H`dSdT @@@ @@@@,I:ddT EN DB edT V >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