L?2Gebauer, R. L. E. Schwinning, S. Ehleringer, J. R.2002SInterspecific competition and resource pulse utilization in a cold desert community 2602-2616Ecology839jAtriplex confertifolia, Chrysothamnus nauseosus, Colorado plateau, desert, Gutierrezia sarothrae, neighbor removal, plant competition, resource pulses, stable isotopes, water uptake carbon-isotope discrimination; n-15 natural-abundance; shortgrass steppe; summer precipitation; chihuahuan desert; arid ecosystem; soil-moisture; root systems; water-uptake; plantsSepIn desert ecosystems a large proportion of water and nitrogen is supplied in rain-induced pulses. It has been suggested that competitive interactions among desert plants would be most intense during these pulse periods of high resource availability. We tested this hypothesis with three cold desert shrub species of the Colorado Plateau (Gutierrezia sarothrae, Atriplex confertifolia. and Chrysothamnus nauseosus), which differ in their distribution of functional roots. In a three-year field study we conducted a neighbor removal experiment in conjunction with simulated 25-mm precipitation events and the addition of a nitrogen pulse. in either spring or summer. We measured predawn water potential (Psi), gas exchange, leaf delta(15)N, carbon isotope discrimination (Delta), and growth of target plants for the duration of the study. We found that G. sarothrae used resource pulses to a larger extent than A. confertifolia, which has more functional roots at depth. In all species, the addition of a water or nitrogen pulse did not significantly affect maximal rates of photosynthesis or branch growth. Contrary to our initial hypothesis, we did not find that pulse use was reduced by the presence of neighboring plants. Nevertheless, there was strong evidence for competitive interactions, which were more likely mediated by water at depth than by nitrogen. in the more deep-rooted species A. confertifolia, neighbor removal affected T, gas exchange, Delta, percentage of carbon, and growth. G. sarothrae, which has a much smaller proportion of roots at depth, was less affected by the removal of neighboring shrubs, and not at all when only predominantly shallow-rooted herbaceous species were removed.These results suggest that shrubs in this cold desert community may primarily compete for water in deeper soil layers, where water depletion is slow and dominated by plant water uptake. There appeared to be little competition for water in shallow soil layers, where depletion is fast and dominated by evaporation.://000178153900023ISI:000178153900023?)Parsons, A. J. Schwinning, S. Carrere, P.2001^Plant growth functions and possible spatial and temporal scaling errors in models of herbivory21-34Grass and Forage Science561herbivory, spatial, scaling, heterogeneity, grazing, model grazing systems; bite dimensions; herbage intake; white clover; sward; grass; pasture; cattle; defoliation; mechanismsMar(Recent studies have provided detail of the mechanisms by which plants and animals interact, but attempts to apply this knowledge to understand function at the scale of whole fields or grazed ecosystems can be fraught with difficulties. Faced with a plethora of detail, and yet demands to make models more comprehensive, modellers face pressure to revert to simplified accounts for what are assumed to be well-established biological phenomena, for example, for describing plant growth and intake, but this raises the risk; that important insights may be lost, or that the analyses may face errors of scaling.The predictions of a previously described spatial model are compared with those of a non-spatial rendition of the same model to identify the differences in predictions and the sources of these differences. In particular, the use of the conventional empirical growth functions and their interaction with temporal and spatial scaling errors are examined. The comparison exposed how substantial Errors could be made in predicting yield and stability under grazing. It is proposed that such errors might be avoided by ensuring that the functional responses used capture the insights of more detailed studies, and by recognizing the difficulties of scaling-up from the level of processes to the field scale and beyond.://000168000200003ISI:000168000200003? Schwinning, S. Rosenzweig, M. L.1990APeriodic Oscillations in an Ideal-Free Predator-Prey Distribution85-91Oikos591Sep://A1990ED44600013ISI:A1990ED44600013?Schwinning, S. Fox, G. A.1995HPopulation-Dynamic Consequences of Competitive Symmetry in Annual Plants422-432Oikos723neighborhood models; size hierarchies; species models; monocultures; interference; variability; coexistence; fitness; environments; germinationAprAsymmetric competition is a form of resource division among plants, in which large plants greatly suppress the growth of smaller neighbors. In annual plants, small size differences between seedlings at the