil. We concluded that hydraulic lift might help snakeweed to alleviate the effect of water stress. Plant Soil 1993 Jun 1531LW775 PLANT SOILISI:A1993LW77500002PSJMb$Nicholson, S. E. Farrar, T. J. 1994The influence of soil type on the relationships between NDVI, rainfall, and soil moisture in semiarid Botswana. 1. NDVI response to rainfallRemote Sens. Environ.n502107-120if_This study examines the variability of the normalized difference vegetation index (NDVI) over Bostwana during the period 1982-1987. Its relationship to rainfall is evaluated from 26 stations representing six vegetation formations and six soil types. The study demonstrates a linear relationship between rainfall and NDVI as long as rainfall does not exceed approximately 500 mm/yr or 50-100 mm/month. Above these limits, a "saturation" response occurs and NDVI increases with rainfall only very slowly. This occurs at much lower thresholds of rainfall over Botswana than over East Africa or the Sahel. The ratio of NDVI to rainfall in Botswana is several times higher than in savannas in these other two regions. Both this ratio, which represents the rain-use efficiency, and the total productivity of vegetation, as assessed from NDVI, vary with vegetation and soil type. The highest productivity and efficiency occur on the clay-rich vertisols, the lowest on the sandy arenosols and solonchaks. In general, the rain-use efficiency appears to be more a function of the underlying soil than of the vegetation formation.iXQWater relations of stem succulent trees in north-central Baja California (Mexico)RKNilsen, E. T. Sharifi, M. R. Rundel, P. W. Forseth, I. N. Ehleringer, J. R. Oecologia823299-303 1990f_Water relations of several stem succulent trees were measured in north-central Baja California in comparisons to other growth forms in the same habitat. Our research concentrated on three stem succulent species (Idria collumnaris, Pachycormus discolor and Bursera microphylla) each with a different succulent stem morphology. The stem succulent trees had 1 5 to 4 kg H-2O/m-3 of trunk while the other trees and shrubs in the same habitat had 0.6 to 0.8 kg H-2O/m-3. The diurnal and seasonal variation in leaf water potential was small for the stem succulent species in comparison to deciduous and evergreen species as a consequence of the stem-water, buffering capacity. In addition, the leaf conductance of the stem succulent species was low (60 mmol m-2 s-1) and yet, the leaf conductance decreased through the day similar to adjacent evergreen and deciduous species. The leaves of the stem succulent trees lost turgor at low saturated water deficits (0.06 to 0.14), had comparatively high osmotic potentials, and high values of leastic modulus in comparison to adjacent evergreen and deciduous species. The stem acts as an important buffering mechanisms allowing for the maintenance of leaf turgor in these stem succulent trees. The low transporiation rates of the stem succulent trees may be a mechanism to minimize leaf saturated water deficit and extend leaf longevity.sxqWater uptake and respiration of root systems of two cacti: Observations and predictions bases on individual rootsa*$Nobel, P. S. Lopez, F. B. Alm, D. M.$Journal of Experimental Botany42 243l 1215-1224r 1991Two species of cactus, Ferocactus acanthodes and Opuntia ficus-indica, were used in glasshouse experiments to test predictions of water uptake and respiration by entire root systems at various soil temperatures (T-soil) and soil water potentials (psi-soil). Predictions were based primarily on values of root hydraulic conductivity for water (L-p; m s-1 MPa-1) and on maintenance respiration (R-M; mol CO-2 kg-1 d-1) previously measured for individual root segments as functions of root age, T-soil, and psi-soil. After the plants had developed new roots for 24 to 36 weeks, root respiration and shoot transpiration were measured at T-soil of 15, 25, and 40 degree C, or psi-soil of -0.10, -0.30, and -0.90 MPa over a 15 d period, followed by root system excavations. Growth respiration accounted for about 12% of the total root respiration under wet soil conditions (psi-soil = -0.10 MPa). Predictions of respiration using R-M, dry weight, growth, and growth respiration of each root age class range from 87% to 111% of observations. Under these same conditions, water uptake predicted using previously measured values of L-p was 2.5-fold greater than observed transpiration, mainly because the root xylem water potential was difficult to quantify by age class. However, incorporating new measurements of L-p for entire roots and using the water potential at the base of the shoot resulted in predictions that ranged from 95% to 120% of observations. The approaches and models developed should assist in the investigation of water gains and carbon losses under natural conditions for root systems of these dicotyledonous desert succulents in particular and for root systems of other species in general.f|uHydraulic conductances of the soil, the root-soil air gap, and the root: Changes for desert succulents in drying soiloNobel, P. S. Cui, M.$Journal of Experimental Botany43 248f319-326l 1992Water movement to and from a root depends on the soil hydraulic conductivity coefficient (L-soil), the distance across any root-soil air gap, and the hydraulic conductivity coefficient of the root (L-p). After analytical equations for the effective conductance of each part of the pathway are developed, the influences of soil drying on the soil water potential and L-soil are described during a 30 d period for a loamy sand in the field. The influences of soil drying on L-p for three desert succulents, Agave deserti, Ferocactus acanthodes, and Opuntia ficus-indica, are also described for a 30 d period. To quantify the effects of soil drying on the development of a root-soil air gap, diameters of 6-week-old roots of the three species were determined at constant water vapour potentials of -1.0 MPa and 10 MPa as well as with the water vapour potential decreasing at the same rate as soil drying during a 30 d period. The shrinkage observed for roots initially 2.0 mm in diameter averaged 19% during the 30 d period. The predominant limiting factor for water movement was L-p of the root for the first 7 d of soil drying, the root-soil air gap for the next 13 d, and L-soil thereafter. Compared with the ease of water uptake from a wet soil, the decrease in conductances during soil drying, especially the decrease in L-soil, caused the overall conductance to decrease by 3 times 10-3-fold during the 30 d period for the three species considered, so relatively little water was lost to the dry soil. Such rectifier-like behaviour of water movement in the soil-root system resulted primarily from changes in L-soil and, presumably, is a general phenomenon among plants, preventing water loss during drought but facilitating water uptake after rainfall.0dLFSeasonal variation in the carbon isotopic composition of desert plants81Ehleringer, J. R. Phillips, S. L. Comstock, J. P.iFunctional Ecology6u4.396-404i 1992Carbon isotope discrimination (DELTA) has been shown to be a valuable tool for long-term estimates of water-use efficiency, because this parameter is influenced by the ratio of intercellular to ambient CO-2 levels, which is in turn related to water-use efficiency. The objective of this study was to examine variation in DELTA-values of field materials and the extent to which changes in DELTA might be interpreted as changes in water-use efficiency. On-line discrimination observations with four desert species confirmed theoretically expected relationships between DELTA and the ratio of intercellular to ambient CO-2 concentrations, a prerequisite to applying DELTA-values to water-use efficiency. Seasonal changes in DELTA-values were measured in 15 desert species over the course of two growing seasons in western Arizona (USA). Two major patterns appeared: in leaves of some species there were 2-3% changes during the year, whereas in others the changes in isotopic composition were less than 1 permill . When carbon isotope analyses are applied to water-use efficiency extrapolations, it has been assumed that leaf temperatures are equivalent among the species being compared. An error analysis of this assumption indicated a near-linear relationship between leaf temperature differential and the difference in isotopic composition necessary to distinguish differences in water-use efficiency. When leaf temperature differentials between plants were ltoreq 2 cntdot 5 degree C, a 1 permill difference in DELTA was sufficient to rank plants correctly and unambiguously with respect to water-use efficiency, indicating the extent to which changes in intercellular CO-2 can offset possible differences in the evaporative gradient among plants when calculating water-use efficiencies. As leaf DELTA-values among different species often range 4% on a given sampling date, carbon isotope discrimination appears to be a feasible approach for ranking relative water-use efficiency differences among arid-land plants.4.Lack of nitrogen cycling in the Atacama DesertXREhleringer, J. R. Mooney, H. A. Rundel, P. W. Evans, R. D. Palma, B. Delatorre, J. Nature 359 6393316-318 1992VOMesquite (Prosopis) trees growing in the rainless region of the Atacama Desert (Chile) produce leaves that abscise and accumulate on a concrete-like carbonate surface, often attaining litter depths of 45 cm. The virtual lack of surface moisture inhibits leaf decomposition, and prevents cycling of nitrogen, the mineral most often limiting plant growth. Leaves in the midpoint of a litter profile were aged to pre-bomb dates (older than 1950) and had both high nitrogen concentrations and a carbon to nitrogen ratio comparable to that of live leaves. The thick carbonate layer prevents root growth into the litter. Prosopis appear to persist by having roots that fix nitrogen in moist subsurface layers and by extracting water and other nutrients from ground water, allowing plants to persist in an ecosystem in which there is no nitrogen cycling.NGEhleringer, James R. Rundel, Philip W. Palma, Beatriz Mooney, Harold A. 1998f_Carbon isotope ratios of Atacama Desert plants reflect hyperaridity of region in northern Chile*#Revista Chilena de Historia Natural17117 79-86e^WLeaf carbon isotope ratios were measured on plants from the coastal portions of the Atacama Desert at Pan de Azucar and Paposo, Chile. Most species possessed C3 photosynthesis, although there were several CAM species, indications of some facultative CAM species, and only one C4 species. The carbon isotope ratios of the C3 plants are unusually high, even for species from and ecosystems. These observations indicated that C3 species of the Atacama Desert were characterized by very low intercellular CO2 concentration that averaged between 159 and 190 ppm, depending on the severity of the drought.e2,Ehleringer, J. R. Schwinning, S. Gebauer, R. 2000(!Water use in arid land ecosystemsf 82Malcolm C. Press Julie D. Scholes Martin G. Barker"Physiological Plant Ecology Boston Blackwell Sciencet347-365i(!Eissenstat, D. M. Caldwell, M. M. 1988nhCompetitive Ability Is Linked to Rates of Water Extraction a Field Study of Two Aridland Tussock Grasses Oecologia751 1-7d]The relative competitive abilities of Agropyron desertorum and Agropyron spicatum under rangeland conditions were compared using Artemisia tridentata ssp. wyomingensis transplants as indicator plants. We found A. desertorum to have substantially greater competitive ability than A. spicatum as manifested by the responses of Artemisia shrubs that were transplanted into nearly monospecific stands of these grass species. The Artemisia indicator plants had lower survival, growth, reproduction, and late-season water potential in the neighborhoods dominated by A. desertorum than in those dominated by A. spicatum. In similar, essentially monospecific grass stands, neutron probe soil moisture measurements showed that stands of A. desertorum extracted water more rapidly from the soil profile than did those of A. spicatum. These differences in extraction rates correlate clearly with the differences in indicator plant success in the respective grass stands. Nitrogen and phosphorus concentrations in Artemisia tissues suggested these nutrients were not limiting plant growth and survival in the A. desertorum plots.e(!Eissenstat, D. M. Caldwell, M. M. 1989nhInvasive Root Growth Into Disturbed Soil of Two Tussock Grasses That Differ in Competitive EffectivenessFunctional Ecology335345-354 The ability to rapidly invade disturbed soil unoccupied by living roots (soil gaps) was examined for two tussock grass species that differ in competitive effectiveness. When soil gaps were created by removing a neighbouring tussock, the rate of root invasion into this space was more rapid for the grass of greater competitive ability, Agropyron desertorum (Fisch. ex Link) Schult, than for Agropyron spicatum (Pursh) Scribn. and Smith. Both species invaded gaps created later in the spring more rapidly than those created soon after snow melt. Acquisition of 32phosphorus placed in a soil gap varied greatly among individual tussocks of the same species that bordered the gap, even though the shoots of the tussocks were approximately the same distance from the radioactive soil and were of similar biomass. Total 32phosphorus acquisition from the gap was similar for the two grass species when the gap was created soon after the snow melted. Radiophosphorus acquisition tended to be greater for A. desertorum when the gap was created later in the spring (P = 0.08). Rapid root growth into soil gaps can allow a plant to occupy a greater soil volume and soil microsites richer in resources. However, greater resource acquisition due to this rapid root growth may not be immediately apparent.'T^ .(Fernandez, Roberto J. Reynolds, James F. 2000ZTPotential growth and drought tolerance of eight desert grasses: Lack of a trade-off?Oecologi 1231 90-98Eight perennial C-4 grasses from the Jornada del Muerto Basin in southern New Mexico show five-fold differences in relative growth rates under well-watered conditions (RGRmax). In a controlled environment, we tested the hypothesis that there is an inverse relationship (trade-off) between RGRmax and the capacity of these species to tolerate drought. We examined both physiological (gas exchange) and morphological (biomass allocation, leaf properties) determinants of growth for these eight species under three steady-state drought treatments (none=control, moderate, and severe). When well watered, the grasses exhibited a large interspecific variation in growth, which was reflected in order-of-magnitude biomass differences after 5 weeks. The species had similar gas-exchange characteristics, but differed in all the measured allocation and morphological characteristics, namely tiller mass and number, root:shoot ratio, dry-matter content, and specific leaf area (SLA). Drought affected tillering, morphology, and allocation, and reduced growth by 50 and 68% (moderate and severe drought, respectively) compared to the well-watered controls. With the exception of SLA, none of these variables showed a significant species-by-treatment interaction. We calculated three indices of drought tolerance, defined as the ratio in final biomass between all the possible "dry"/"wet" treatment pairs: severe/moderate, moderate/control, and severe/control. We found no significant correlation between these drought tolerance indices, on the one hand, and three indices of growth potential (greenhouse RGRmax, final biomass in the control treatment, and final:initial biomass ratio in controls), on the other. Based on these controlled-environment results, we hypothesize that the commonly reported correlation between plant growth potential and drought tolerance in the field may in some cases be explained by differential effects of plants on soil-water content rather than by differences in species responses to drought.  Fisher, S. G. Grimm, N. B. 1985{Hydrologic and Material Budgets For a Small Sonoran Desert Watershed Arizona Usa During Three Consecutive Cloudburst Floods"Journal of Arid Environments9i2s105-118Precipitation and runoff chemistry were monitored during three successive summer storms on a small Sonoran Desert catchment. Both nitrogen and phosphorus concentrations in precipitation were high (1.38 and 0.21 mg/l, respectively) yet lower than in runoff water (2.77 and 0.39 mg/l). However, 75-90 per cent of precipitation did not run off and the storm events represented net inputs of dissolved N and P to the watershed. Antecedent storms influence both water chemistry and nutrient budgets and greatly depleted transportable particulates. Because precipitation in the Sonoran Desert is variable both temporally and spatially, caution should be exercised in attempting to construct 'typical year' watershed nutrient budgets. Despite low rainfall and sporadic runoff, hydrologic fluxes of nitrogen and phosphorus in desert watersheds are comparable to those of mesic regions. Chloride, H+, and SO42- were retained by the watershed during these storms while HCO3- and dissolved and particulate organic matter were exported at rates higher than input by precipitation. Total dissolved solids entering in precipitation were three times export in stream flow while total particulate output greatly exceeded input in rain.i Fitter, A. H. 1994voArchitecture and biomass allocation as components of the plastic response of root systems to soil heterogeneity4 "M. M. Caldwell R. W. PearcynhExploitation of environmental heterogeneity by plants. Ecophysiological processes above and below ground  San Diego Academic Press Inc.305-3220*Flanagan, Lawrence B. Ehleringer, James R. 1998d]Ecosystem-atmosphere CO2 exchange: Interpreting signals of change using stable isotope ratiosB&Trends in Ecology and Evolution-131 10-14A"Fowler, A.M. Hennessy, K.J.i 1995`YPotential impacts of global warming on the frequency and magnitude of heavy precipitationNatural Hazards11283-303 679-690$://000169690600002d*$Williams, M. Bond, B. J. Ryan, M. G.~Evaluating different soil and plant hydraulic constraints on tree function using a model and sap flow data from ponderosa pine Plant Cell and Environmentyseasonal-variation; general-model; water-use; conductance; forest; xylem; transpiration; productivity; exchange; nitrogen Relationships between tree size and physiological processes such as transpiration mag have important implications for plant and ecosystem function, but as yet are poorly understood. We used a process-based model of the soil-plant-atmosphere continuum to investigate patterns of whole-tree sap flow in ponderosa pine trees of different size and age (36 m and similar to 250 years versus 13 m and 10-50 years) over a developing summer drought, We examined three different hypothetical controls on hydraulic resistance, and found that size-related differences in sap flow could be best explained by absolute differences in plant resistance related to path length (hypothesis 1) rather than through different dynamic relationships between plant resistance and leaf water potential (hypothesis 2), or alterations in rates of cumulative inducement and repair of cavitation (hypothesis 3). Reductions in sap flow over time could be best explained by rising soil- root resistance (hypothesis 1), rather than by a combination of rising plant and soil-root resistance (hypothesis 2), or by rising plant resistance alone (hypothesis 3), Comparing hourly predictions with observed sap dow we found that a direct relationship between plant resistance and leaf water potential (hypothesis 2) led to unrealistic bimodal patterns of sap flow within a day, Explaining seasonal reduction in sap flow purely through rising plant resistance (hypothesis 3) was effective but failed to explain the observed decline in pre-dawn leaf water potential for small trees. Thus, hypothesis 1 was best corroborated. A sensitivity analysis revealed a significant difference in the response to drought-relieving rains; precipitation induced a strong recovery in sap flow in the hypothetical case of limiting soil-root resistance (hypothesis 1), and an insignificant response in the case of limiting plant resistance (hypothesis 3), Longer term monitoring and manipulation experiments are thus likely to resolve the uncertainties in hydraulic constraints on plant function.Plant Cell Environ. 2001 Jul247449MT PLANT CELL ENVIRONISI:000169690600002ysiological processes above and below ground  San Diego Academic Press Inc.305-3220*Flanagan, Lawrence B. Ehleringer, James R. 1998d]Ecosystem-atmosphere CO2 exchange: Interpreting signals of change using stable isotope ratiosB&Trends in Ecology and Evolution-131 10-14A"Fowler, A.M. Hennessy, K.J.i 1995`YPotential impacts of global warming on the frequency and magnitude of heavy precipitationNatural Hazards11283-303D pTngPhenotypic variance of adaptation for a desert shrub species that spans a rainfall and drought gradientt(!Sandquist, Darren Ehleringer, Jim 19954-Bulletin of the Ecological Society of America,762 SUPPL. PART 2w 236e Using Smart Source Parsing0)Sandquist, Darren R. Ehleringer, James R. 1995f_Carbon isotope discrimination in the C-4 shrub Atriplex confertifolia along a salinity gradientGreat Basin Naturalist552o135-141aCarbon isotope discrimination (DELTA) was measured for leaves of Atriplex confertifolia along a salinity gradient in northern Utah. Over this gradient, the variation of DELTA values was high for a C-4 species, and the DELTA values were positively correlated with salinity in both years of the study. Of the possible explanations for this pattern, the DELTA results are consistent with the notion that salinity induces an increase in the bundle sheath leakiness of these C-4 plants.h`ZSoil moisture content and plant transpiration in the Chihuahuan Desert of New Mexico (USA)6/Schlesinger, W. H. Fonteyn, P. J. Marion, G. M."Journal of Arid Environments122l119-126t 1987rlThermocouple hygrometers were used to monitor soil water potential in control and harvested (bare) plots in the Chihuahuan Desert of New Mexico (U.S.A.) from June 1983 to June 1984. Soil water potentials were converted to water contents using the moisture tension release characteristics for each soil horizon. Comparisons of seasonal fluctuations in soil water storage on these plots suggest that 72% of incident precipitation is removed by plant transpiration in this ecosystem. Plant uptake of water from the lower soil profile is an important factor affecting the rate and depth of CaCO-3 deposition in desert soils.