onset of competition are magnified into large differences in seed-set by asymmetric competition. We formulate a novel neighborhood model, which reflects this seedling size effect as modified by the type of competitive symmetry. In the model, competition type is represented by a single, biologically meaningful parameter. We implement the model in a population growth model for two species, one at low density (the invader), and one at high density (the resident). The species are the same, except for their seedling biomass distributions. Under these conditions, we find that asymmetric competition always favors invasion by the species with lager average seedling size, but impairs invasion by the other species. Based on this invasibility criterion, we conclude that asymmetric competition always favors competitive exclusion in our model. However, by modifying some of the model assumptions, we suggest scenarios in which asymmetric competition may promote coexistence.://A1995RH16000015ISI:A1995RH16000015 ?Schwinning, S. Parsons, A. J.1996QAnalysis of the coexistence mechanisms for grasses and legumes in grazing systems799-813Journal of Ecology846$diet selection, nitrogen cycle, pasture composition, plant population dynamics, spatial heterogeneity continuously grazed mixture; temperate grassland sward; plant animal interactions; white clover; diet selection; perennial ryegrass; mixed swards; nitrogen-fixation; crop growth; sheep urineDecm1 It is widely assumed that grass-legume associations offer a way to sustainable, low input land use, with reduced environmental impact. However, a combination of both ecological and physiological principles may be needed to understand the sustainability of species balances.2 To increase understanding of grass-legume dynamics, we developed a model that extends a recently proposed pasture model (Thornley, Bergelson & Parsons: Annals of Botany 1995, 75, 79-94) by including selective grazing and spatial considerations, Population oscillations were shown to stem from the way grasses can exploit leguminous N fixation. If the legume is a relatively good competitor for light, populations do not oscillate near equilibrium, but in the converse case, populations do oscillate.3 Large amplitude oscillations can arise when there are sufficiently long time delays in the plant populations' responses to changes in the competitive environment. In the present model, these stem from variable internal substrate pools (of C and N), which uncouple biosynthesis from resource uptake, but other time delay mechanisms are easily envisaged.4 Urine deposits prevent the establishment of equilibrium within patches, but spatially random urine deposition stabilizes population fluctuations at the field scale. This is because perturbations to local N cycles desynchronize patches with regard to the grass-legume population cycle.5 Differences in the soil N environment (fertilizer input, leaching rate) determine whether the species can coexist, but where coexistence is possible, species composition regulates soil mineral N.6 Selective grazing (herbivory) does not essentially alter the grass-legume interaction, but complex foraging trade-offs lead to herbivory effects that may seem counterintuitive. The model has important implications for attempts to control the legume content of mixed species communities.://A1996WB58300001ISI:A1996WB58300001 ?Schwinning, S. Parsons, A. J.1996cA spatially explicit population model of stoloniferous N-fixing legumes in mixed pasture with grass815-826Journal of Ecology846cellular automaton, metapopulation dynamics, pattern formation, population oscillations trifolium-repens l; white clover; dynamics; sheep; pattern; environments; competition; systems; quality; growthDec1 In a previous paper, we outlined the physiological prerequisites for population oscillations between a grass and a nitrogen-fixing legume, such as clover. Here, we examine the field-scale consequences of patch-scale oscillations in legume content, using a cellular automaton with variable hierarchy between the two species.2 We define cell states in the automaton by species content and soil N status. Grass-legume oscillations at the patch scale are represented as an alternation between states of grass dominance (high N) and legume dominance (low N). To this physiologically based population oscillation, we add local extinctions of legume and state-dependent success in legume invasion.3 Legume populations oscillate at the field scale, given arbitrary initial conditions. However, spatially random perturbations to the soil N status (e.g. urine) establishes a pasture structure that dampens the field scale oscillation. The stabilizing pasture structure comprises moving patches of legume dominance. This pattern was not predicted by our previous, purely physiological model.4 The model highlights that a patchy species distribution does not in itself mean the species is dispersal limited. In this model, changing the dispersal ability of legumes plays only a limited role in determining their proportion in the mixture. Legume abundance depends as much on the rate at which favourable (low N) sites become available for invasion.5 Seasonal disturbances that act uniformly across the field, such as winter (legume) mortality and/or springtime fertilizer application, can lead to sustained field scale variations in legume content that are only partly explained by the level of seasonal disturbance itself. Another large part is explained by previous years' legume contents. Pastures may therefore exhibit a 'memory' for legume performance which helps to explain the perceived 'unpredictability' of some grass-legume associations.6 We argue that legume dynamics in mixed pastures cannot be fully understood without combining ecological and physiological concepts of species interactions at three different scales: competitive interactions at the patch-scale, dispersal at the between-patch scale, and seasonality at the field-scale.://A1996WB58300002ISI:A1996WB58300002?Schwinning, S.1996JDecomposition analysis of competitive symmetry and size structure dynamics47-57Annals of Botany771Pennisetum americanum 'Custer', mode of competition, size structure dynamics, plant growth analysis plant-populations; asymmetric competition; growth; variability; model; density; monocultures; patterns; light; photosynthesisJangAn analysis is introduced, based on the decomposition of relative growth rates, to examine the mode of competition (i.e. whether competition is symmetric or asymmetric), the size-dependence of growth, and their interdependence. In particular, the basis for two commonly held Views is examined: (1) that the type of resource limitation determines the mode of competition, and (2) that asymmetric competition always leads to size-divergence between unequal competitors. It is shown that in held-grown miller plants, competition for light was symmetric at low density and asymmetric at high density. However, size variation at low density decreased during growth, because small plants had greater relative growth rates than larger plants. Size variation stayed constant at high density, since plants of all sizes had equal average relative growth rates. Based on these results and a general discussion, it is proposed that the type of resource limitation does not determine the mode of competition. Competition for light can be symmetric, and foraging for heterogeneously distributed soil resources can produce asymmetric competition below-ground. Furthermore, the mode of competition alone does not determine size structure dynamics. Size-dependence of resource conversion efficiency and allocation can modify the effects of resource uptake on growth. (C) 1996 Annals of Botany Company://A1996TR78600006ISI:A1996TR78600006?Schwinning, S. Weiner, J.1998OMechanisms determining the degree of size asymmetry in competition among plants447-455 Oecologia1134Zresource competition, allometry of growth and resource uptake, plasticity, spatial patterns, competition in clonal plants soil nutrient heterogeneity; cold-desert perennials; morphological plasticity; seedling establishment; resource acquisition; abutilon-theophrasti; local interference; root communication; structure dynamics; individual growthFebWhen plants are competing, larger individuals often obtain a disproportionate share of the contested resources and suppress the growth of their smaller neighbors, a phenomenon called size-asymmetric competition. We review what is known about the mechanisms that give rise to and modify the degree of size asymmetry in competition among plants, and attempt to clarify some of the confusion in the literature on size asymmetry. We broadly distinguish between mechanisms determined primarily by characteristics of contested resource from those that are influenced by the growth and behavior of the plants themselves. To generate size asymmetric resource competition, a resource must be "pre-emptable." Because of its directionality, light is the primary, but perhaps not the only, example of a pre-emptable resource. The available data. suggest that competition for mineral nutrients is often size symmetric (i.e., contested resources are divided in proportion to competitor sizes), but the potential role of patchily and/or episodically supplied nutrients in causing size asymmetry is largely unexplored. Virtually nothing is known about the size symmetry of competition for water. Plasticity in morphology and physiology acts to reduce the degree of size asymmetry in competition. We argue that an allometric perspective on growth, allocation, resource uptake, and resource utilization can help us understand and quantify the mechanisms through which plants compete.://000072196300001ISI:000072196300001? Schwinning, S. Parsons, A. J.1999XThe stability of grazing systems