\VSeasonally fluctuating resources and temporal variability of interspecific competition$Schmitt, R. J. Holbrook, S. J. Oecologia691 1-11 1986The influence of seasonal availability of two critical resources (food and substrates from which food was harvested) on interspecific competition between striped surfperch (Embiotoca lateralis) and black surfperch (Embiotoca jacksoni) was examined. There was a strong depth-related gradient in density of prey and in cover of foliose algae; both declined with increasing bottom depth. Density of prey was reduced 5-10 fold during the winter season, but cover of substrates remained constant throughout the year. Although both fishes co-occurred throughout the same depth range, striped surfperch were more common in shallow habitats and black surfperch were more abundant deeper. Local abundance and distribution patterns of both surfperch species did not change seasonally. Stepwise regression analyses suggested that availability of favored substrates was a proximate influence on local patterns of surfperch distribution and abundance, and that interspecific competition depressed abundance of the two species to the same degree. Removal experiments conducted during the cold-water season revealed that interspecific competition influenced depth distribution of black surfperch but not striped surfperch. Seasonal change in density of prey was accompanied by marked changes in overlap in use of foraging substrates by the surfperches. The pattern of change in interspecific overlap suggested that surfperch competed for food only when prey were seasonally scarce. There was no difference in the agonistic tendencies of the two fishes, and the absolute and relative frequency of interspecific chases was independent of food level. These results have important implications regarding the impact of temporal variability of interspecific competition in natural communities. In the surfperch system, competition was characterized by constant and time-varying elements that had symmetrical and asymmetrical effects and involved both interference and exploitation mechanisms.Schulze, E. D. Mooney, H. A. Sala, O. E. Jobbagy, E. Buchmann, N. Bauer, G. Canadell, J. Jackson, R. B. Loreti, J. Oesterheld, M. Ehleringer, J. R. 1996d^Rooting depth, water availability, and vegetation cover along an aridity gradient in Patagonia Oecologiaa 10873503-511nAbove- and belowground biomass distribution, isotopic composition of soil and xylem water, and carbon isotope ratios were studied along an aridity gradient in Patagonia (44-45 degree S). Sites, ranging from those with Nothofagus forest with high annual rainfall (770 mm) to Nothofagus scrub (520 mm), Festuca (290 mm) and Stipa (160 mm) grasslands and into desert vegetation (125 mm), were chosen to test whether rooting depth compensates for low rainfall. Along this gradient, both mean above- and belowground biomass and leaf area index decreased, but average carbon isotope ratios of sun leaves remained constant (at -27 permill ), indicating no major differences in the ratio of assimilation to stomatal conductance at the time of leaf growth. The depth of the soil horizon that contained 90% of the root biomass was similar for forests and grasslands (about 0.80-0.50 m), but was shallower in the desert (0.30 m). In all habitats, roots reached water-saturated soils or ground water at 2-3 m depth. The depth profile of oxygen and hydrogen isotope ratios of soil water corresponded inversely to volumetric soil water contents and showed distinct patterns throughout the soil profile due to evaporation, water uptake and rainfall events of the past year. The isotope ratios of soil water indicated that high soil moisture at 2-3 m soil depth had originated from rainy periods earlier in the season or even from past rainy seasons. Hydrogen and oxygen isotope ratios of xylem water revealed that all plants used water from recent rain events in the topsoil and not from water-saturated soils at greater depth. However, this study cannot explain the vegetation zonation along the transect on the basis of water supply to the existing plant cover. Although water was accessible to roots in deeper soil layers in all habitats, as demonstrated by high soil moisture, earlier rain events were not fully utilized by the current plant cover during summer drought. The role of seedling establishment in determining species composition and vegetation type, and the indirect effect of seedling establishment on the use of water by fully developed plant cover, are discussed in relation to climate change and vegetation modelling. ZTMU. G. Hacke J. S. Sperry B. E. Ewers D. S. Ellsworth K. V. R. Schfer R. Oren  2000f6Influence of soil porosity on water use in Pinus taeda +  Oecologian 124495-505ZTXylem cavitation Soil water transport Root-shoot relations Stomatal regulation Abstract We analyzed the hydraulic constraints imposed on water uptake from soils of different porosities in loblolly pine (Pinus taeda L.) by comparing genetically related and even-aged plantations growing in loam versus sand soil. Water use was evaluated relative to the maximum transpiration rate (Ecrit) allowed by the soil-leaf continuum. We expected that trees on both soils would approach Ecrit during drought. Trees in sand, however, should face greater drought limitation because of steeply declining hydraulic conductivity in sand at high soil water potential (S). Transport considerations suggest that trees in sand should have higher root to leaf area ratios (AR:AL), less negative leaf xylem pressure (L), and be more vulnerable to xylem cavitation than trees in loam. The AR:AL was greater in sand versus loam (9.8 vs 1.7, respectively). This adjustment maintained about 86% of the water extraction potential for both soils. Trees in sand were more deeply rooted (>1.9 m) than in loam (95% of roots <0.2 m), allowing them to shift water uptake to deeper layers during drought and avoid hydraulic failure. Midday L was constant for days of high evaporative demand, but was less negative in sand (-1.6 MPa) versus loam (-2.1 MPa). Xylem was more vulnerable to cavitation in sand versus loam trees. Roots in both soils were more vulnerable than stems, and experienced the greatest predicted loss of conductivity during drought. Trees on both soils approached Ecrit during drought, but at much higher S in sand (<-0.4 MPa) than in loam (<-1.0 MPa). Results suggest considerable phenotypic plasticity in water use traits for P. taeda which are adaptive to differences in soil porosity.   |  - .   ; @            p u     u w x ~ ztHandley, L. L. Austin, A. T. Robinson, D. Scrimgeour, C. M. Raven, J. A. Heaton, T. H. E. Schmidt, S. Stewart, G. R. 1999haThe 15N natural abundance (delta15N) of ecosystem samples reflects measures of water availabilityO,&Australian Journal of Plant Physiology2621185-199.rkWe assembled a globally-derived data set for site-averaged foliar delta15N, the delta15N of whole surface mineral soil and corresponding site factors (mean annual rainfall and temperature, latitude, altitude and soil pH). The delta15N of whole soil was related to all of the site variables (including foliar delta15N) except altitude and, when regressed on latitude and rainfall, provided the best model of these data, accounting for 49% of the variation in whole soil delta15N. As single linear regressions, site-averaged foliar delta15N was more strongly related to rainfall than was whole soil delta15N. A smaller data set showed similar, negative correlations between whole soil delta15N, site-averaged foliar delta15N and soil moisture variations during a single growing season. The negative correlation between water availability (measured here by rainfall and temperature) and soil or plant delta15N fails at the landscape scale, where wet spots are delta15N-enriched relative to their drier surroundings. Here we present global and seasonal data, postulate a proximate mechanism for the overall relationship between water availability and ecosystem delta15N and, newly, a mechanism accounting for the highly delta15N-depleted values found in the foliage and soils of many wet/cold ecosystems. These hypotheses are complemented by documentation of the present gaps in knowledge, suggesting lines of research which will provide new insights into terrestrial N-cycling. Our conclusions are consistent with those of Austin and Vitousek (1998) that foliar (and soil) delta15N appear to be related to the residence time of whole ecosystem N.y3e("Aguiar, Martin R. Sala, Osvaldo E. 1999VPPatch structure, dynamics and implications for the functioning of arid ecosytems&Trends in Ecology and Evolution147273-27770)Alder, N. N. Sperry, J. S. Pockman, W. T. 1996Root and stem xylem embolism, stomatal conductance, and leaf turgor in Acer grandidentatum populations along a soil moisture gradientW Oecologia. 105.3293-301The objective of this study was to determine how adjustment in stomatal conductance (g-s) and turgor loss point (PSI-tlp) between riparian (wet) and neighboring slope (dry) populations of Acer grandidentum Nutt. was associated with the susceptibility of root versus stem xylem to embolism. Over two summers of study (1993-1994), the slope site had substantially lower xylem pressures (PSI-px) and g-s than the riparian site, particularly during the drought year of 1994. The PSI-tlp also lower at the slope (-2.9 +- 0.1 MPa; all errors 95% confidence limits) than at riparian sites (-1.9 +- 0.2 MPa); but it did not drop in response to the 1994 drought. Stem xylem did not differ in vulnerability to embolism between sites. Although slope-site stems lost a greater percentage of hydraulic conductance to embolism than riparian stems during the 1994 drought (46 +- 11% versus 27 +- 3%), they still maintained a safety margin of at least 1.7 MPa between midday PSI-px and the critical pressure triggering catastrophic xylem' embolism (TpxCT). Root xylem was more susceptible to embolism than stem xylem, and there were significant differences between sites: riparian roots were completely cavitated at -1.75 MPa, compared with -2.75 MPa for slope roots. Vulnerability to embolism was related to pore sizes in intervessel pit membranes and bore no simple relationship to vessel diameter. Safety margins from PSI-pxCT averaged less than 0.6 MPa in roots at both the riparian and slope sites. Minimal safety margins at the slope site during the drought of 1994 may have led to the almost complete closure of stomata (g, = 9 +- 2 versus 79 +- 15 mmol m-2 s-1 at riparian site) and made any further osmotic adjustment of PSI-tlp non-adaptive. Embolism in roots was at least partially reversed after fall rains. Although catastrophic embolism in roots may limit the minimum PSI for gas exchange, partial (and reversible) root embolism may be adaptive in limiting water use as soil water is exhausted. Using Smart Source Parsing   & f<'N\['=- i A h n *Lk +{ M  #  Am  ^U   /  C Z EV;/2   R  u nZ 7 V6 3d ; [ :P #xVH6 M[u+'Q? x %~ f;  }e `~ : 0 s  Bm  9 ! N  ; F Dh@9Wolters, Volkmar Silver Whendee, L. Bignell David, E. Coleman David, C. Lavelle, Patrick van der Putten Wim, H. de Ruiter, Peter Rusek, Josef Wall Diana, H. Wardle David, A. Brussaard, Lijbert Dangerfield John, M. Brown Valerie, K. Giller Ken, E. Hooper David, U. Sala, Osvaldo Tiedje, James van Veen Johannes, A.s 2000Effects of global changes on above- and belowground biodiversity in terrestrial ecosystems: Implications for ecosystem functioning Bioscience5012 1089-10982&Wondzell, Steve Ludwig, John A.m 1995ZSCommunity dynamics of desert grasslands: Influence of climate, landforms, and soils$Journal of Vegetation Science!63377-3903 v oPermanently marked vegetation transects in Big Bend National Park, Texas, USA were monitored to follow temporal dynamics of desert grassland communities on a variety of landforms and soil types over a 26-yr period after the removal of domestic livestock. Historic records indicate that the park area was severely overgazed prior to its establishment, and our results show that the species present increased in both cover and density after the removal of livestock. However, the timing of recovery corresponded to multi-year periods of above-average precipitation. Little change was observed in between 1955 and 1960, a period dominated by several consecutive years of drought. The cover of two large shrubs common to the Chihuahuan Desert. Larrea tridentata and Flourensia cernua, increased from 1960 to 1967, a period dominated by summer drought and frequent wet winters. The cover and density of forbs, perennial grasses. and most shrubs increased on nearly all landforms between 1967 and 1981. when summers were wetter than average. In contrast, the cover of Larrea tridentata decreased during this period. Comparisons among the plant communities on each landform showed that they diverged through time after domestic livestock were removed. Presumably, differences in topographic position and soil texture influence water availability which was reflected in the species composition on each soil series. Unfortunately, we cannot isolate the effects of recovery from grazing from the effects of climate because the study design did not include control plots located within grazed pastures. Certainly, the directional trajectory of change and the regrowth of grasses into inter-shrub spaces. must, at least in part, be the result of recovery from grazing. However, our data also indicate that the desert grassland communities are sensitive to multi-year periods of above- or below-average precipitation. Clearly, the dynamics between shrubs and grasses cannot be explained by a simple successional paradigm that views increased shrub dominance as retrogression from a climax grassland. Many alternate hypotheses have been forwarded to explain the dynamics that control the vegetation composition in the desert and desert grassland region of North America. Experimental tests of these hypotheses are needed to identify the interactions between biotic and abiotic factors that control dominance by shrubs or grasses.B;Wondzell, Steven M. Cunningham, Gary L. Bachelet, Dominiqueo 1996}Relationships between landforms, geomorphic processes, and plant communities on a watershed in the northern Chihuahuan DesertLandscape Ecology116i351-362 > 7The close correlation of plant communities to landforms and geomorphic surfaces resulted from differences in the redistribution of water and organic matter between landforms in the northern Chihuahuan Desert. Biotic processes are limited by water and nitrogen, and the interactions between landforms, geomorphic processes, soils, and plant communities control the redistribution of these limiting resources within internally drained catchments. Geomorphic processes are regulated by the geologic structure and gross topographic relief of internally drained catchments over geological time scales. Land forming processes can be viewed as static at time scales of 10's to 100's of years, with individual landforms regulating geomorphic processes, namely erosion and deposition resulting from the horizontal redistribution of water within the catchment. The vegetation composition is a critical feedback, reinforcing the erosional or depositional geomorphic processes that dominate each landform. The Jornada Long-Term Ecological Research site may be one of the simplest cases in which to decipher the relationship between landforms, geomorphic processes and plant communities. However, these geomorphic processes are common to all internally drained catchments throughout the Basin and Range Province, and result in the development of characteristic landforms and associated vegetation communities. Although the patterns may be modified by differences in parent material, watershed site, and land use history - erosional, depositional, and transportational landforms can still be identified. The sharpness of ecotones between plant communities on individual landforms is related to the degree to which landforms are linked through the flow of water and sediment. Sharp ecotones occurred at the transition from depositional to erosional landforms where little material was transferred and steep environmental gradients are maintained. Gradual ecotones occurred at the transition from erosional to depositional landforms where large quantities of material were transferred leading to the development of a gradual environmental gradient. The relationships between geomorphic processes and vegetation communities that we describe have important implications for understanding the desertification of grasslands throughout semi-arid regions of North America. Disturbances such as grazing and climate change alter the composition of plant communities, thereby affecting the feedbacks to geomorphic processes, eventually changing drainage patterns and the spatial patterns of plant communities supported within the landscape. 4.Wraith, Jon M. Baker, John M. Blake, Thomas K. 1995^XWater uptake resumption following soil drought: A comparison among four barley genotypes$Journal of Experimental Botany46 288t873-880w$Soil water resulting from episodic growing season rainfall evaporates rapidly in semi-arid regions. Plants may not benefit from such water additions if near-surface roots are unable to resume water uptake rapidly following periods of soil water deficit. Our objectives were to develop a means of quantifying root uptake responses in the upper soil layer following rewetting after soil water deficit, and to evaluate the existence of genotypic differences among four diverse barley (Hordeum vulgare L.) genotypes in this regard. Plants were grown in replicate soil columns having hydraulically isolated surface and subsoil layers, and instrumented with time-domain reflectometry (TDR) waveguides. The upper 0.05 m soil layer was allowed to dry to -1.8 to -3.0 MPa for 10-14 d, during which time subsoil wetness was maintained at about -0.6 to -0.7 MPa. The time-course of soil water uptake was monitored at 0.5 h intervals following rewetting of the surface layer. Substantial water uptake began 1 d after rewetting following 10 d, and 2-3 d after rewetting following 14 d of water deficit. Rate of water uptake was more rapid in response to a second rewetting 5-7 d later. Consistent genotypic responses in terms of cumulative water uptake on a whole plant and leaf area-specific basis were observed during each trial. These results have application to evaluating drought hardiness and interspecific competitive ability under semi-arid conditions, and to investigations of root physiological and morphological changes that contribute to recovery from water deficit. (t>B;The limits to xylem embolism recovery in Pinus sylvestris L,&Sobrado, M. A. Grace, J. Jarvis, P. G.$Journal of Experimental Botany43 251831-836i 1992In this studys we test the hypothesis that, when water supply is under tension, reversal of cavitation can occur as long as water continuity is maintained in the vicinity of tracheids. The experiments were conducted on young branches, 7-8 mm diameter, of Pinus sylvestris L., freshly collected and allowed to lose water on the bench after being debarked. During dehydration, the volumetric fractions of water (V-w) and gas (V-g) changed steadily as relative water content (theta) declined. Meanwhile, ultrasonic emission (UAE) started after a threshold theta = 90% was reached and were maximal at theta = 75%. Before and after dehydration, branch segments were connected to water-filled tubing and placed from 0.2 to 3.6 m above a water source and water inflow and outflow were recorded. These distances provided a source of water at a potential of -2.0 to -36 kPa. We considered that the segment water potential would be a function of the surface tension across the water meniscii at the ends of the embolized tracheids. Thus, water potentials calculated from tracheid dimensions would be as low as -43 kPa. Water inflow to segments declined when the distacnce from the source was increased or the segments were very dehydrated. Increasing the distance above the water source would be expected to increase the water potential difference but to reduce water uptake. The most dehydrated segments absorbed water faster at the beginning of the refilling period ( simeq 2 h), but at the end of 16 h, theta- was lower and V-g larger than in less embolized tissue. Recovery of water flow followed a similar trend, and was lost when embolisms increased. For a narrow range of theta, hydraulic conductance was reduced sharply, indicating that wide tracheids were still gas-filled. Thus, the number of tracheids remaining embolized increased when the source water potential was low and there was severe embolism. We conclude that embolism can be reversed in P. sylvestris at a rate depending on the water potential of the source, severity of embolism and hydraulic conductivity.