revisited: spatial models and the role of heterogeneity737-747Functional Ecology136temperate grassland sward; bite dimensions; lolium-perenne; diet selection; herbage intake; cattle; defoliation; ecosystems; herbivores; physiologyDec://000084788600001ISI:000084788600001 ? Schwinning, S. Ehleringer, J. R.2001LWater use trade-offs and optimal adaptations to pulse-driven arid ecosystems464-480Journal of Ecology893desert, genetic algorithm, plant functional types, whole-plant carbon gain desert shrubs; summer precipitation; shortgrass steppe; plants; soil; community; annuals; availability; temperature; succulentsJun1 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 C-3 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.://000169689900013ISI:000169689900013 ? 9Schwinning, S. Davis, K. Richardson, L. Ehleringer, J. R.2002rDeuterium enriched irrigation indicates different forms of rain use in shrub/grass species of the Colorado Plateau345-355 Oecologia1303arid ecosystems, plant functional types, precipitation pulse use, stable isotope label, water partitioning cold desert community; soil-water dynamics; summer precipitation; shortgrass steppe; shrubs; trees; vegetation; ecosystem; patterns; grassesFebWe contrasted the seasonal use of simulated large rain events (24 mm) by three native species of the and 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.://000174215200004ISI:000174215200004? -Schwinning, S. Starr, B. I. Ehleringer, J. R.2003TDominant cold desert plants do not partition warm season precipitation by event size252-260 Oecologia1362deuterium-labeled irrigation, niche separation, precipitation change, rain use efficiency rainfall events; photosynthetic capacity; stomatal conductance; heavy precipitation; shortgrass steppe; colorado plateau; water; community; nitrogen; shrubsJulWe conducted experiments to examine the quantitative relationships between rainfall event size and rainwater uptake and use by four common native plant species of the Colorado Plateau, including two perennial grasses, Hilaria jamesii (C-4) and Oryzopsis hymenoides (C-3), and two shrubs, Ceratoides lanata (C-3), and Gutierrezia sarothrae (C-3). Specifically, we tested the hypothesis that grasses use small rainfall events more efficiently than shrubs and lose this advantage when events are large. Rainfall events between 2 and 20 mm were simulated in spring and summer by applying pulses of deuterium-labeled irrigation water. Afterwards, pulse water fractions in stems and the rates of leaf gas exchange were monitored for 9 days. Cumulative pulse water uptake over this interval (estimated by integrating the product of pulse fraction in stem water and daytime transpiration rate over time) was approximately linearly related to the amount of pulse water added to the ground in all four species. Across species, consistently more pulse water was taken up in summer than in spring. Relative to their leaf areas, the two grass species took up more pulse water than the two shrub species, across all event sizes and in both seasons, thus refuting the initial hypothesis. In spring, pulse water uptake did not significantly increase photosynthetic rates and in summer, pulse water uptake had similar, but relatively small effects on the photosynthetic rates of the three C-3 plants, and a larger effect on the C-4 plant H. jamesii. Based on these data, we introduce an alternative hypothesis for the responses of plant functional types to rainfall events of different sizes, building on cost-benefit considerations for active physiological responses to sudden, unpredictable changes in water availability.://000184092500010ISI:000184092500010b? Weltzin, J. F. Loik, M. E. Schwinning, S. Williams, D. G. Fay, P. A. Haddad, B. M. Harte, J. Huxman, T. E. Knapp, A. K. Lin, G. H. Pockman, W. T. Shaw, M. R. Small, E. E. Smith, M. D. Smith, S. D. Tissue, D. T. Zak, J. C.2003VAssessing the response of terrestrial ecosystems to potential changes in precipitation941-952 Bioscience5310global change, community, ecosystem, precipitation, soil moisture climate-change; biosphere model; united-states; elevated co2; vegetation distribution; primary productivity; species-diversity; temporal dynamics; sonoran desert; water-useOctChanges in Earth's surface temperatures caused by anthropogenic emissions of greenhouse gases are expected to affect global and regional precipitation regimes. Interactions between changing precipitation regimes and other aspects of global change are likely to affect natural and managed terrestrial ecosystems as well as human society. Although much recent research has focused on assessing the responses of terrestrial ecosystems to rising carbon dioxide or temperature, relatively little research has focused on understanding how ecosystems respond to changes in precipitation regimes. Here we review predicted changes in global and regional precipitation regimes, outline the consequences of precipitation change for natural ecosystems and human activities, and discuss approaches to improving understanding of ecosystem responses to changing precipitation. Further, we introduce the Precipitation and Ecosystem Change Research Network (PrecipNet), a new interdisciplinary research network assembled to encourage and foster communication and collaboration across research groups with common interests in the impacts of global change on precipitation regimes, ecosystem structure and function, and the human enterprise.://000185816100007ISI:000185816100007D?Schwinning, Susanne1994@Effects of competitive symmetry on populations of annual plants. Ecology and Evolutionary BiologyTucsonUniversity of ArizonaPh.D.?