&Sparks, J. P. Ehleringer, J. R. 1997lfLeaf carbon isotope discrimination and nitrogen content for riparian trees along elevational transects Oecologia 1093362-367aLeaf carbon isotope discrimination (DELTA), seasonal estimates of the leaf-to-air water vapor gradient on a molar basis (omega), and leaf nitrogen contents were examined in three riparian tree species (Populus fremontii, P. angustifolia and Salix exigua) along elevational transects in northern and southern Utah USA (1500-2670 m and 600-1820 m elevational gradients, respectively). The co values decreased with elevation for all species along transects. Plants growing at higher elevations exhibited lower A values than plants at lower elevations (P. fremontii, 22.9%. and 19.5 permill , respectively; P. angustifolia, 23.2 permill and 19.2 permill , respectively; and S. exigua, 21.1 permill and 19.1 permill , respectively). Leaf nitrogen content increased with elevation for all species, suggesting that photosynthetic capacity at a given intercellular carbon dioxide concentration was greater at higher elevations. Leaf A and nitrogen content values were highly correlated, implying that leaves with higher photosynthetic capacities also had lower intercellular carbon dioxide concentrations. No significant interannual differences were detected in carbon isotope discrimination. Sperry, J. S. Tyree, M. T. 198860Mechanism of water stress-induced xylem embolismPlant Physiology883 581-587 F@acer saccharum. xylem . water stress. air . pressure . tension . Sperry, J. S. Tyree, M. T. 1990F@Water-stress-induced xylem embolism in three species of conifers"Plant, Cell and Environmentg135s427-436picea rubens. abies balsamea. juniperus virginiana. water stress. hydraulic conductivity. plant disorders. air . tree injection. tracheids . wood anatomy. tissue ultrastructure. regulation . oxalic acid. calcium . vermont . ID: inter tracheid pit membranes. air bubbles.a0*Sperry, J. S. Alder, N. N. Eastlack, S. E. 1993^XThe effect of reduced hydraulic conductance on stomatal conductance and xylem cavitation$Journal of Experimental Botany44 263h 1075-1082pThe vulnerability of xylem conduits to cavitation theoretically determines the maximum flow rate of water through plants, and hence maximum transpiration (E), stomatal conductance (g-s), and leaf area (A-1). Field-grown Betula occidentalis with a favourable water supply exhibit midday xylem pressures approaching the cavitation-inducing range (- 1.42 to - 2.2 MPa). We studied the ability of the stomata to prevent cavitation-inducing pressures when whole-plant hydraulic conductance per leaf area (k-1) was reduced by making overlapping transverse cuts in the main stem. Controls were intact, or had only the phloem cut in the same pattern. Reducing k-1 caused two responses: (1) variable g-s, or E with psi-px falling into the cavitation range causing up to 98% embolism and 100% leaf death, (2) decreased g-s and E with psi-px remaining above the cavitation point and no leaf death or induction of cavitation. Shoots avoiding cavitation either produced new xylem and returned to control values of k-1, g-s, and E (experiments in June and July), or they showed continued decline in g-s, and E associated with increasing psi-px and eventual premature senescence of leaves (experiments in August). Whether embolism occurred after reducing k, probably depended on the response time of stomata, and the proximity of psi-px to the cavitation range when the xylem was cut. Stomata probably responded indirectly to reduced k via small changes in leaf psi; root signalling was unlikely because of the constant rooting environment.hXGMcCully, Margaret E. 1999Root xylem embolisms and refilling. Relation to water potentials of soil, roots, and leaves, and osmotic potentials of root xylem sapPlant Physiology 11939 1001-1008Embolism and refilling of vessels was monitored directly by cryo-microscopy of field-grown corn (Zea mays L.) roots. To test the reliability of an earlier study showing embolism refilling in roots at negative leaf water potentials, embolisms were counted, and root water potentials (PSIroot) and osmotic potentials of exuded xylem sap from the same roots were measured by isopiestic psychrometry. All vessels were full at dawn (PSIroot-0.1 MPa). Embolisms were first seen in late metaxylem vessels at 8 AM. Embolized late metaxylem vessels peaked at 50% at 10 AM (PSIroot-0.1 MPa), fell to 44% by 12 PM (PSIroot-0.23 MPa), then dropped steadily to zero by early evening (PSIroot-0.28 MPa). Transpiration was highest (8.5 mug cm-2 s-1) between 12 and 2 PM when the percentage of vessels embolized was falling. Embolized vessels were refilled by liquid moving through their lateral walls. Xylem sap was very low in solutes. The mechanism of vessel refilling, when PSIroot is negative, requires further investigation. Daily embolism and refilling in roots of well-watered plants is a normal occurrence and may be a component of an important hydraulic signaling mechanism between roots and shoots.u81McIvor, John G. Williams, John Gardener, Chris J. 1995VOPasture management influences runoff and soil movement in the semi-arid tropics4.Australian Journal of Experimental Agriculture351w 55-65iThe influence of pasture management (sowing introduced species, timber treatment, clearing and cultivation, stocking rate) on runoff and soil movement was measured on a neutral red duplex soil at Cardigan, near Charters Towers. Runoff and soil movement were greatest in native woodlands and least in developed pastures, although the differences were often not significant. Runoff and soil movement were related to cover levels on the plots. In small rainfall events (total lt 50 mm and intensity lt 15 mm/h), runoff and soil movement decreased rapidly as cover increased, and only small cover levels (40%) were needed to reduce them to a low level. As the size of the storm increased, greater cover levels were required, and for very large events (total gt 100 mm and intensity gt 45 mm/h) cover had no effect on runoff although it still reduced soil movement. On neutral red duplex soils, managers should maintain at least 40% groundcover but this would still allow large losses of suspended sediment in large storms.`ZResponses to nutrient pulses of two colonizers requiring different disturbance frequencies Miao, S. L. Bazzaz, F. A.zEcologyc716c 2166-2178 1990Using a congeneric pair of colonizing perennial species, we tested the prediction that Plantago major, a species from frequently disturbed habitats, would respond to pulses of nutrient availability with greater increases of growth and shifts in biomass allocation to reproduction than would P. rugelii, a species from less frequently disturbed habitats. The two species were maintained at two nutrient levels, and plants were pulsed once each at five different times during the course of the experiment. Net photosynthesis was measured before plants were harvested at four intervals. It was found that the timing of nutrient pulses affected leaf and and root relative growth rates (RGR) of P. major plants differently. At the high nutrient level, nutrient pulses given to P. major plants before the reproductive stage resulted in increased leaf RGR at the expense of root growth, whereas the opposite was observed at the low nutrient level. However, P. rugelii generally exhibited similar change in leaf and root RGR following nutrient pulses. P. major pulsed during the reproductive stage exhibited higher increases of RGR, total biomass, and leaf/root ratio, and different patterns from those plants pulsed before the reproductive stage. In contrast, P. rugelii generally exhibited similar change in these characters. At the final harvest, P. major plants significantly increased allocation to spike biomass in response to nutrient pulses, whereas P. rugelii did so only at specific times. For P. major, there were significant negative correlations between proportional allocation to vegetative (both leaves and roots) and reproductive organs, while for P. rugelii, the negative correlation was found only between proportional allocation to roots and spikes. The differences in response to nutrient availability of the two colonizers are commensurate with the degree and frequency of disturbance of the sites on which they grow in the field.N RA finite-element model of radial and axial conductivities for individual roots: Development and validation for two desert succulents*$Alm, D. J. Cavelier, J. Nobel, P. S.Annals of Botany691t 87-92H 1992A morphologically explicit numerical model for analysing water uptake by individual roots was developed based on a conductor network, with specific conductors representing axial or radial conductivities for discrete root segments. Hydraulic conductivity (L-p; m s-1 MPa-1) was measured for roots of Agave deserti Engelm. and Opuntia ficus-indica (L.) Miller by applying a partial vacuum to the proximal ends of excised roots in solution. L-P was also measured for 40- to 80-mm segments along a root, followed by measurements of axial conductivity and calculation of radial conductivity. Predicted values of L-P for entire roots based on two to ten segments per root averaged 1.04+-0.07 (mean+-s.e. mean for n = 3) of the measured L-P for A. deserti and 1.06+-0.10 for O. ficus-indica. The model also closely predicted the drop in water potential along the root xylem (DELTA--psi-xylem); when a tension of 50 kPa was applied to the proximal ends of 0.2 m-long roots of A. deserti and O. ficus-indica, the measured DELTA--psi-xylem to midroot averaged 30 kPa compared with a predicted decrease of 36 kPa. Such steep gradients in psi-xylem suggest that the driving force for water movement from the soil to young distal roots may be relatively small. The model, which agreed with an analytical solution for a simple hypothetical situation, can quantify situations without analytical solutions, such as when root and soil properties vary arbitrarily along a root.s<6Anthoni, Peter M. Law, Beverly E. Unsworth, Michael H. 1999RLCarbon and water vapor exchange of an open-canopied ponderosa pine ecosystem*#Agricultural and Forest Meteorologyf953151-1688Eddy covariance measurements of carbon dioxide and water vapor exchange were made above a ponderosa pine (Pinus ponderosa Dougl. ex P. and C. Laws.) forest located in a semiarid environment in central Oregon. The stand is a mixture of old-growth and young trees. Annual net carbon gain by the ecosystem (NEE) was 320 +- 170 gC m-2 year-1 in 1996 and 270 +- 180 gC m-2 year-1 in 1997. Compared to boreal evergreen forest at higher latitudes, the pine forest has a substantial net carbon gain (150 +- 80 gC m-2 year-1 in 1996 and 180 +- 80 gC m-2 year-1 in 1997) outside the traditionally defined growing season (from bud swell in early May (Day 125) to partial leaf-off in late September (Day 275)). Carbon assimilation continued to occur in the relatively mild winters, though at a slower rate (April, maximum leaf level assimilation (Amax) of 6-9.5 mumol m-2 leaf s-1), and ecosystem respiration was relatively low (apprx1.6 +- 0.1 gC m-2 day-1). In the growing season, although photosynthetic capacity was large (July, Amax = 16-21 mumol m-2 leaf s-1), carbon assimilation was constrained by partial stomatal closure to maintain a sustainable water flow through the soil-plant system, and ecosystem respiration was large (3.5 +- 0.1 and 4.3 +- 0.1 gC m-2 day-1 in growing season of 1996 and 1997, respectively) because of high air and soil temperatures. Despite large changes in evaporative demand over just a few days (VPD changing from 0.5 to 3.5 kPa), the ecosystem water use was remarkably constant in summer (apprx1.6-1.7 mm day-1). Such homeostasis is most likely another result of stomatal control. Interannual variations in climate had a large influence on the ecosystem carbon balance. In summer 1997, an El Nino year, precipitation was more frequent (17 days with 33 mm of rain) than in summer 1996 (5 days with 5 mm of rain), and the net ecosystem exchange was substantially lower in July to September 1997 (10 +- 60 gC m-2) than during the equivalent period in 1996 (100 +- 60 gC m-2). Although temperatures between years were similar, the carbon assimilation in 1997 was offset by increased respiration, probably because soils were more frequently wet, encouraging microbial respiration.<5Archer, Steve Schimel David, S. Holland Elisabeth, A. 1995D=Mechanisms of shrubland expansion: Land use, climate or CO-2?Climatic Change291 91-99followed by measurements of axial conductivity and calculation of radial conductivity. Predicted values of L-P for entire roots based on two to ten segments per root averaged 1.04+-0.07 (mean+-s.e. mean for n = 3) of the measured L-P for A. deserti and 1.06+-0.10 for O. ficus-indica. The model also closely predicted the drop in water potential along the root xylem (DELTA--psi-xylem); when a tension of 50 kPa was applied to the proximal ends of 0.2 m-long roots of A. deserti and O. ficus-indica, the measured DELTA--psi-xylem to midroot averaged 30 kPa compared with a predicted decrease of 36 kPa. Such steep gradients in psi-xylem suggest that the driving force for water movement from the soil to young distal roots may be relatively small. The model, which agreed with an analytical solution for a simple hypothetical situation, can quantify situations without analytical solutions, such as when root and soil properties vary arbitrarily along a root.s<6Anthoni, Peter M. Law, Beverly E. Unsworth, Michael H. 1999RLCarbon and water vapor exchange of an open-canopied ponderosa pine ecosystem*#Agricultural and Forest Meteorologyf953151-1688Eddy covariance measurements of carbon dioxide and water vapor exchange were made above a ponderosa pine (Pinus ponderosa Dougl. ex P. and C. Laws.) forest located in a semiarid environment in central Oregon. The stand is a mixture of old-growth and young trees. Annual net carbon gain by the ecosystem (NEE) was 320 +- 170 gC m-2 year-1 in 1996 and 270 +- 180 gC m-2 year-1 in 1997. Compared to boreal evergreen forest at higher latitudes, the pine forest has a substantial net carbon gain (150 +- 80 gC m-2 year-1 in 1996 and 180 +- 80 gC m-2 year-1 in 1997) outside the traditionally defined growing season (from bud swell in early May (Day 125) to partial leaf-off in late September (Day 275)). Carbon assimilation continued to occur in the relatively mild winters, though at a slower rate (April, maximum leaf level assimilation (Amax) of 6-9.5 mumol m-2 leaf s-1), and ecosystem respiration was relatively low (apprx1.6 +- 0.1 gC m-2 day-1). In the growing season, although photosynthetic capacity was large (July, Amax = 16-21 mumol m-2 leaf s-1), carbon assimilation was constrained by partial stomatal closure to maintain a sustainable water flow through the soil-plant system, and ecosystem respiration was large (3.5 +- 0.1 and 4.3 +- 0.1 gC m-2 day-1 in growing season of 1996 and 1997, respectively) because of high air and soil temperatures. Despite large changes in evaporative demand over just a few days (VPD changing from 0.5 to 3.5 kPa), the ecosystem water use was remarkably constant in summer (apprx1.6-1.7 mm day-1). Such homeostasis is most likely another result of stomatal control. Interannual variations in climate had a large influence on the ecosystem carbon balance. In summer 1997, an El Nino year, precipitation was more frequent (17 days with 33 mm of rain) than in summer 1996 (5 days with 5 mm of rain), and the net ecosystem exchange was substantially lower in July to September 1997 (10 +- 60 gC m-2) than during the equivalent period in 1996 (100 +- 60 gC m-2). Although temperatures between years were similar, the carbon assimilation in 1997 was offset by increased respiration, probably because soils were more frequently wet, encouraging microbial respiration.<5Archer, Steve Schimel David, S. Holland Elisabeth, A. 1995D=Mechanisms of shrubland expansion: Land use, climate or CO-2?Climatic Change291 91-999dD=Kalin, Arroyo M. T. Squeo, F. A. Armesto, J. J. Villagran, C. 1988jdEffects of aridity on plant diversity in the northern Chilean Andes: Results of a natural experiment.'Annals of the Missouri Botanical Gardenh751  55-78`ZHyperarid climates in western South America from 15 degree S to 29 degree S, extending up to 3,000 m in the northern Chilean Andes, result primarily from the Andes intercepting precipitation from the Intertropical Convergence and the drying effect of the equatorward-running Humboldt Current bathing much of the Pacific coast of South America. Paleobotanical, paleontological, and geological evidence suggests that such harsh climatic conditions developed very recently in Holocene times, following a turbulent Pleistocene history of alternating wet/cold and warm/dry periods. Seasonal climates probably first emerged in the Pliocene after a long, warm/wet Miocene during which precipitation is thought to have increased from east to west, as opposed to west to east, as occurs north of 25 degree S today. For the three remaining intact vegetation belts (desert scrub, Andean, and high Andean) in the northern Chilean Andes above the Atacama Desert, surveys on six transects, each approximately one-fourth of a degree latitude wide, located at 18 degree S, 19 degree S, 21 degree S, 24 degree S, 26 degree S, 28 degree S, revealed only 769 species of vascular plants in 290 genera. Altitudinally, species richness decreases with elevation in the winter rainfall zone, where precipitation is received from the southwest ("invierno chileno"; transects 26 degree S, 28 degree S), to peak at mid elevations in the summer rainfall zone where precipitation is received from the east ("invierno boliviano"; transects 18 degree S, 19 degree S, 21 degree S, 24 degree S). Species richness decreases by 80% and cover by 50% over the very severe rainfall gradient from 18-24 degree S. In spite of its greater reliance on water, the azonal bog flora has been less affected than the zonal flora by aridity, probably because of greater chances of reintroduction of species following their loss through long-distance dispersal. Species richness, when considered on a broad biogeographical scale, was significantly positively correlated with mean annual rainfall, as it was for most 500-m elevational levels when considered separately. The effect of cold temperatures at the higher elevations was completely obliterated by aridity on a biogeographical scale. Although diversity (measured as a synthetic characteristic combining richness and abundance) and species richness showed similar trends as aridity increases, loss of diversity tended to be more gradual in comparison with reductions in species richness. Aridity leads to community features analogous to those seen in the early stages of primary succession, maintained on a permanent basis. Losses in species richness with aridity along the western side of the Andes and from east to west across the Andes were overlain by highly characteristic life form tendencies. Perennial herbs are proportionately most abundant in areas of highest rainfall, annual herbs gain greatest prominence in areas of intermediate aridity, while woody species were proportionately most strongly represented under extreme dry/warm and extreme dry/cold conditions. The woody habit is generally more common in the northern Andes than in some similar North American plant communities. The maintenance of considerable diversity (in spite of severe loss in species richness) coupled with emphasis on long-lived woody species in the harshest environments in the northern Chilean Andes are tendencies also seen in many species-rich, climatically benign tropical plant communities. The hypothesis is developed that convergence in these features results from the similar selective effects of certain abiotic and biotic factors in the harsh arid environments of the northern Chilen Andes and in species-rich tropical communities. The hypothesis is developed that convergence in these features results from the similar selective effects of certain abiotic and biotic factors in the harsh arid environments of the northern Chilean Andes and in species-rich tropical communities, respectively. We predict that additional life-history trait similarities (e.g., in breeding systems) will emerge for the plant species of abiotically and biotically "harsh" environments, respectively.s than do subhumid grasslands. Consequently, changes in vegetation may influence the distribution of soil carbon and nutrients over time (perhaps decades to centuries). Shifts in the water balance are typically much more rapid. Catchment studies indicate that the water yield decreases 25-40 mm for each 10% increase in tree cover, and increases in transpiration of water taken up by deep roots may account for as much as 50% of observed responses. Because models are increasingly important for predicting the consequences of vegetation change, we discuss the treatment of belowground processes and how different treatments affect model outputs. Whether models are parameterized by biome or plant life form (or neither), use single or multiple soil layers, or include N and water limitation will all affect predicted outcomes. Acknowledging and understanding such differences should help constrain predictions of vegetation change.tJDJeltsch, Florian Stephan, Thomas Wiegand, Thorsten Weber Gerhard, E. 2001ZSArid rangeland management supported by dynamic spatially explicit simulation models 60Breckle, Siegmar W. Veste, Maik Wucherer, Walter& Sustainable land use in deserts "Heidelber, Berlin, New Yorks Springer Verlago229-240 Effects of nutrient depletion on growth of Holcus lanatus L. and Festuca ovina L. and on the ability of their roots to absorb nitrogen at warm and cool temperaturesKachi, N. Rorison, I. H.New Phytologistt 115m3r531-538i 1990|,*#Watson, I.W. Westoby, M. Holm, A.M. 1997{Continuous and episodic components of demographic change in two Eremophila species from arid Western Australia - Appendicese@JJournal of Ecology856833-8464("Weltzin, Jake F. McPherson, Guy R. 1997ztSpatial and temporal soil moisture resource partitioning by trees and grasses in a temperature savanna, Arizona, USA Oecologia 112f2e156-164 Stable isotope analysis was used to determine sources of water used by coexisting trees and grasses in a temperate savanna dominated by Quercus emoryi Torr. We predicted that (1) tree seedlings and bunchgrasses utilize shallow sources of soil water, (2) mature savanna trees use deeper sources of water, and (3) trees switch from shallow to deep water sources within 1 year of germination. We found that Q. emoryi trees, saplings, and seedlings (about 2 months, 1 year, and 2 years old), and the dominant bunchgrass (Trachypogon montufari (H.B.K.) Nees.) utilized seasonally available moisture from different depths within the soil profile depending on size/age relationships. Sapling and mature Q. emoryi acquired water from gt 50 cm deep, 2-month-old seedlings utilized water from lt 15 cm, and 1- and 2-year-old seedlings and grasses used water from between 20 cm and 35 cm. This suggests that very young seedlings are decoupled from grasses in this system, which may facilitate germination and early establishment of Q. emoryi within extant stands of native grasses. The potential for subsequent interaction between Q. emoryi and native grasses was evidenced by similar patterns of soil water use by 1- and 2-year-old seedlings and grasses. Q. emoryi seedlings did not switch from shallow to deep sources of soil water within 2 years of germination: water use by these seedlings apparently becomes independent of water use by grasses after 2 years of age. Finally, older trees (saplings, mature trees) use water from deeper soil layers than grasses, which may facilitate the stable coexistence of mature trees and grasses. Potential shifts in the seasonality of precipitation may alter interactions between woody plants and grasses within temperate savannas characterized by bimodal precipitation regimes: reductions in summer precipitation or soil moisture may be particularly detrimental to warm-season grasses and seedlings of Q. emoryi. ("Weltzin, Jake F. McPherson, Guy R. 1999^XFacilitation of conspecific seeding recruitment and shifts in temperate savanna ecotonesEcological Monographsr694e513-534e In contrast to documented increases in woody plant dominance of savannas and grasslands of North America, oak (Quercus L.) savannas that form lower tree lines in the southwestern United States and northwestern Mexico have been stable over the last several centuries. We sought to identify potential biotic and abiotic constraints on seedling recruitment of Quercus emoryi within the context of potential shifts in lower tree line. We used field surveys to describe seedling distribution at and below lower tree line, and to determine the potential for acorn dispersal from lower tree line into adjacent grassland. Field and greenhouse experiments were used to test explanatory hypotheses generated by descriptive surveys. Q. emoryi seedlings were located almost exclusively beneath mature, conspecific tree canopies within the woodland and savanna and were absent from adjacent semidesert grassland in 1993 and 1995. Seed bank surveys indicated that acorns were concentrated beneath tree canopie s and were dispersed into adjacent grassland in low numbers. Although soil N, C, and P were about two times greater beneath trees than in adjacent grassland, experimental nutrient amendments to subcanopy and grassland soils indicated that soil nutrients did not limit Q. emoryi growth. Reciprocal transfers of subcanopy and grassland soil to subcanopy and grassland microsites indicated that microsite was more important than soil source for seedling growth. Overstory shade was important at all stages of seedling development investigated: the provision of artificial or natural shade increased rates of seedling emergence and subsequent survival as much as 19-fold and increased recruitment rates between 30-and 60-fold. We conclude that rates of Q. emoryi recruitment within grassland below tree line are relatively low and are constrained by low rates of seed dispersal coupled with a low probability of seedling emergence. In contrast, large numbers of acorns are dispersed directly beneath Q. emoryi trees, where they have a higher probability of emergence than in adjacent grassland. Survival rates of emerged seedlings were low, regardless of landscape position. Thus, observed patterns of seedling distribution on the landscape resulted from interactions between seed dispersal and habitat-specific response of seedlings to environmental variation. Results of this and complementary research suggest that the lower tree line in southern Arizona is stabilized by self-enhancing feedback mechanisms of overstory shade, seed dispersal, and seedling establishment, coupled with strong abiotic constraints beyond the current ecotone. These processes stabilize the woodland-grassland ecotone both spatially and temporally, consistent with Wilson and Agnew's one-sided positive feedback switch. Although this switch would not produce an indefinitely stable vegetation mosaic, upslope or downslope shifts in lower tree line are apparently resistant to decadal or even century-scale climatic pe rturbation. The observed shift in tree line in the last millennium was less likely the result of slow, spatial progression of autogenic safe sites than the result of episodic and infrequent allogenic processes that simulated or negated the importance of conspecific, biogenic safe sites.N>7Cayan, D. R. Dettinger, M. D. Diaz, H. F. Graham, N. E.1 1998F?Decadal Variability of Precipitation over Western North America&Journal of Climate1112 3148-3166PIDecadal (>7- yr period) variations of precipitation over western North America account for 20%-50% of the variance of annual precipitation. Spatially, the decadal variability is broken into several regional [O(1000 km)] components. These decadal variations are contributed by fluctuations in precipitation from seasons of the year that vary from region to region and that are not necessarily concentrated in the wettest season(s) alone. The precipitation variations are linked to various decadal atmospheric circulation and SST anomaly patterns where scales range from regional to global scales and that emphasize tropical or extratropical connections, depending upon which precipitation region is considered. Further, wet or dry decades are associated with changes in frequency of at least a few short-period circulation "modes" such as the Pacific-North American pattern. Precipitation fluctuations over the southwestern United States and the Saskatchewan region of western Canada are associated with extensive shifts of sea level pressure and SST anomalies, suggesting that they are components of low-frequency precipitation variability from global-scale climate processes. Consistent with the global scale of its pressure and SST connection, the Southwest decadal precipitation is aligned with opposing precipitation fluctuations in northern Africa.g0)Cayan, D. R. Redmond, K. T. Riddle, L. G.a 1999@9ENSO and Hydrologic Extremes in the Western United StatesJournal of Climate129 2881-2893D=Frequency distributions of daily precipitation in winter and daily stream flow from late winter to early summer, at several hundred sites in the western United States, exhibit strong and systematic responses to the two phases of ENSO. Most of the stream flows considered are driven by snowmelt. The Southern Oscillation index (SOI) is used as the ENSO phase indicator. Both modest (median) and larger (90th percentile) events were considered. In years with negative SOI values (El Nino), days with high daily precipitation and stream flow are more frequent than average over the Southwest and less frequent over the Northwest. During years with positive SOI values (La Nina), a nearly opposite pattern is seen. A more pronounced increase is seen in the number of days exceeding climatological 90th percentile values than in the number exceeding climatological 50th percentile values, for both precipitation and stream flow. Stream flow responses to ENSO extremes are accentuated over precipitation responses. Evidence suggests that the mechanism for this amplification involves ENSO-phase differences in the persistence and duration of wet episodes, affecting the efficiency of the process by which precipitation is converted to runoff. The SOI leads the precipitation events by several months, and hydrologic lags (mostly through snowmelt) delay the stream flow response by several more months. The combined 6-12-month predictive aspect of this relationship should be of significant benefit in responding to flood (or drought) risk and in improving overall water management in the western states. Coexistence of competitors in spatially and temporally varying environments: A look at the combined effects of different sorts of variabilityChesson, P. L.$Theoretical Population Biology283,263-287b 1985A stochastic model is developed for competition among organisms living in a patchy and varying environment. The model is designed to be suitable for species with sedentary adults and widely dispersing larvae or propagules, and applies best to marine systems but may also be adequate for some terrestrial systems. Three kinds of environmental variation are incorporated simultaneously in the model. These are pure spatial variation, pure temporal variation, and the space times time interaction. All three kinds of variation can promote coexistence, and when variation is restricted to immigration rates, all three kinds act very similarly. Moreover, for long-lived organisms their action is nearly identical, and their effects, when present together, combine equivalently. For short-lived organisms, however, pure temporal variation is a less effective promoter of coexistence. Variation in death rates acts quite differently from variation in birth rates for it may demote coexistence in some circumstances, while promoting coexistence in other circumstances. Furthermore, pure spatial variation in death rates has quite different effects than other kinds of death-rate variation. In addition to conditions for coexistence, information is given on population fluctuation, convergence to stationary distributions, and asymptotic distributions for long-lived organisms. While the model is presented as an ecological model, a genetical interpretation is also possible. This leads to new suggested mechanisms for the maintenance of polymorphisms in populations.NGCommunity consequences of life-history traits in a variable environment Chesson, P. L. Huntly, N.o Annales Zoologici Fennicio251r 5-16 1988@9Short-term instabilities and long-term community dynamicsChesson, P. Huntly, N.$Trends in Ecology & Evolutiono410293-298o 1989F@Geometry, heterogeneity and competition in variable environmentsChesson, P. L.VOPhilosophical Transactions of the Royal Society of London B Biological Sciences 330 1257165-173a 1990pjThe effects of environmental fluctuations on coexistence of competing species can be understood by a new geometric analysis. This analysis shows how a species at low density gains and average growrth rate advantage when the environment fluctuates and all species have growth rates of the particular geometric form called subadditive. This low density advantage opposes competitive exclusion. Additive growth rates confer no such low density advantage, while superadditive growth rates promote competitive exclusion. Growth-rate geometry can be understood in terms of heterogeneity within populations. Total population growth is divided into different components, such as may be contributed by different life-history stages, phenotypes, or subpopulations in different microhabitats. The relevant aspects of such within-population heterogeneity can be displayed as a scatter plot of sensitivities of different components of population growth to environmental and competitive factors, and can be measured quantitatively as a covariance. A three-factor model aids the conceptual division of population growth into suitable components.\\Owen, A. G. Jones, D. L. 2001Competition for amino acids between wheat roots and rhizosphere microorganisms and the role of amino acids in plant N acquisition$Soil Biology and Biochemistry333 4-5651-657i$The direct uptake of organic nitrogen compounds from the soil solution by plant roots has been hypothesised to constitute a significant source of N to the plant particularly in N limiting ecosystems. The experiments undertaken here were designed to test whether wheat roots could out-compete the rhizosphere microflora for a pulse addition of organic N in the form of three contrasting amino acids, namely lysine, glycine and glutamate. Amino acids were added at a concentration reflecting reported soil solution concentrations (100 muM) and the uptake into either plant biomass or respiration or microbial biomass and respiration determined over a 24 h chase period. The results showed that the plant roots could only capture on average 6% of the added amino acid with the remainder captured by the microbial biomass. We therefore present direct in vivo evidence to support earlier work which has hypothesised that organic N may be of only limited consequence in high input agricultural systems. We suggest that this is a result of the higher concentrations of NO3- in agricultural soil solutions, the slow movement of amino acids in soil relative to NO3-, the rapid turnover of amino acids by soil microorganisms, and the poor competitive ability of plant roots to capture amino acids from the soil solution.4-Pandey, G. R. Cayan, D. R. Georgakakos, K. P.  1999NHPrecipitation structure in the Sierra Nevada of California during winter6/Journal of Geophysical Research. D. Atmospheres  104l D10e 019-12Influences of upper air characteristics along the coast of California upon wintertime (November-April) precipitation in the Sierra Nevada are investigated. Precipitation events in the Sierra Nevada region occur mostly during wintertime, irrespective of station location (leeside or windside) and elevation. Most precipitation episodes in the region are associated with moist southwesterly winds (coming from the southwest direction) and also tend to occur when the 700-mbar temperature at the upwind direction is close to -2 degree C. This favored wind direction and temperature signify the importance of both moisture transport and orographic lifting in augmenting precipitation in the region. By utilizing the observed dependency of the precipitation upon the upper air conditions, a linear model is formulated to quantify the precipitation observed at different sites as a function of moisture transport. The skill of the model increases with timescale of aggregation, reaching more than 50% variance explained at an aggregation period of 5-7 days. This indicates that upstream air moisture transport can be used to estimate the precipitation totals in the Sierra Nevada region.epjFactors determining seed persistence of Chondrilla juncea L. (skeleton weed) in southern Western AustraliaPanetta, F. D.$Australian Journal of Ecology132211-224l 1988Except when sown into non-wetting soils, buried seeds (achenes) of Chondrilla juncea germinated readily following summer rainfall events of less than 10 mm. Seeds lying on the surface were much less likely to germinate in response to small rainfall events, but were prone to predation by seed-harvesting ants. Although seedlings were capable of resuming growth following temporary dehydration at the earliest stages of emergence from the pericarp, very few of these were able to establish. Establishment from summer germination was virtually nil. The capacity to germinate at higher temperatures was increased following storage for seeds which had been produced in February and March. However, this was considered to be only a weak expression of a release from relative dormancy, compared to the marked changes in temperature response which occur during the after-ripening if many winter annuals. Differences in the germination behaviour and persistence of seeds of the narrow-leafed and broadleafed forms of C. juncea when evident, were usually slight. On the basis of an analysis of climatic records for 13 wheatbelt sites, obtained over the period for which C. juncea has been known to occur in Western Australia, it is argued that more than two germinating events could be expected to occur, on average, during the summer months. The relative absence of mechanisms which might confer protection from the effects of unseasonal rainfall, in conjunction with evidence for high levels of seed predation, point to considerable seed losses. Thus the abundance of the species elsewhere in Australia appears to have resulted from its capacity for vegetative regeneration.(!Paruelo, Jose M. Sala, Osvaldo E. 1995B;Water losses in the patagonian steppe: A modelling approachEcology762510-520 In this paper we sought to answer questions related to the long-term soil water dynamics of the Patagonian steppe: What are the magnitude and seasonal dynamics of transpiration, evaporation, and deep percolation? How do these fluxes respond to fluctuations in annual precipitation? What is the pattern of soil water availability? We developed a soil water model for the steppe with a daily time step. The model gives weekly cumulative values of transpiration, evaporation, and drainage, maximum and minimum water content for the different soil layers in each week, and the weekly frequency of days with soil water potential higher than -1 MPa for each layer. The model was tested against three sets of experimental data. Simulated data of total water losses were significantly correlated with observed data, and the slope did not differ significantly from 1 nor the gamma-intercept from 0. On a long term basis, evaporation accounted for 56% of total water loss, transpiration 34%, and deep percolation the remaining 10%. Transpiration and evaporation had asynchronic dynamics. Evaporation was high during the coldest and wettest months of the year (mainly winter months). Transpiration, on the contrary, reached maximum values when energy and water availability were simultaneously high in late spring-early summer. Drainage took place during the coldest months, when most of precipitation occurred, and the soil remained near field capacity. Both evaporation and transpiration had a positive response to an increase in precipitation. However, the proportion of total water loss following these pathways decreased with increasing precipitation. Drainage had a positive exponential relationship with winter precipitation. Probabilities of soil water potential higher than -1 MPa in the upper soil layer were very low during most of the warm season (P lt 0.15). At the beginning of the growing season the wettest layer was located at an intermediate depth (10-20 and 20-40 cm), and moved downward so at the end, only deep roots had high soil water availability. 2+Carrera, A. L. Sain, C. L. Bertiller, M. B.l 2000`ZPatterns of nitrogen conservation in shrubs and grasses in the Patagonian Monte, ArgentinaPlant and Soil 22426185-193 We analysed the main plant strategies to conserve nitrogen in the Patagonian Monte. We hypothesized that the two main plant functional groups (xerophytic evergreen shrubs and mesophytic perennial grasses) display different mechanisms of nitrogen conservation related to their structural and functional characteristics. Evergreen shrubs are deep-rooted species, which develop vegetative and reproductive growth from spring to late summer coupled with high temperatures, independently from water inputs. In contrast, perennial grasses are shallow-rooted species with high leaf turnover, which display vegetative growth from autumn to spring and reproductive activity from mid-spring to early-summer, coupled with precipitation inputs. We selected three evergreen shrubs (Larrea divaricata Cav., Atriplex lampa Gill. ex Moq. and Junellia seriphioides (Gilles and Hook.) Moldenke) and three perennial grasses (Stipa tenuis Phil., S. speciosa Trin. and Rupr. and Poa ligularis Nees ex Steud.), characteristic of undisturbed and disturbed areas of the Patagonian Monte. N concentration in expanded green and senesced leaves was estimated in December 1997 (late spring) and June 1998 (late autumn). Deep-rooted evergreen shrubs displayed small differences in N concentration between green and senesced leaves (low N-resorption efficiency), having high N concentration in senesced leaves (low N-resorption proficiency). Shallow-rooted perennial grasses, conversely, showed high N-resorption efficiency and high N-resorption proficiency (large differences in N concentration between green and senesced leaves and very low N concentration in senesced leaves, respectively). The lack of a strong mechanism of N resorption in evergreen shrubs apparently does not agree with their ability to colonize N-poor soils. These results, however, may be explained by lower N requirements in evergreen shrubs resulting from lower growth rates, lower N concentrations in green leaves, and lower leaf turnover as compared with perennial grasses. Long-lasting N-poor green tissues may, therefore, be considered an efficient mechanism to conserve N in evergreen shrubs in contrast with the mechanism of strong N resorption from transient N-rich tissues displayed by perennial grasses. Evergreen shrubs with low N-resorption efficiency provide a more N-rich substrate, with probably higher capability of N mineralization than that of perennial grasses, which may eventually enhance N fertility and N availability in N-poor soils.D.R. Cayan R.H. Webb 1992NHEl Nino/Southern Oscillation and streamflow in the western United States H.F. Diaz V. MarkgrafNHEl Nino Historical and Paleoclimatic Aspects of the Southern Oscillation  Cambridget Cambridge University Press 29-69b   27-36$://000084493300003tb\BassiriRad, H. Tremmel, D. C. Virginia, R. A. Reynolds, J. F. de Soyza, A. G. Brunell, M. H.rlShort-term patterns in water and nitrogen acquisition by two desert shrubs following a simulated summer rain Plant Ecologyrdelta N-15; desert shrubs; nitrogen uptake; rain; rapid response; water uptake fertile-soil microsites; root-growth; osmotic adjustment; larrea-tridentata; ecosystems; nitrate; field; n-15; availability; succulentsB://000089385100009a"Pockman, W. T. Sperry, J. S.ZSVulnerability to xylem cavitation and the distribution of Sonoran desert vegetation American Journal of BotanyLEcomparative approach; Sonoran desert vegetation; species distribution; xylem cavitation; xylem conducting efficiency; water relations stomatal conductance; hydraulic conductance; water-stress; betula-occidentalis; artemisia-tridentata; populus-balsamifera; drought tolerance; chaparral shrubs; seasonal-changes; air- embolismrWe studied 15 riparian and upland Sonoran desert species to evaluate how the limitation of xylem pressure (Psi(x)) by cavitation corresponded with plant distribution along a moisture gradient. Riparian species were obligate riparian trees (Fraxinus velutina. Populus fremontii, and Salix gooddingii), native shrubs (Baccharis spp.), and an exotic shrub (Tamarix ramosissima). Upland species were evergreen (Juniperus monosperma, Larrea tridentata), drought-deciduous (Ambrosia dumosa, Encelia farinosa, Fouquieria splendens, Cercidium microphyllum), and winter-deciduous (Acacia spp., Prosopis velutina) trees and shrubs. For each species, we measured the "vulnerability curve" of stem xylem, which shows the decrease in hydraulic conductance from cavitation as a function of Psi(x) and the Psi(erit) representing the pressure at complete loss of transport. We also measured minimum in situ Psi(x)(Psi(xmin)) during the summer drought. Species in desert upland sites were uniformly less vulnerable to cavitation and exhibited lower Psi(xmin) than riparian species. Values of Psi(erit) were correlated with minimum Psi(x). Safety margins (Psi(xmin)-Psi(erit)) tended to increase with decreasing Psi(xmin) and were small enough that the relatively vulnerable riparian species could not have conducted water at the Psi(x) experienced in upland habitats (-4 to -10 MPa). Maintenance of positive safety margins in riparian and upland habitats was associated with minimal to no increase in stem cavitation during the summer drought. The absence of less vulnerable species from the riparian zone may have resulted in parr from a weak bur significant trade-off between decreasing vulnerability to cavitation and conducting efficiency. These data suggest that cavitation vulnerability limits plant distribution by defining maximum drought tolerance across habitats and influencing competitive ability of drought tolerant species in mesic habitats. Am. J. Bot.e 2000 Sept879y355KP AMER J BOTISI:000089385100009e>8Porporato, A D'Odorico, P Ridolfi, L Rodriguez-Iturbe, I 2000hb A Spatial Model for Soil-Atmosphere Interaction: Model Construction and Linear Stability Analysis"Journal of Hydrometeorologyp1l 61-74 A simple model is developed to investigate the role of spatial dynamics in the soil-atmosphere system. The model is constructed by considering the mass and energy balance equations for soil and atmosphere, closed with a two-dimensional, corrected quasigeostrophic approximation for large-scale atmospheric motions, and a suitable model for rainfall occurrence. The main result presented concerns the linear stability analyses of the homogenous equilibrium solutions for dry and wet climates. In the first case (desert) the system is stable and no spatial perturbation internal to the system can produce spatial heterogeneities. In the second case (wet climate) the dynamics can produce spatial instabilities of several scales, resulting in likely patterns of wet and dry zones. A key role in triggering this instability is played by the sensible heat flux to the atmosphere, which in turn is linked to soil moisture.r:3Porporato, A Laio, F Ridolfi, L Rodriguez-Iturbe, I 2001Plants in water-controlled ecosystems: active role in hydrologic processes and response to water stress III. Vegetation water stress"Advances in Water Resources24725-744The reduction of soil moisture content during droughts lowers the plant water potential and decreases transpiration; this in turn causes a reduction of cell turgor and relative water content which brings about a sequence of damages of increasing seriousness. A review of the literature on plant physiology and water stress shows that vegetation water stress can be assumed to start at the soil moisture level corresponding to incipient stomatal closure and reach a maximum intensity at the wilting point. The mean crossing properties of these soil moisture levels crucial for water stress are derived analytically for the stochastic model of soil moisture dynamics described in Part II (F. Laio, A. Porporato, A. Ridolfi, I. Rodriguez-Iturbe. Adv. Water Res. 24 (7) (2001) (07-723). These properties are then used to propose a measure of vegetation water stress which combines the mean intensity, duration, and frequency of periods of soil water deficit. The characteristics of vegetation water stress are then studied under different climatic conditions, showing how the interplay between plant, soil, and environment can lead to optimal conditions for vegetation.>7Pregitzer, Kurt S. Hendrick, Ronald L. Fogel, Robert T. 1993PIThe demography of fine roots in response to patches of water and nitrogenNew Phytologist 1253t575-580Fine root demography was quantified in response to patches of increased water and nitrogen availability in a natural, second-growth, mixed hardwood forest in northern Michigan, USA. As expected, the addition of water and water plus nitrogen resulted in a significant overall increase in the production of new fine roots. New root production was much greater in response to water plus nitrogen when compared with water alone, and the duration of new root production was related to the length of resource addition in the water plus nitrogen treatments; the average difference in new root length between the 20 vs. 40 d additions of water plus nitrogen amounted to almost 600%. Roots produced in response to the additions of water and water plus nitrogen lived longer than roots in the control treatments. Thus, additions of water and water plus nitrogen influenced both the proliferation of new roots and their longevity, with both proliferation and longevity related to the type and duration of resource supply. Results suggest that root longevity and mortality may be plastic in response to changes in soil resource availability, as is well known for root proliferation.Bnv <486-489$://A1987K285600002s&Richards, J. H. Caldwell, M. M.nhHydraulic Lift - Substantial Nocturnal Water Transport between Soil Layers by Artemisia-Tridentata Roots Oecologia Oecologia 1987 Oct734xK2856 OECOLOGIAISI:A1987K285600002>8Ridolfi, L D'Odorico, P Porporato, A Rodriguez-Iturbe, I 2000PIDuration and frequency of water stress in vegetation: An analytical modelWater Resources Research36 2297-2307o$Robinson, J.V. C.D. Sandgren 1983pjThe effect of temporal environmental heterogeneity on community structure: a replicated experimental study Oecologiac57 98-102>8Rodriguez-Iturbe, I D'Odorico, P Porporato, A Ridolfi, L 1999VPOn the spatial and temporal links between vegetation, climate, and soil moistureWater Resources Research35 3709-3722:3Rodriguez-Iturbe, I Porporato, A Laio, F Ridolfi, L 2001 Plants in water-controlled ecosystems: active role in hydrologic processes and response to water stress I. Scope and general outlinet"Advances in Water Resourcesv24697-705 d]This series of four papers studies the complex dynamics of water-controlled ecosystems from the hydro-ecological point of view [e.g., I. Rodriguez-Iturbe, Water Resour. Res. 36 (1) (2000)[3-9]. After this general outline, the role of climate, soil, and vegetation is modeled in Part II [F. Laio, A. Porporato, L. Ridolfi, I. Rodriguez-Iturbe, Adv. Water Res. 24 (7) (2001) [707-723] to investigate the probabilistic structure of soil moisture dynamics and the water balance. Particular attention is given to the impact of timing and amount of rainfall, plant physiology, and soil properties. From the statistical characterization of the crossing properties of arbitrary levels of soil moisture, Part III develops an expression for vegetation water stress [A. Porporato, F. Laio, L. Ridolfi, I. Rodriguez-Iturbe, Adv. Water Res. 24 (7) (2001) [25-744]. This measure of stress is then employed to quantify the response of plants to soil moisture deficit as well as to infer plant suitability to given environmental conditions and understand some of the reasons for possible coexistence of different species. Detailed applications of these concepts are developed in Part IV [F. Laio, A. Porporato, C.P. Fernandez-Illescas, I. Rodriguez-Iturbe, Adv. Water Res. 24 (7) (2001) 745-762], where we investigate the dynamics of three different water-controlled ecosystems.t<5Rothstein, David E. Zak, Donald R. Pregitzer, Kurt S.f 1996jcNitrate deposition in northern hardwood forests and the nitrogen metabolism of Acer saccharum marsh Oecologia 108a2s338-344EIt is generally assumed that plant assimilation constitutes the major sink for anthropogenic Nitrate NO-3- deposited in temperate forests because plant growth is usually limited by nitrogen (N) availability. Nevertheless, plants are known to vary widely in their capacity for NO-3- uptake and assimilation, and few studies have directly measured these parameters for overstory trees. Using a combination of field and greenhouse experiments, we studied the N nutrition of Acer saccharum Marsh. in four northern hardwood forests receiving experimental NO-3- additions equivalent to 30 kg N ha-1 year-1. We measured leaf and fine-root nitrate reductase activity (NRA) of overstory trees using an in vivo assay and used 15N to determine the kinetic parameters of NO-3- uptake by excised fine roots. In two greenhouse experiments, we measured leaf and root NRA in A. saccharum seedlings fertilized with 0-3.5 g NO-3--N m-2 and determined the kinetic parameters of NO-3- and NH-4+ uptake in excised roots of seedlings. In both overstory trees and seedlings, rates of leaf and fine root NRA were substantially lower than previously reported rates for most woody plants and showed no response to NO-3- fertilization (range = non-detectable to 33 nmol NO-2- g-1 h-1). Maximal rates of NO-3- uptake in overstory trees also were low, ranging from 0.2 to 1.0 mu-mol g-1 h-1. In seedlings, the mean V-max for NO-3- uptake in fine roots (1 mu-mol g-1 h-1) was approximately 30 times lower than the V-max for NH-4+ uptake (33 mu-mol g-1 h-1). Our results suggest that A. saccharum satisfies its N demand through rapid NH-4+ uptake and may have a limited capacity to serve as a direct sink for atmospheric additions of NO-3-.n"Roundy, B. A. Dobrenz, A. K. 1989voHerbivory and Plant Water Status of Jojoba Simmondsia-Chinensis Link Schn. in the Sonoran Desert in Arizona UsaO"Journal of Arid Environments163283-292-Herbivory may alter the ability of a plant to compete with ungrazed neighbors for limited resources such as water in arid environments. Water status parameters of grazed and ungrazed jojoba [Simmondsia chinensis (Link) Schn.], a drought-tolerant, evergreen shrub of the Sonoran Desert, were compared over a 2-year period in Arizona. Ungrazed shrubs were larger than grazed shrubs but had similar canopy leaf area to larger grazed shrubs and about four times the leaf area of smaller grazed shrubs. Xylem water potential, transpiration rate, and stomatal resistance were similar for grazed and ungrazed shrubs both in early spring and later in early summer at the peak of the drought period. Xylem water potential of jojoba was generally as low or lower than wolfberry (Lycium exsertum A. Gray) and was much lower than paloverde [Cercidium microphyllum (Torey) Rose & Johnston], which apparently had deep roots in contact with wetter soil than jojoba. The ability of jojoba to transpire and take up water while enduring low plant water potential probably allows it to reduce the soil matric potential around its roots below that available to possible competitors. As evaporative demand increased and water potential decreased into the drought period, jojoba transpiration was greatly reduced by stomatal closure in mid-afternoon which resulted in increased xylem water potential as water uptake allowed recovery of transpirational water loss. The ability of jojoba to grow during short seasonal periods of water availability in the Sonoran Desert and its ability to endure low plant water potential while continuing limited phostosynthesis are suggested as important factors in jojoba tolerance to herbivory as well as drought."<226-236$://000086182600005 F@Adiku, S. G. K. Rose, C. W. Braddock, R. D. Ozier-Lafontaine, H.\VOn the simulation of root water extraction: Examination of a minimum energy hypothesis Soil Sciencenhmaize; modelling; root distribution; root water uptake; sorghum; transpiration soil; conductivity; plantAn alternative procedure for calculating root water extraction from different depths of a soil profile is proposed, The procedure is based on the concept that the root water extraction entails energy expenditure by the plant and that the plant seeks to minimize the total rate of energy expenditure during water uptake. The model, therefore, considers root water extraction to be a minimization problem whose solution can be sought using a dynamic programming framework. We tested the model by simulating the variations of the soil water content using the time and depth of a maize-sorghum intercrop experiment reported for an 8-day drying cycle by Ozier- Lafontaine et al, (Plant and Soil 204:183-201, 1998), Simulated patterns follow the observed water content distribution quite well. Furthermore, results from numerical experiments show that the model is capable of simulating a range of water extraction patterns in a realistic manner. Patterns of water extraction from uniformly wet soil profiles follow those of the root distribution with depth. The extraction rate is highest in the section where the root length density is also highest. Once the soil profile dries out, water extraction patterns cease to bear any similarity to root distribution. Model simulations also show increased root activity at greater depths when the top sections of the soil dry out. Generally, the model avoids the need to make any prior assumptions about the pattern of the root water extraction.  Soil Sci.h 2000 Mar  165b3w299EJ SOIL SCIISI:000086182600005hAgnew, A. D. Q. 19976/Switches, pulses and grazing in arid vegetation"Journal of Arid Environments374609-617This paper examines the possible effect of plants of drylands on their own environment and on neighbouring individuals, in particular the potential for positive feedback between individuals and populations. There are many examples both in shrubby desert vegetation and in desert grassland, but it is in the grassland that feedback is especially associated with small rainfall pulses. I suggest that boundaries between grassland and shrubland are likely to be abrupt in space and time because desert grassland vegetation has a more positive reaction to rainfall change and to its own effects on its environment. Grazing has an important part to play in this relationship..'Aguado, Santacruz G. A. Garcia, Moya E. 1998Environmental factors and community dynamics at the southernmost part of the North American Graminetum: I. On the contribution of climatic factors to temporal variation in species compositiona Plant Ecology 1351 13-29Compositional patterns of vegetation and their relationship to temporal and spatial environmental variation, with emphasis on climatic factors, were investigated in plant communities located in the southernmost portion of the North American Graminetum in central Mexico. Data from 353 samples, obtained in four ecologically contrasting plant communities during 11 years, were analyzed by partial canonical correspondence analysis. Eight climatic variables and eighteen covariables (seventeen edaphic and one resource management) were included in the ordination. A relationship between floristic change and weather variation, once covariables effects were fitted, was examined. Despite a strong contrast in ecological conditions among study sites, a set of four climatic variables was finally found in which each variable contributed independently and statistically (P < 0.01) to the total variance in the vegetation data. Thus, environmental variables other than climatic could not conceal the important role of weather as a mediator of floristic change through time. Summer precipitation and summer maximum temperature showed the highest correlations with the first two species axes, 0.77 and -0.39, respectively. Contribution of these two climatic variables to variance in the vegetation data explained by environmental variables was approximately 81%. Annual species were abundant during rainy years, while abundance of perenial grasses and shrubs showed no clear relationship to weather variation. This study explicitly probes the important role of rain patterns in shaping structure and composition of semiarid communities at the southernmost part of the North American Graminetum.("Aguiar, Martin R. Sala, Osvaldo E. 1994d]Competition, facilitation, seed distribution and the origin of patches in a Patagonian steppe4 Oikos-70 26-34The Patagonian steppe is composed mainly of shrubs and tussock grasses organized in two types of patches: (1) scattered grass tussocks in a matrix of bare soil and (2) shrubs, each surrounded by a dense ring of grass tussocks. We analyzed the variation of competition, facilitation, and seed distribution through space and time as major driving forces in the development of this patch structure. Emergence and survival of grass seedlings increased with distance from shrubs when the ring of grasses was left intact. On the contrary, when the ring of grasses was experimentally removed, seedling survival decreased with distance from the shrub. Differences in root density, soil water potential, and evaporation accounted for these patterns. Density of naturally dispersed seeds of grasses decreased with distance from the shrub. Simulated recruitment near shrubs without the ring of grasses was twice as high as near shrubs with the ring. High root competition near the shrubs surrounded by a ring of grasses decreased the survivability of seedlings and overshadowed the aerial protection provided by the shrubs. We suggest that when shrubs are young and small, facilitation is more important than competition which results in the formation of the dense ring of grasses. When the shrub becomes large and the ring complete, competition overshadows facilitation. After shrub death, the ring may disintegrate and remnant tussocks may form the other patch type, scattered grass tussocks.sPIAguiar, Martin R. Paruelo, Jose M. Sala, Osvaldo E. Lauenroth, William K.  1996pjEcosystem responses to changes in plant functional type composition: An example from the Patagonian steppe$Journal of Vegetation Scienceo7o381-390Grass cover along a grazing intensity gradient in Patagonia decreases, whereas bare soil and shrub cover increases. Our objective was to study the effect of a change in the dominant plant functional type on soil water balance, primary production, herbivore biomass, roughness, and albedo. Using a soil water balance model, we found increases in evaporation and deep drainage, and a decrease in total transpiration along the grazing intensity gradient. Above-ground primary production, estimated from transpiration, decreased along the grazing intensity gradient because shrubs did not fully compensate for the decrease in grass production. Using a statistical model, we calculated herbivore biomass from estimates of above-ground primary production. Estimated herbivore biomass was lowest in the shrub-dominated extreme of the grazing gradient. Roughness increased from the grass-dominated to the shrub-dominated community. Albedo had a maximum at an intermediate position along the gradient. Our results suggest that changes in plant functional type composition, independent of changes in biomass, affect ecosystem functioning and the exchange of energy and material with the atmosphere. Grasses and shrubs proved to be appropriate plant functional types to link structure and function of ecosystems.ekClark, James S. Carpenter, Steven R. Barber, Mary Collins, Scott Dobson, Andy Foley, Jonathan A. Lodge, David M. Pascual, Mercedes Pielke, Roger, Jr. Pizer, William Pringle, Cathy Reid, Walter V. Rose, Kenneth A. Sala, Osvaldo Schlesinger, William H. Wall, Diana H. Wear, DavidS 20012,Ecological forecasts: An emerging imperativeSciencen 293i 5530657-660Planning and decision-making can be improved by access to reliable forecasts of ecosystem state, ecosystem services, and natural capital. Availability of new data sets, together with progress in computation and statistics, will increase our ability to forecast ecosystem change. An agenda that would lead toward a capacity to produce, evaluate, and communicate forecasts of critical ecosystem services requires a process that engages scientists and decision-makers. Interdisciplinary linkages are necessary because of the climate and societal controls on ecosystems, the feedbacks involving social change, and the decision-making relevance of forecasts.n534-543$://A1997XV92100002a"Clothier, B. E. Green, S. R.:3Roots: The big movers or water and chemical in soilf Soil Science4.time-domain reflectometry; sap flow; kiwifruitRoot water uptake and the dynamic availability of water to plants is a phenomenon that tends to be overlooked by soil scientists, despite the often dominant role of roots as a sink for water in the soil. Water taken up by roots is vital for plant growth and the productive management of soils. In addition, trapping and consuming water in the rootzone means there is less remaining to act as a vehicle to carry chemicals beyond the grasp of roots and consign them to either receiving groundwater bodies or surface reserves. Here we review landmark developments in the theoretical description of the availability of water to roots. By the 1960s, thanks to Wilford Gardner and others, we had achieved a good theoretical understanding of the interplay between soil characteristics and water availability for a single, isolated, semiinfinite root embedded in a cylinder of soil. We discuss the incorporation of these simple theoretical notions into the multitude of comprehensive simulation models of rootzone functioning that followed the democratic spread of computers throughout the 1970s. Much still remains to be done in linking rootzone form to root functioning, but technology is coming to the rescue. New techniques are providing improved means by which we can better observe both the changing spatial form of roots and the temporal pattern of their functioning. Time Domain Reflectometry for measuring soil water content close to roots and near the soil surface, in tandem with instantaneous monitoring of sap flow directly within roots, is providing a sharper view of root functioning. Meanwhile improved rhizotrons, along with new techniques of image analysis and topological description, are providing better descriptions of root system form. We feel that progress in understanding the role of roots as the big movers of water and chemical in soil will, in the near future, remain driven by improvements in our ability to observe the link between rootzone form and function. Soil Sci. 1997 Aug 1628XV921 SOIL SCIISI:A1997XV92100002 Cody, M. L. 1986,&Structural niches in plant communities J. Diamond T. J. CaseCommunity Ecology New York  Harper & Row381-405on was advanced in two ways: on the one hand, a geographic model was elaborated; and on the other, a temporal sequence for various climatic situations was developed. During the last 1000 yr, two significant events related to global changes occurred: the Medieval Warm Period (MWP) and the Little Ice Age (LIA). The Medieval Warm Period was characterized by a humid and warm climate in the plains and recession of the Andean glaciers. In contrast, during the Little Ice Age the plains had temperate, semi-arid to arid climates, and Andean glaciers advanced. In the western region, the fluvial-lacustrine systems were more extensive during cold events (LIA) and contracted during warm events (MWP). In contrast, in the eastern region the fluvial- lacustrine systems showed a diminution during cold events and increased their extent during warm episodes. During the LIA, the occurrence of two cold pulses separated by an intermediate period has been established. The first cold pulse extended from the beginning of the XV century to the end of the XVI century; the second cold pulse (the main one) began at the beginning of the XVIII century and lasted until the beginning of the XIX century. Both cold pulses can be related to the Sporer and Maunder Minimums respectively. These climatic changes modified the landforms, influenced the vegetation distribution and were one of the main factors for control of human activities during the last 1000 yr. (C) 2000 Elsevier Science Ltd and INQUA. All rights reserved. Quat. Int. 199962296XK QUATERN INTISI:000086051900005Clark, James S. Carpenter, Steven R. Barber, Mary Collins, Scott Dobson, Andy Foley, Jonathan A. Lodge, David M. Pascual, Mercedes Pielke, Roger, Jr. Pizer, William Pringle, Cathy Reid, Walter V. Rose, Kenneth A. Sala, Osvaldo Schlesinger, William H. Wall, Diana H. Wear, DavidS 20012,Ecological forecasts: An emerging imperativeSciencen 293i 5530657-660Planning and decision-making can be improved by access to reliable forecasts of ecosystem state, ecosystem services, and natural capital. Availability of new data sets, together with progress in computation and statistics, will increase our ability to forecast ecosystem change. An agenda that would lead toward a capacity to produce, evaluate, and communicate forecasts of critical ecosystem services requires a process that engages scientists and decision-makers. Interdisciplinary linkages are necessary because of the climate and societal controls on ecosystems, the feedbacks involving social change, and the decision-making relevance of forecasts.n534-543$://A1997XV92100002a"Clothier, B. E. Green, S. R.:3Roots: The big movers or water and chemical in soilf Soil Science4.time-domain reflectometry; sap flow; kiwifruitRoot water uptake and the dynamic availability of water to plants is a phenomenon that tends to be overlooked by soil scientists, despite the often dominant role of roots as a sink for water in the soil. Water taken up by roots is vital for plant growth and the productive management of soils. In addition, trapping and consuming water in the rootzone means there is less remaining to act as a vehicle to carry chemicals beyond the grasp of roots and consign them to either receiving groundwater bodies or surface reserves. Here we review landmark developments in the theoretical description of the availability of water to roots. By the 1960s, thanks to Wilford Gardner and others, we had achieved a good theoretical understanding of the interplay between soil characteristics and water availability for a single, isolated, semiinfinite root embedded in a cylinder of soil. We discuss the incorporation of these simple theoretical notions into the multitude of comprehensive simulation models of rootzone functioning that followed the democratic spread of computers throughout the 1970s. Much still remains to be done in linking rootzone form to root functioning, but technology is coming to the rescue. New techniques are providing improved means by which we can better observe both the changing spatial form of roots and the temporal pattern of their functioning. Time Domain Reflectometry for measuring soil water content close to roots and near the soil surface, in tandem with instantaneous monitoring of sap flow directly within roots, is providing a sharper view of root functioning. Meanwhile improved rhizotrons, along with new techniques of image analysis and topological description, are providing better descriptions of root system form. We feel that progress in understanding the role of roots as the big movers of water and chemical in soil will, in the near future, remain driven by improvements in our ability to observe the link between rootzone form and function. Soil Sci. 1997 Aug 1628XV921 SOIL SCIISI:A1997XV92100002 Cody, M. L. 1986,&Structural niches in plant communities J. Diamond T. J. CaseCommunity Ecology New York  Harper & Row381-405h` 23-38$://A1996UW52800003t>8Kremer, R. G. Hunt, E. R. Running, S. W. Coughlan, J. C.Simulating vegetational and hydrologic responses to natural climatic variation and GCM-predicted climate change in a semi- arid ecosystem in Washington, USA"Journal of Arid EnvironmentsJ. Arid. Environ. 1996 May331PUW528 J ARID ENVIRONISI:A1996UW52800003427-439$://000084682100009aLai, C. T. Katul, G.ZSThe dynamic role of root-water uptake in coupling potential to actual transpirationm"Advances in Water Resourcesiroot water uptake; root density; root efficiency; transpiration; potential evaporation; infiltration heat-flux estimation; soil-water; extraction patterns; model; plant; moisture; growth; evaporation; sensitivity; simulationThe relationship between actual (E-act) and potential (E-p) transpiration above a grass-covered forest clearing was investigated numerically and experimentally from simultaneous measurements of soil moisture content profiles, mean meteorological conditions, turbulent heat and water vapor fluxes in the atmospheric surface layer, and soil hydraulic properties for two drying periods. The relationship between E- act/E-p was found to be approximately constant and insensitive to variability in near-surface soil moisture content. To explore this near-constant E-act/E-p, a model that relates potential and actual transpiration and accounts for root-uptake efficiency, potential transpiration rate, and root-density distribution was proposed and field-tested. The total amount of water consumed by the root system was integrated and compared with eddy-correlation latent heat flux measurements (field scale) and total water storage changes (local scale). Model calculations suggested that the deeper and more efficient roots are primarily responsible for the total water loss within the root zone when the near-surface soil layer approaches their wilting point. (C) 1999 Elsevier Science Ltd. All rights reserved.iAdv. Water Resour. 2000 Jan 11234t272ZV ADV WATER RESOURISI:000084682100009o:3Laio, F Porporato, A Ridolfi, L Rodriguez-Iturbe, I 2001Plants in water-controlled ecosystems: active role in hydrologic processes and response to water stress II. Probabilistic soil moisture dynamics"Advances in Water Resources!24707-723LFA stochastic model for soil moisture dynamics at a point is studied in detail. Rainfall is described as a marked Poisson process, producing a state-dependent infiltration into the soil. Losses due to leakage and evapotranspiration also depend on the existing level of soil moisture through a simplifying but realistic representation of plant physiological characteristics and soil properties. The analytic solution of the steady-state probability distributions is investigated to assess the role of climate, soil, and vegetation in soil moisture dynamics and water balance.F?Laio, F Porporato, A Fernandez-Illescas, CP Rodriguez-Iturbe, I 2001Plants in water-controlled ecosystems: active role in hydrologic processes and response to water stress IV. Discussion of real cases" Advances in Water Resources24745-762TMThree water-controlled ecosystems are studied here using the stochastic description of soil moisture dynamics and vegetation water stress proposed in Part II (F. Laio, A. Porporato, L. Ridolfi, I. Rodriguez-Iturbe, Adv. Water Res. 24 (7) (2001) 707-723) and Part III (A. Porporato, F. Laio, L. Ridolfi, I. Rodriguez-Iturbe, Adv. Water Res. 24 (7) (2001) 725-744) of this series of papers. In the savanna of Nylsvley (South Africa) the very diverse physiological characteristics of the existing plants give rise to different strategies of soil moisture exploitation. Notwithstanding these differences, the vegetation water stress for all the species turns out to be very similar, suggesting that coexistence might be attained also through differentiation of water use. The case of the savanna of Southern Texas points out how rooting depth and interannual rainfall variability can impact soil moisture dynamics and vegetation water stress. Because of the different responses to water stress of trees and grasses, external climatic forcing could be at the origin of the dynamic equilibrium allowing coexistence in this ecosystem. Finally, the analysis of a short grass steppe in Colorado provides an interesting example of the so-called inverse texture effect, whereby preferential conditions for vegetation are dependent on soil texture and rainfall. Sites which are more favorable during wet conditions may become less suitable to the same vegetation type during drier years. Such an effect is important to explain the predominance of existing species, as well as to investigate their reproductive strategies.eAN ,d&Moorhead, D. L. Reynolds, J. F. 1991PIA General Model of Litter Decomposition in the Northern Chihuahuan DesertEcological Modelling56 1-4197-220.Numerous empirical studies have described the pathways of mass, C and N flows during decomposition, but there remains a paucity of data on underlying mechanisms in arid ecosystems. In the northern Chihuahuan Desert, termites remove large quantities of litter and act as carbon and nitrogen sinks, contributing to low soil fertility. In their absence, decomposition at the soil surface is primarily driven by abiotic weathering, but studies suggest buried litter decay occurs through microbiological activities. We develop a general, synthetic model to examine the interactions between buried litter, decomposer microorganisms, and C and N pools in this ecosystem. Our goal is to explore the mechanisms underlying observed patterns of decomposition in arid systems using a modelig approach that balances simplicity with enough detail to suggest the reasons for system behavior. To this end, we utilize elements of existing models, interfacing microbial physiology and population dynamics with empirical observations of C and N pool dynamics, litter mass loss and changing C:N ratios. Good agreement was achieved between simulated and observed patterns of mass loss and nitrogen concentrations once a time lag describing the microbial colonization of litter was induced. Model results indicate nutrient availabilities may be determined by relatively short-term carbon dynamics mediated by microflora since soil organic matter and nitrogen content are low. Model behavior also suggests decomposer organisms immobilize nitrogen from surrounding soils, accounting for the elevated quantities observed within decaying materials. Past hypotheses have proposed that soil flora and fauna partially decouple decomposition processes from abiotic constraints in this system. This study indicates that the pattern of microbial activities, accounting for the decomposition of buried materials in the absence of termites, is primarily determined by climatic conditions.273-286$://000085378000018r"Musters, P. A. D. Bouten, W.|vA method for identifying optimum strategies of measuring soil water contents for calibrating a root water uptake modelJournal of Hydrologytnevapotranspiration; soil moisture; unsaturated zone; root water uptake; calibration dynamics; stand; ecosystem  Measurements of soil water contents, such as with Time Domain Reflectometry (TDR) are essential to calibrate models that estimate the vertical distribution of root water uptake by simulating the unsaturated flow of soil water. This study presents a method to determine how the number of TDR probes used, their orientation and depth of installation affect uncertainties in calibrating a Root Water Uptake Model (SWIF); thus the suggested method identifies optimal placement of TDR sensors for the calibration of a root water uptake model. Our approach is based on estimating efficiencies of different monitoring strategies with the help of a data set of 3000 simulations with different parameter sets. Parameter sets are accepted if simulations reproduced the 'measured' soil water dynamics of a reference run at specific depths within an accuracy of 0.01 m(3) m(-3); a value representing acceptable error intervals in measured soil water contents. The smaller the number of simulations that meet this criterion, the more information measured soil water dynamics contain to identify model parameters and thus the more effective a measurement strategy for model calibration. The presented method is illustrated with results for a pine stand on a sandy soil with a rooting depth of 1.5 m. For this situation, uncertainties in simulated water contents are already less than 0.01 m3 m-3 if SWIF is calibrated against measurements with a relatively small number (5) of TDR probes installed horizontally. Installation depths are critical when using a lower number of probes. Vertical profiles of measured soil water contents, measured with non-automated techniques, are as well effective for model calibration, especially in combination with measurements from permanently installed TDR probes. These profiles contain most information for model calibration if measured 5 days after a clear rain fall event preceded by a dry period of 20 days or more with a relatively high water demand. Uncertainty intervals in simulated distributions of yearly root water uptake were highest in the topsoil (about 5.0 x 10(-4) m(3) m(-3) day(-1) for z > -0.3 m), regardless of measurement strategy and amounted up to 30% of mean yearly uptake. Yet, in general, these uncertainties were less than 10% of the uncertainties in soil water contents. Additional measurements, like root observations and sap flow measurements that reduce ranges of model parameters, are therefore necessary to confine intervals in simulated uptake distributions. Also, error intervals can be reduced by calibrating parameters one by one, instead of tuning all parameters at the same time. Overall, results for the case study clearly demonstrate the potentials of using model simulations for identifying measurement strategies. (C) 2000 Elsevier Science B.V. All rights reserved.b J. Hydrol. 2000 Jan 31 227 1-4x285FA J HYDROLISI:000085378000018a"Nardini, Andrea Pitt, Franco 1999~Drought resistance of Quercus pubescens as a function of root hydraulic conductance, xylem embolism and hydraulic architectureNew Phytologistr 143e39485-493f_Water relations, xylem embolism, root and shoot hydraulic conductance of both young plants in the field and potted seedlings of Quercus pubescens have been studied with the aim of investigating whether these variables may account for the well known adaptation of this oak species to arid habitats. Our data revealed that Q. pubescens is able to maintain high leaf relative water contents under water stress conditions. In fact, relative water contents measured in summer (July) did not differ from those recorded in April. This was apparently achieved by compensating water loss by an equal amount of water uptake. Such a drought avoidance strategy was made possible by the recorded high hydraulic efficiency of stems and roots under water stress. In fact, root hydraulic conductance of field-grown plants was maintained high in summer when the percentage loss of hydraulic conductance of stems was lowest. The hydraulic architecture of young plants of Q. pubescens measured in terms of partitioning of hydraulic resistances along the water pathway revealed that the highest hydraulic resistance was located in stems of the current year's growth. This hydraulic architecture is interpreted as consistent with the adaptation of Q. pubescens to arid habitats as a consequence of the recorded seasonal changes in water relation parameters as well as in root and stem hydraulics.cOXa> 14839-14842e$://000077436700046$Allen, C. D. Breshears, D. D.7haDrought-induced shift of a forest-woodland ecotone: Rapid landscape response to climate variationeVOProceedings of the National Academy of Sciences of the United States of Americav0*vegetation; ecosystems; record; time; lagslfIn coming decades, global climate changes are expected to produce large shifts in vegetation distributions at unprecedented rates. These shifts are expected to be most rapid and extreme at ecotones, the boundaries between ecosystems, particularly those in semiarid landscapes. However, current models do not adequately provide for such rapid effects- particularly those caused by mortality-largely because of the lack of data from field studies. Here we report the most rapid landscape-scale shift of a woody ecotone ever documented: in northern New Mexico in the 1950s, the ecotone between semiarid ponderosa pine forest and pinon-juniper woodland shifted extensively (2 km or more) and rapidly (<5 years) through mortality of ponderosa pines in response to a severe drought. This shift has persisted for 40 years. Forest patches within the shift zone became much more fragmented, and soil erosion greatly accelerated. The rapidity and the complex dynamics of the persistent shift point to the need to represent more accurately these dynamics, especially the mortality factor, in assessments of the effects of climate change.&Proc. Natl. Acad. Sci. U. S. A. 1998 Dec 89525"146NJ PROC NAT ACAD SCI USAISI:000077436700046haDrought-induced shift of a forest-woodland ecotone: Rapid landscape response to climate variationa*#Allen Craig, D. Breshears David, D.a 1998\UProceedings of the National Academy of Sciences of the United States of America. Dec.o9525 14839-14842 Using Smart Source ParsingleIn coming decades, global climate changes are expected to produce large shifts in vegetation distributions at unprecedented rates. These shifts are expected to be most rapid and extreme at ecotones, the boundaries between ecosystems, particularly those in semiarid landscapes. However, current models do not adequately provide for such rapid effects-particularly those caused by mortality-largely because of the lack of data from field studies. Here we report the most rapid landscape-scale shift of a woody ecotone ever documented: in northern New Mexico in the 1950s, the ecotone between semiarid ponderosa pine forest and pinon-juniper woodland shifted extensively (2 km or more) and rapidly (<5 years) through mortality of ponderosa pines in response to a severe drought. This shift has persisted for 40 years. Forest patches within the shift zone became much more fragmented, and soil erosion greatly accelerated. The rapidity and the complex dynamics of the persistent shift point to the need to represent more accurately these dynamics, especially the mortality factor, in assessments of the effects of climate change.Root system water uptake and respiration for Agave deserti: Observations and predictions using a model based on individual rootsAlm, D. M. Nobel, P. S.Annals of Botany671  59-66  1991vpWater uptake and root respiration for the desert succulent Agave deserti Engelm. were predicted in the laboratory and in the field using a model based on root hydraulic conductivity (L-P) and the rate of maintenance respiration previously determined in the laboratory for individual roots as non-linear functions of root age, soil temperatures (T-soil), and soil water potential (PSI-soil). Root carbon dioxide loss and shoot transpiration were measured in the laboratory for plants with root systems in solution and in water-saturated soil, where T-soil was varied from 10 degree C to 40 degree C. In the field, T-soil was also varied (from 11 degree C to 28 degree C) while PSI-soil was maintained at -0.01 or lt -5 MPa, or T-soil was allowed to oscillate naturally while PSI-soil declined from -0.05 to -0.89 MPa over a 4-d period. After each experimental treatment, roots were divided into 15 age classes (1 week to 3 years), and the root xylem water potential was measured. Averaging over all experiments for the entire root systems, the predicted total daily water uptake was 102% +- 4% of the observed transpiration, and the predicted total daily root respiration was 96% +- 5% of that observed (mean +- s.e. mean, for n = 11 d). Thus, the respiratory maintenance cost of root system water acquisition can be accurately predicted based on the behaviour integrated for individual roots.ght through enhanced physiology (especially Prosopis) and growth (especially Larrea) in the season following drought. With regard to the second hypothesis, we again found more similarities than differences between the different aged (young vs. mature) islands. The stage of maturity of a resource island complex did not seem to be a significant factor to the growth and physiological activity of the shrub. Ecol. Monogr. 1999 Feb691164VU ECOL MONOGRISI:000078484900004,LiW 3J*$Novoplansky, Ariel Goldberg, Deborah 2001HBInteractions between neighbour environments and drought resistance"Journal of Arid Environments471 11-32The effects of neighbours on drought resistance were tested for two perennial bunchgrasses that co-occur throughout a small-scale productivity gradient in a desert grassland in New Mexico. Seedlings of the relatively mesic Sporobolus airoides and the relatively xeric Scleropogon brevifolius were grown alone and in intra- and interspecific pairs for 45 days after which they were left unwatered. Time to shoot death and subsequent revival success were monitored. Regardless of treatment, Scleropogon brevifolius was smaller at the end of the pre-drought period, survived longer into a drought and was more likely to revive after apparent above-ground death. Interspecific (but not intraspecific) neighbours strongly influenced relative biomass allocation to roots. It is concluded that the lack of effects of neighbours on total growth does not necessarily indicate that neighbours do not affect fitness by triggering morphological shifts. The study demonstrates the need both for incorporation of competition into studies of the physiology of drought resistance and plasticity and their possible implications in plant competition and community organization.e Noy-Meir, I. 19732,Desert Ecosystems: Environment and Producers.(Annual Review of Ecology and Systematics4} 25-51 I. Noy-Meir 1985.(Desert Ecosystems structure and function ,%M. Evenari I. Noy-Meir D.W. Goodall Ecosystems of the World  Amsterdam Elsevier 12a 92-103HAOesterheld, Martin Loreti, Juan Semmartin, Maria Sala, Osvaldo E. 2001piInter-annual variation in primary production of a semi-arid grassland related to previous-year productionn$Journal of Vegetation Science1210137-142 Mean annual precipitation accounts for a large proportion of the variation in mean above-ground net primary production (ANPP) of grasslands worldwide. However, the inter-annual variation in production in any grassland site is only loosely correlated with precipitation. The longest record of variation in production and precipitation for a site corresponds to a shortgrass steppe in Colorado, USA. A previous study of this record showed that current-year precipitation accounted for 39% of the inter-annual variation in ANPP. In this note, we show that ca. one third of the unexplained variation is related to previous-year ANPP: ANPP per mm of precipitation was higher in years preceded by wet, more productive years than in years preceded by average years; similarly, ANPP per mm of precipitation was lower in years preceded by dry, less productive years than in years preceded by average years. Since previous-year ANPP was, in turn, associated with precipitation of a year before, current-year ANPP was also explained by precipitation of two previous years. Our finding not only increases our predictive ability, but it also changes our understanding of how ANPP responds to fluctuations in precipitation. If ANPP is thought to vary according to current-year precipitation only, it will simply track annual precipitation in time. According to this new result, however, ANPP fluctuations are buffered if wet, more productive years alternate with dry, less productive years, and they are amplified if wet or dry sequences of several years take place.198-202$://A1996UY00700003a Ojha, C. S. P. Rai, A. K.s(!Nonlinear root-water uptake model;:3Journal of Irrigation and Drainage Engineering-Asce,%estimating soil-water; plant; systemslSound irrigation management requires information about root- water uptake patterns of different crops. In the existing literature, the extraction rate of sail moisture by the roots is assumed to be either constant or varying linearly with the root depth, during the crop period. However, both these models have limited application. With this in view, an empirical root- water uptake model in which the extraction rate varies non- linearly with the root depth is proposed. Using experimentally obtained soil-moisture depletion values for non-limiting soil- moisture conditions, it is found that the proposed model performs better than the constant rate models and linearly varying rate models of root-water uptake. Also, constant as well as linear extraction-rate models are found to be particular cases of the proposed model. Based on simulations :or limited soil-moisture conditions, it is also found that the frequency of irrigation is sensitive to the use of root-water uptake models."J. Irrig. Drainage Eng-ASCE 1996Jul-Aug 1224e"UY007 J IRRIG DRAIN ENG-ASCEISI:A1996UY00700003w990-1002$://000077129600007r<5Oren, R. Ewers, B. E. Todd, P. Phillips, N. Katul, G.ZTWater balance delineates the soil layer in which moisture affects canopy conductanceEcological Applications canopy; evapotranspiration; Pinus taeda; soil water balance; subcanopy time domain reflectometry; douglas-fir-forest; xylem sap flow; stomatal conductance; oak stand; quercus-petraea; hydraulic lift; climate-change; tree growth; leaf-areaTo link variation in canopy conductance to soil moisture in the rooting zone, measurements of throughfall (P-T), volumetric soil moisture (theta) to 0.7 m, transpiration from trees >10 mm in diameter (E-C), and vapor pressure deficit (D) were made in a forest dominated by Pinus taeda. Total evapotranspiration (E- T) was estimated from P-T, changes in volumetric soil water content within a defined soil volume (BS), and drainage out of that volume (Q), calculated from unsaturated soil hydraulic conductivity and theta. Our calculations suggest that over 145 growing-season days, Q was negligible, and most of P-T, averaging similar to 2 mm/d, was partitioned between soil moisture recharge (0.4 mm/d) and E-T (1.6 mm/d, not including similar to 0.4 mm/d of interception, I, by canopy trees), of which E-C was estimated from direct measurements at nearly 1.2 mm/d. Evapotranspiration by the subcanopy component accounted for slightly over 0.4 mm/d, about a third of E-T (a fourth if I is included). Most of the water used for E-T (>90%) was taken from the upper soil layer (top 0.35 m, or less). Canopy leaf conductance, calculated from E-C. D, and canopy leaf area, was strongly related to a in the upper soil layer once theta decreased below 0.22.h Ecol. Appl.t 1998 Nov 8t4t141EL ECOL APPLlISI:000077129600007e*#Ostfeld, Richard S. Keesing, Felici 2000TNPulsed resources and community dynamics of consumers in terrestrial ecosystems&Trends in Ecology and Evolution156232-237 ,145-159$://000165790700012o2+Reynolds, J. F. Kemp, P. R. Tenhunen, J. D. Effects of long-term rainfall variability on evapotranspiration and soil water distribution in the Chihuahuan Desert: A modeling analysiss Plant Ecology  arid rangeland; Bouteloua eriopoda; climate variability; desertification; Jornada Basin; Larrea tridentata; long-term; simulation model mojave desert; shrub communities; patagonian steppe; root distribution; plant; competition; patterns; grasses; vegetation; southwest We used the patch arid land simulator (PALS-FT) - a simple, mechanistic ecosystem model - to explore long-term variation in evapotranspiration (ET) as a function of variability in rainfall and plant functional type (FT) at a warm desert site in southern New Mexico. PALS-FT predicts soil evaporation and plant transpiration of a canopy composed of five principal plant FTs: annuals, perennial forbs, C-4 grasses, sub-shrubs, and evergreen shrubs. For each FT, the fractional contribution to transpiration depends upon phenological activity and cover as well as daily leaf stomatal conductance, which is a function of plant water potential, calculated from root-weighted soil water potential in six soil layers. Simulations of water loss from two plant community types (grass- vs. shrub-dominated) were carried out for the Jornada Basin, New Mexico, using 100 years of daily precipitation data (1891-1990). In order to emphasize variability associated with rainfall and fundamental differences in FT composition between communities, the seasonal patterns cover of perennials were held constant from year to year. Because the relative amount of year to year cover of winter and summer annual species is highly variable in this ecosystem, we examined their influence on model predictions of ET by allowing their cover to be variable, fixed, or absent. Over the entire 100-yr period, total annual ET is highly correlated with total annual rainfall in both community types, although T and E alone are less strongly correlated with rainfall, and variation in transpiration is nearly 3 times greater than evaporation and 2 times greater than variation in rainfall (CV of rainfall = 35%). Water use shows a relatively high similarity between the grass- and shrub-dominated communities, with a 100-yr average T/ET of 34% for both communities. However, based on a year-by-year comparison between communities, T/ET was significantly greater in the grass-dominated community, reflecting the fact that over the long term more than half of the rain occurs in the summer and is used slightly more efficiently (T ?E) by the C-4-grass community than the shrub community, although we found some rainfall patterns that resulted in much greater T/ET in the shrub community in a given year. Percent of water lost as transpiration (T/ET) suggests that while there is a general trend toward increased T/ET with rainfall in both community types, T/ET is extremely variable over the 100-yr simulation, especially for normal and below normal amounts of rainfall (T/ET values range from 1 to 58% for the grass-dominated site and 6 to 60% for the shrub-dominated site). These predictions suggest that because of the relatively shallow distribution of soil water, there is little opportunity for vertical partitioning of the soil water resource by differential rooting depths of the plant FTs, in contrast to the two-layer hypothesis of Walter (1971). However, functional types may avoid competition by keying on particular 'windows' of moisture availability via differences in phenologies. We found very little differences in average, long-term model predictions of T, E, and ET when annual plant cover was variable, fixed, or absent. The results of our simulations help reconcile some of the disparate conclusions drawn from experimental studies about the relative contribution of transpiration vs. evaporation to total evapotranspiration, primarily by revealing the great year-to-year variability that is possible. Plant Ecol. 2000 Oct 150 1-2381YB PLANT ECOLISI:000165790700012&Reznick, David Yang Anthony, P.D 1993voThe influence of fluctuating resources on life history: Patterns of allocation and plasticity in female guppiesmEcologyr747 2011-2019  6 /We investigated how resources are allocated to reproduction and how variations in resource availability influence reproductive allocation, offspring number, and offspring size in guppies (Poecilia reticulata). Our goal was to evaluate how plastic these variables are in response to environmental variation and to characterize the nature of this plasticity. Female guppies which had just given birth (litter 1) were assigned to either high or low levels of food availability until they gave birth to their next litter (litter 2, interval 1). They were then randomly reassigned to either high or low food with the constraint that there be equal numbers of individuals in each of four treatments: high-high, high-low, low-high, and low-low. They were maintained on this level of food availability until they produced their next litter (litter 3, interval 2). We analyzed variables that characterized the female after the birth of the third litter and the offspring in the third litter. These were two-way analyses, with intervals 1 and 2 as the main effects and high vs. low food as the levels of each effect. The qualities of the third litter were influenced by both interbrood intervals, indicating that the resources used for producing the litter were derived from both intervals. Specifically, higher food availability during either interval resulted in a significant increase in the number of offspring in litter 3, independent of the size of the mother. This result indicates that the number of offspring produced in a litter will be a function of both the immediate and the past environment. Lower food during either interval resulted in an increase in the number of days between the second and third litters, indicating that, if resource availability is low, the female may delay the initiation of the next litter, allowing her to acquire more resources. Resource availability during both intervals also influenced how resources were allocated to individual offspring. Females responded to low food during the first interbrood interval by producing heavier offspring in litter 3. This increase in mass was almost entirely attributable to an increase in fat reserves. Such a result could represent adaptive plasticity, if it can be demonstrated that maternal fitness increases through the production of heavier offspring in a low-food environment.fs r.'Wan, C. Sosebee, R. E. McMichael, B. L.r 1994haHydraulic properties of shallow vs. deep lateral roots in a semiarid shrub, Gutierrezia sarothrae"American Midland Naturalista 131d1t120-127Hydraulic conductivity of the deep and shallow lateral roots in the xerophyte Gutierrezia sarothrae Shinners (broom snakeweed) was measured for plants grown under two soil-water regimes. Hydraulic conductivity was 3.4-fold greater in the white, nonsuberized deep roots than in the brown, suberized shallow roots in dry soil; but was only 67% higher under favorable soil moisture conditions. Deep roots had 40% more large xylem vessels (diameters gtoreq 20 mu-m) than shallow roots in the plants growing in the field during the normal summer dry season. This may result in lower axial resistance to water flow in the xylem conduits in deep roots, but theoretical axial resistances calculated with the Poiseuille-Hagen equation could only explain 23% of the difference in hydraulic conductivity between deep and shallow roots. Apparently, increased radial resistance in the suberized shallow roots was mainly responsible for the low hydraulic conductivity observed during the dry season. Suberized roots of G. sarothrae regained high hydraulic conductivity 1 day after soil rewetting. This indicates a rectifier-like behavior of root systems in this half-shrub, as has been documented for desert succulents.o 1-7$://A1995QP93100001e0*Wan, C. G. Sosebee, R. E. McMichael, B. L.rkWater Acquisition and Rooting Characteristics in Northern and Southern-Populations of Gutierrezia-Sarothrae9,%Environmental and Experimental Botany broom snakeweed; ecotype; root growth; drought; desert shrub; water use dryas-octopetala ecotypes; experimental ecology; drought resistance; tussock grasses; hydraulic lift; deep roots; soil; growth; photosynthesis; extractionPJSeeds from northern (Idaho) and southern (New Mexico and Texas) populations of broom snakeweed (Gutierrezia sarothrae) were germinated in a greenhouse and the seedlings grown in a soil mixture in 30-cm deep (19-1) plastic pots for 50 days. The pots were then fastened onto the top of similar pots and the seedling root systems were allowed to grow into the lower pots for 4 weeks through punctures in the bottoms of the upper pots. Soil water extraction from four different depths was measured using time domain reflectometry (TDR) 2 weeks after roots had begun to grow into the lower pots. The two New Mexico populations (Jornada and Mountainair) and a population from Plains, Texas extracted more water from deeper (greater than or equal to 38 cm) soil layers than the Idaho population (P < 0.05). Root length in the deeper soil layers (55-60 cm) was greater (P < 0.05) for the Jornada desert population than for the Idaho and Mountainair populations. The Plains population had shorter roots and extracted less water in the upper soil layers than the other populations (P < 0.05). Specific root length in the Idaho population was at least 67% greater (P < 0.01) than those in southern populations, indicating smaller root diameters. The patterns of soil water extraction and root penetration were correlated with the seasonal distribution of precipitation in the plants' natural habitats. The Idaho seedlings had roots confined primarily to the topsoil to secure water from spring precipitation. In contrast, some southern seedlings were able to develop an extensive deep root system to avoid spring drought.Environ. Exp. Bot. 1995 Jan351QP931 ENVIRON EXP BOTISI:A1995QP93100001:4Wan, Changgui Sosebee, Ronald E. McMichael, Bobby L. 1996f_Lateral root development and hydraulic conductance in four populations of Gutierrezia sarothrae,%Environmental and Experimental Botany362157-165n " We hypothesize that drought-avoidance in Gutierrezia sarothrae populations is characterized by well-developed lateral roots. Root growth and hydraulic conductance were studied in four populations of Gutierrezia sarothrae. Seedlings from a Malta, Idaho (ID) seed source had four times higher root/shoot ratio (P lt 0.05), but only 17% hydraulic conductance of those from Tahoka, Texas (TA) seed source. Consequently, transpiration surface area of the ID seedlings was 17% that of the TA seedlings. There was no difference in total root length between the seedlings, but hydraulic conductance per unit root length was 3.8 fold greater (P lt 0.05) in the TA seedlings, which was accompanied by a lower specific lateral root length (P lt 0.05), thus greater lateral root diameters. A parallel experiment with adult plants from New Mexico (NM) and Plains, Texas (PL) and the above two populations showed that whole-plant hydraulic conductance was ordered as NM gt TA=PL gt ID. The NM and TA populations had lower specific lateral root length (P lt 0.05), thus larger-diameter laterals, than the ID population. The NM population had similar transpiration surface area and total root length as that of the TA population, but a greater lateral root biomass (P lt 0.05) than the latter. Because of higher hydraulic conductance in the NM population, its xylem water potential declined more slowly than that of the Texas and Idaho populations as soil moisture deficit developed. Stem hydraulic conductance was positively correlated with xylem water potential. Therefore, the NM population was able to maintain a higher stem conductance during drought, which was associated with its higher whole-plant hydraulic conductance and greater carbon partitioning in the stems. The drought avoidance mechanism in the NM plants was characterized by an efficient water transport system due to greater lateral root biomass and more stems per unit leaf area. The higher root/shoot ratio in desert populations (NM and ID) as compared with that of the semiarid rangeland populations (TA and PL) does not ensure more effective water acquisition. Rather, root morphological modifications such as accelerated secondary thickening of lateral roots may play an important role in enhancing water-acquisition capability of the root systems of Gutierrezia sarothrae. .'Williams, David G. Ehleringer, James R. 2000b[Intra- and interspecific variation for summer precipitation use in pinyon-juniper woodlandsEcological Monographs704517-537-In the arid southwest of North America, winter precipitation penetrates to deep soil layers, whereas summer "monsoon" precipitation generally wets only surface layers. Use of these spatially separated water sources was determined for three dominant tree species of the pinyon-juniper ecosystem at six sites along a gradient of increasing summer precipitation in Utah and Arizona. Mean summer precipitation ranged from 79 to 286 mm, or from 18% to 60% of the annual total across the gradient. We predicted that, along this summer rainfall gradient, populations of dominant tree species would exhibit a clinal off-on response for use of water from upper soil layers, responding at particular threshold levels of summer precipitation input. This prediction was largely supported by our observations of tree water source use over a two-year period and from irrigation experiments. Hydrogen and oxygen stable isotope ratios (deltaD and delta18O) of tree xylem water were compared to that of precipitation, groundwater, and deep and shallow soil water to distinguish among possible tree water sources. deltaD-delta18O relationships and seasonal xylem water potential changes revealed that trees of this ecosystem used a mixture of soil water and recent precipitation, but not groundwater. During the monsoon period, a large proportion of xylem water in Pinus edulis and Juniperus osteosperma was from monsoon precipitation, but use of this precipitation declined sharply with decreasing summer rain input at sites near the regional monsoon boundary in Utah. Quercus gambelii at most sites along the gradient used only deep soil water even following substantial inputs of summer rain. Populations of Quercus at sites with the highest average summer precipitation input, however, predominantly used water in upper soil layers from recent summer rain events. Soil temperature correlated with patterns of summer precipitation use across the gradient; high soil temperatures north of the monsoon boundary may have inhibited surface root activity for some or all of the three tree species. Irrigation experiments with deuterium-labeled water revealed that Quercus gambelii in northern Arizona and southern Utah did not use water from surface layers. In contrast, Juniperus osteosperma at these sites responded significantly to the irrigations: between 37% and 41% of xylem water originated from irrigations that wetted only the top 30 cm of soil. Responses by Pinus edulis to these irrigations were variable; uptake of labeled water by this species was greater in September at the end of the summer than during the hot midsummer period. Inactivity of Pinus roots in midsummer supports the hypothesis that root activity in this species is sensitive to soil temperature. Seasonal patterns of leaf gas exchange and plant water potential corresponded to the seasonality of rainfall at different sites. However, no correlation between a species' ability to use summer rainfall and its tolerance to water deficits at the leaf level was found. Midday stomatal conductance (gs) for Pinus needles approached zero at predawn water potentials near -2 MPa, whereas gs in Quercus and Juniperus declined to zero at -2.8 and -3.7 MPa, respectively. The relationship between photosynthesis (A) and gs was similar among the three species, although Quercus maintained higher overall rates of gas exchange and tended to operate higher on the A/gs curve than the two conifers. At sites in eastern Arizona where Quercus fully used moisture from summer rains, leaf gas exchange characteristics were similar to those of Pinus and Juniperus.a121-129$://000087409200002a(!Williams, D. G. Ehleringer, J. R.if`Carbon isotope discrimination and water relations of oak hybrid populations in southwestern Utah(!Western North American Naturalist,$oak hybrids; Quercus turbinella; Quercus gambelii; leaf structure; leaf nitrogen; carbon isotope discrimination; delta C-13; delta D; water potential hawaiian metrosideros-polymorpha; xylem embolism; gas-exchange; photosynthesis; quercus; plants; trees; biogeography; woodland; drought The evergreen oak Quercus turbinella and the deciduous Q. gambelii form natural hybrids in southwestern Utah and northern Arizona. Hybrid individuals also are found in northern Utah in a region where only Q. gambelii currently exists, indicating that Q. turbinella has recently retreated southward. Our objectives were to (1) examine the ecophysiology of parental taxa and hybrids under natural conditions in southeastern Utah, and (2) investigate the level of integration between leaf carbon isotope discrimination (a synthetic gas exchange trait) and structural and chemical traits of leaves in morphologically variable hybrid populations. Leaf length, width, mass-to-area ratio (LMA, g m(-2)), and nitrogen concentration (N, g g(-1)) within 2 hybrid populations near New Harmony, Utah, were highly intercorrelated. Variation within the hybrid populations spanned mean values for these traits observed in parental taxa from adjacent "pure populations of each species. Carbon isotope discrimination (Delta), an integrated measure of the ratio of intercellular to ambient CO2 concentration, ranged from 16.1 parts per thousand to 19.6 parts per thousand within the 2 hybrid populations and was positively correlated with leaf nitrogen concentration and negatively correlated with LMA; individuals in hybrid populations with leaves resembling Q. gambelii had the highest leaf Delta and N concentrations and lowest LMA compared with leaves from plants that resembled Q. turbinella. CO2 uptake is limited by stomatal conductance and possibly by mesophyll resistance to a greater extent in Q. turbinella phenotypes than in intermediate or Q. gambelii phenotypes. delta D of stem xylem water (an indication of active rooting depth) and predawn water potential during the peak monsoon period in August were not correlated to leaf Delta values within the hybrid populations. Several individuals that were morphologically similar to Q. turbinella in the hybrid populations maintained high predawn water potentials and derived moisture from winter recharge that presumably was taken from deep soil layers. Apparently, a few adult individuals of the Q. turbinella phenotype in hybrid populations accessed water from deep in the soil profile, which enabled them to avoid summer drought. Reduced monsoonal activity may have been an important, but not the single, determinant of Q. turbinella's retreat from northern Utah during the recent Holocene. West. North Am. Naturalist 2000 Aprr602v$320PH WEST NORTH AM NATURALISTISI:000087409200002a 1201-1215i$://A1997YD79000015m>7Breshears, D. D. Rich, P. M. Barnes, F. J. Campbell, K.rb[Overstory-imposed heterogeneity in solar radiation and soil moisture in a semiarid woodlandaEcological Applications D>Bouteloua gracilis; canopy and intercanopy gaps; Juniperus monosperma; land degradation; microclimate; overstory; Pinus edulis; solar radiation and moisture; woodland northern new-mexico; pinyon-juniper woodlands; water-balance; spatial autocorrelation; desert ecosystems; vegetation; forest; model; dynamics; patterns Degradation of semiarid ecosystems is a major environmental problem worldwide, characterized by a reduction in the ratio of herbaceous to woody plant biomass. These ecosystems can be described as a set of canopy patches comprising woody plants and the intercanopy patches that separate them, yielding an overstory with intermediate closure. Field measurements of microclimate at the scale of canopy patches, particularly for near-ground solar radiation and soil moisture, are largely lacking from both nondegraded and degraded ecosystems. We tested for relationships among spatial patterns of the overstory, near-ground solar radiation, and soil moisture in a semiarid pinon-juniper woodland in northern New Mexico that had a highly heterogeneous overstory (approximate to 50% canopy cover) and was not degraded with respect to ground cover and erosion rates. We used measurements taken every 1 m along a 102-m transect-solar radiation indices were estimated monthly and annually using hemispherical photographs, and soil moisture was measured over 4 yr using time-domain reflectometry (TDR)- and analyzed the data using general least squares linear models that accounted for spatial autocorrelation and temporal heteroscedasticity. Time-averages of solar radiation and of soil moisture both were spatially autocorrelated at scales of up to 4 m (P < 0.05), corresponding approximately to the average lengths of both canopy and intercanopy patches and to the scale of spatial autocorrelation in the canopy/intercanopy pattern of the overstory (3 m; P < 0.05). For near-ground solar radiation, we found expected spatial variation between patches (canopy < intercanopy; P < 0.0001) and within patches for centers vs. edges (canopy center < canopy edge and intercanopy center > intercanopy edge; P < 0.0001) and for north vs. south edges (canopy north edge < canopy south edge and intercanopy south edge < intercanopy north edge; P < 0.0001). For soil moisture, canopy locations were significantly drier than intercanopy locations (P < 0.0001), and edge locations were significantly wetter than center locations both overall and within both patch types (P < 0.0001). Spatial heterogeneity in soil mositure was attributed primarily to canopy interception and drip on the basis of large differences a in snow cover between canopy and intercanopy locations. Spatial autocorrelation in the residuals for soil moisture of up to 7 m was attributed to transpiration by woody plants at scales corresponding to belowground root distributions. The spatial heterogeneities in near-ground solar radiation and soil moisture are of sufficient magnitude to affect biotic processes of woody and herbaceous plants, such as growth and seedling establishment. Because land degradation problems in semiarid shrublands and woodlands appear to result from differential impacts to intercanopy vs. canopy patches, our results can be used to help design effective mitigation and remediation strategies. More generally, our results demonstrate how the physical presence of woody canopies reinforces spatial heterogeneity in microclimate and, because our site has intermediate closure of the overstory, bridge the gap along a grassland-forest continuum between related studies in relatively open savannas and in forests with nearly closed canopies. Ecol. Appl. 1997 Nov74YD790 ECOL APPLISI:A1997YD79000015d shrubs become less coupled to short-term fluctuations in precipitation and more resistant to long-term drought or climate shifts. With regard to the first hypothesis, we conclude that the two species are relatively similar in function despite the different phenological "strategies" of Larrea (evergreen) and Prosopis (winter deciduous), In the absence of drought, both species exhibited maximal rates of shoot and root growth, as well as high photosynthesis and transpiration, in late spring. This remained as the period for maximal growth and physiological activity for Prosopis shrubs that experienced drought in either summer or winter/spring. On the other hand, Larrea shrubs that experienced drought in winter/spring had maximal growth and activity shifted to the summer period, and in the absence of drought, Larrea shrubs also exhibited high physiological activity during the summer (especially following high rainfall). Thus, Larrea appears to have a greater capacity for shifting its activity patterns to alternate periods to take advantage of changes in resource availability. Shrubs of both species appeared well adapted to withstand season-long droughts. Mechanisms for survival include the following capacities: (1) to shift growth and physiological activity to utilize different temporal moisture (Larrea); (2) to utilize different levels of soil water (both species); (3) to carry out limited physiological activity and growth during drought (especially Larrea); and (4) to compensate for some negative impacts of drought through enhanced physiology (especially Prosopis) and growth (especially Larrea) in the season following drought. With regard to the second hypothesis, we again found more similarities than differences between the different aged (young vs. mature) islands. The stage of maturity of a resource island complex did not seem to be a significant factor to the growth and physiological activity of the shrub. Ecol. Monogr. 1999 Feb691164VU ECOL MONOGRISI:000078484900004j  1869-1879$://000085517800052y>8Reid, K. D. Wilcox, B. P. Breshears, D. D. MacDonald, L.VORunoff and erosion in a pinon-juniper woodland: Influence of vegetation patchesd.'Soil Science Society of America Journal ysemiarid woodland; spatial pattern; soil-moisture; new-mexico; infiltration; rainfall; australia; dynamics; desert; modelpIn many semiarid regions, runoff and erosion differ according to vegetation patch type. These differences, although hypothesized to fundamentally affect ecological processes, have been poorly quantified. In a semiarid pinon-juniper woodland [Pinus edulis Engelm, and Juniperus monosperma (Engelm.) Sarg.] in northern New Mexico, He measured runoff and erosion from the three patch types that compose these woodlands: Canopy patches (those beneath woody plants), vegetated patches in intercanopy areas, and bare patches in intercanopy areas. The bare intercanopy patches exhibited the highest rates, followed by vegetated intercanopy patches and then by canopy patches. Large convective summer storms, though relatively infrequent, generated much of the runoff and most of the sediment; prolonged frontal storms were capable of generating considerable runoff but little sediment. A portion of the runoff and most of the sediment generated from bare intercanopy patches was redistributed down-slope, probably to adjacent vegetated intercanopy patches, demonstrating connectivity between these two patch types. Our results indicate that there are significant and important differences in runoff and sediment production from the three patch types; that bare intercanopy patches act as sources of both water and sediment for the vegetated intercanopy patches; and that the transfer of water and sediment at small scales is both frequent enough and substantial enough to be considered ecologically significant.Soil Sci. Soc. Am. J. 1999Nov-Dec636 287QT SOIL SCI SOC AMER JISI:000085517800052 69-106$://000078484900004gPJReynolds, J. F. Virginia, R. A. Kemp, P. R. de Soyza, A. G. Tremmel, D. C.jdImpact of drought on desert shrubs: Effects of seasonality and degree of resource island developmentEcological MonographsrRKdesertification; drought response; Jornada LTER; Larrea tridentata; Prosopis glandulosa; resource island formation; shrub physiology; soil nitrogen; soil water southern new-mexico; chihuahuan desert; water relations; larrea-tridentata; soil-moisture; sonoran desert; root-growth; nitrogen mineralization; use efficiency; plant sizenLarge areas of semiarid grasslands in the southwestern United States have been virtually replaced by shrubs during the past century. Understanding the causes and consequences of such vegetation dynamics requires that we elucidate the interplay between external forces of change (e.g., climate, human impacts) and the internal forces within these ecosystems that foster resilience and/or stability. Several conceptual models of arid ecosystems address this interplay by including the potential role of autogenic shrub effects on ecosystem processes, which lead to the formation of "resource islands" and tend to promote shrub persistence. Specifically, during the process of shrub establishment and maturation, the cycling of nutrients is progressively confined to the zones of litter accumulation beneath shrubs, while bare intershrub spaces become increasingly nutrient poor. As shrub resource islands develop, there is increased interception and stemflow by shrub canopies, confining infiltration of nutrient-enriched rainfall directly beneath the shrubs; the barren intershrub spaces generate overland flow, soil erosion by wind and water, and nutrient losses. These islands are preferred sites for the regeneration of shrubs and herbaceous plants and are correlated with spatial variation in soil microbial populations and soil microfauna that promote nutrient cycling. Lf further changes in the transition between grassland and shrubland are to be correctly predicted-or if we wish to intervene and redirect transitions-we must develop a greater mechanistic understanding of the structural and functional relationships between shrubs and the resource islands associated with them. We conducted a 3-yr field study in the Jornada Basin of southern New Mexico to explore the relationships between seasonal manipulations of soil water and its impact on soil nutrient dynamics of resource islands and shrub growth and physiology. At our study site, where total annual precipitation is similar to 230 mm (similar to 65% falls during the summer period), we simulated seasonal drought in summer (1 June-30 September) and winter/spring (1 October-31 May) by constructing large rainfall-exclusion shelters over shrub resource islands at different stages of development. Our experiment tests two principal hypotheses. The first is that the two major shrub species in the Jornada Basin, creosotebush (Larrea tridentata) and mesquite (Prosopis glandulosa), have different growth phenologies, rooting patterns, and physiological responses to resource availability (primarily water). The second is that different size classes of shrubs ("small" and "large") represent distinct stages of resource island development (i.e., "young" and "mature," respectively) and, hence, different stabilities-that is, as islands develop, their associated shrubs become less coupled to short-term fluctuations in precipitation and more resistant to long-term drought or climate shifts. With regard to the first hypothesis, we conclude that the two species are relatively similar in function despite the different phenological "strategies" of Larrea (evergreen) and Prosopis (winter deciduous), In the absence of drought, both species exhibited maximal rates of shoot and root growth, as well as high photosynthesis and transpiration, in late spring. This remained as the period for maximal growth and physiological activity for Prosopis shrubs that experienced drought in either summer or winter/spring. On the other hand, Larrea shrubs that experienced drought in winter/spring had maximal growth and activity shifted to the summer period, and in the absence of drought, Larrea shrubs also exhibited high physiological activity during the summer (especially following high rainfall). Thus, Larrea appears to have a greater capacity for shifting its activity patterns to alternate periods to take advantage of changes in resource availability. Shrubs of both species appeared well adapted to withstand season-long droughts. Mechanisms for survival include the following capacities: (1) to shift growth and physiological activity to utilize different temporal moisture (Larrea); (2) to utilize different levels of soil water (both species); (3) to carry out limited physiological activity and growth during drought (especially Larrea); and (4) to compensate for some negative impacts of drought through enhanced physiology (especially Prosopis) and growth (especially Larrea) in the season following drought. With regard to the second hypothesis, we again found more similarities than differences between the different aged (young vs. mature) islands. The stage of maturity of a resource island complex did not seem to be a significant factor to the growth and physiological activity of the shrub. Ecol. Monogr. 1999 Feb691164VU ECOL MONOGRISI:000078484900004%\ ~$Cui, Muyi Caldwell, Martyn M.S 1998Nitrate and phosphate uptake by Agropyron desertorum and Artemisia tridentata from soil patches with balanced and unbalanced nitrate and phosphate supply New Phytologist 139823267-272mTo test the ability of plants to integrate small-scale imbalances in soil nitrate and phosphate patches, plant growth and acquisition of nitrate and phosphate were measured for the perennial grass Agropyron desertorum (Fisch. ex Link) Schult. and the shrub Artemisia tridentata Nutt. ssp. vaseyana (Rydb.) Beetle in soil where the principal supply of nitrate and phosphate came from two enriched patches. The soil was calcareous loamy-skeletal Typic Haploxerolls. These patches were applied in two treatments: either nitrate and phosphate were applied in both patches (balanced treatment) or one patch contained only nitrate and the other only phosphate (unbalanced treatment). The same total quantity of nutrients was applied in both treatments and these included 15N and 32P tracers. The plants were in large pots in open field conditions. There were no significant differences in total biomass production and nitrogen concentration between the two treatments, indicating that both species had the physiological ability to integrate soil nutrient resources. Artemisia was able to acquire more phosphate in the unbalanced treatment, probably due to the high local solution phosphate concentration. Generally Artemisia acquired more N and P than did Agropyron.>8D'Odorico, P Ridolfi, L Porporato, A Rodriguez-Iturbe, I 2000XQPreferential states of seasonal soil moisture: The impact of climate fluctuationsaWater Resources Research36 2209-2219c231-240$://000072673700017rB://A1997XK15200003tNHBreshears, D. D. Myers, O. B. Johnson, S. R. Meyer, C. W. Martens, S. N.~Differential use of spatially heterogeneous soil moisture by two semiarid woody species: Pinus edulis and Juniperus monospermaJournal of Ecologycompetition; juniper; pinon; plant water potential; woodland northern new-mexico; water-use; herbaceous vegetation; catastrophe-theory; use efficiency; great-basin; woodlands; dynamics; gradient; sierran, 1 Soil moisture in semiarid woodlands varies both vertically with depth and horizontally between canopy patches beneath woody plants and the intercanopy patches that separate them, such that shallow soil layers in intercanopy locations are wettest, yet few studies have considered both dimensions of spatial variability in testing for acquisition of resources by plants. 2 Three hypotheses were tested relative to the use of shallow water in intercanopy locations by two coexisting semiarid-woodland tree species, Pinus edulis (a pinon) and Juniperus monosperma (a juniper): (i) both P. edulis and J. monosperma can use shallow water from intercanopy locations;. (ii) J. monosperma is able to obtain more shallow water from intercanopy locations than P. edulis, and (iii) the spatial arrangement of the trees influences the amount of water they obtain. Soil moisture and plant water potential (i.e. plant water stress) were measured before and after the addition of water to shallow depths (0-30 cm) of intercanopy locations for trees of both species in two spatial arrangements: isolated and paired with a contiguous tree of the other species. 3 Both species responded to the addition of shallow water in intercanopy locations, as measured by plant water potential. The response of J. monosperma was significantly greater than that of P. edulis, as measured by depletion of shallow soil moisture in intercanopy locations and by change in plant water potential per unit change in soil water potential (the difference was not detectable on the basis of plant water potential alone); in addition, the amount of depletion was correlated with basal area of J. monosperma but not of P. edulis. The responses were not influenced by spatial arrangement (isolated vs. paired with a contiguous tree of the other species). 4 The results of this study are consistent with differences in the relative abundances of the two species across locations, suggesting that species differences in ability to use shallow water in intercanopy locations is important in structuring semiarid woodlands. Further, the results suggest that current theoretical concepts for semiarid ecosystems, which ignore either vertical or horizontal variability in soil moisture, may be inadequate for predicting changes in the ratio of woody to herbaceous plant biomass, particularly for plant communities with co-dominant woody species that differ in ability to acquire spatially heterogeneous resources.J. Ecol. 1997 Jun853 XK152 J ECOLISI:A1997XK15200003 1010-1017o$://000077495000013m>7Breshears, D. D. Nyhan, J. W. Heil, C. E. Wilcox, B. P.Effects of woody plants on microclimate in a semiarid woodland: Soil temperature and evaporation in canopy and intercanopy patches.'International Journal of Plant Sciencesctmpinus-edulis; new-mexico; grasses; juniper; shrubs; heterogeneity; germination; grassland; moisture; dynamicse>8The canopies of woody plants in semiarid ecosystems modify the microclimate beneath and around them, with canopy patches usually having lower soil temperatures than intercanopy patches. However, lacking are studies that have evaluated how heterogeneity in soil temperature, induced by woody plant canopies, influences soil evaporation rates and the consequent effects on plant-available water. Soil temperatures were measured and soil evaporation rates were estimated for canopy and intercanopy patches in a semiarid pinon-juniper woodland (Pinus edulis and Juniperus monosperma) in northern New Mexico. Soil temperature was measured at 2-cm depths in four canopy and four intercanopy locations during 1994. Maximum soil temperature in intercanopy patches was greater than in canopy patches between May and September, by as much as 10 degrees C, while soil temperatures in intercanopy patches were lower than in canopy patches during colder parts of the day in the fall and winter months. Equations for soil drying rates for sandy loam soil samples were determined in laboratory experiments over a range of temperatures and soil water contents. Drying rates were disproportionately greater at high soil moisture and high soil temperature. Intercanopy patches were predicted to dry more than canopy patches for days in April through September by as much as 2% volumetric soil water content per day. The difference between patches was amplified at lower soil water contents when expressed as soil water potential, which more directly determines plant-available water. Our results quantify the effects of woody plants on the microclimate with respect to soil temperature and evaporation, which in turn affect herbaceous and woody plants by modifying factors such as germination, the potential for facilitation, and the amount of plant-available water.Int. J. Plant Sci. 1998 Nov 1596148FU INT J PLANT SCIISI:000077495000013