*Chapman, D.F Parsons, A.J. Schwinning, S.1996UManagement of clover in grazed pastures: expectations, limitations and opportunities.55-64aWhite Clover: New Zealand's Competitive Edge. Symposium of the New Zealand Grassland Association. Lincoln, N.Z,?)Parsons, A.J. Carrere, P. Schwinning, S.1999,Dynamics of heterogeneity in a grazed sward.187-214FInternational Symposium on Grassland Ecophysiology and Grazing EcologyLdeMoraes, A. Nabinger, C. de Faccio, P.C. Alves, S.J. Campos Lustosa, S.B.Curitiba, Parana, BrazilD?Schwinning, Susanne1984<Isolation of phaseic acid and test of its effect on stomata.Department of Botany Goettingen%Gearg August University of GoettingenDiplom?/Ehleringer, J.R. Schwinning, S. Gebauer, R.L.E.1999"Water use in arid land ecosystems.347-365!Advances in Physiological EcologyOxfordBlackwell Science?Schwinning, S. Parsons, A.J.1996[Interactions between grasses and legumes: understanding variability in species composition.153-163{Legumes in Sustainable Farming: Proceedings of the Sustainable Farming Systems/ British Grassland Society Joint Conference.3x?Peter Chesson Renate L. E. Gebauer Susanne Schwinning Nancy Huntly Kerstin Wiegand Morgan S. K. Ernest Anna Sher Ariel Novoplansky Jake F. Weltzin2004cResource pulses, species interactions, and diversity maintenance in arid and semi-arid environments236-253 Oecologia`Coexistence -eq(?"Susanne Schwinning Osvaldo E. Sala2004JHierarchy of responses to resource pulses in arid and semi-arid ecosystems211-220 OecologiakClimate chF?jSeyfried, M. S. Schwinning, S. Walvoord, M. A. Pockman, W. T. Newman, B. D. Jackson, R. B. Phillips, F. M.2004EEcohydrological control of deep drainage in arid and semiarid regionsEcology (in press)?Huxman, T. E. Smith, M. D. Fay, P. A. Knapp, A. K. Shaw, M. R. Loik, M. E. Smith, S. D. Tissue, D. T. Zak, J. C. Weltzin, J. F. Pockman, W. T. Sala, O. E. Haddad, B. M. Harte, J. Koch, G. W. Schwinning, S. Small, E. E. Williams, D. G.20049Convergence across biomes to a common rain-use efficiency651-654Nature4296992YUnited-states; elevated co2; ecosystems; productivity; dynamics; gradient; shrubs; steppeJun 10Water availability limits plant growth and production in almost all terrestrial ecosystems(1-5). However, biomes differ substantially in sensitivity of aboveground net primary production ( ANPP) to between-year variation in precipitation(6-8). Average rain-use efficiency ( RUE; ANPP/precipitation) also varies between biomes, supposedly because of differences in vegetation structure and/or biogeochemical constraints(8). Here we show that RUE decreases across biomes as mean annual precipitation increases. However, during the driest years at each site, there is convergence to a common maximum RUE (RUEmax) that is typical of arid ecosystems. RUEmax was also identified by experimentally altering the degree of limitation by water and other resources. Thus, in years when water is most limiting, deserts, grasslands and forests all exhibit the same rate of biomass production per unit rainfall, despite differences in physiognomy and site-level RUE. Global climate models(9,10) predict increased between-year variability in precipitation, more frequent extreme drought events, and changes in temperature. Forecasts of future ecosystem behaviour should take into account this convergent feature of terrestrial biomes.://000221912600038 ISI:000221912600038?8Schwinning, S. Sala, O. E. Loik, M. E. Ehleringer, J. R.2004^Thresholds, memory, and seasonality: understanding pulse dynamics in arid/semi-arid ecosystems191-193 Oecologia141 ange - Ecosystem structure - Precipitation thresholds - Precipitation variability - Rainfall sizeZIn arid/semi-arid ecosystems, biological resources, such as water, soil nutrients, and plant biomass, typically go through periods of high and low abundance. Short periods of high resource abundance are usually triggered by rainfall events, which, despite of the overall scarcity of rain, can saturate the resource demand of some biological processes for a time. This review develops the idea that there exists a hierarchy of soil moisture pulse events with a corresponding hierarchy of ecological responses, such that small pulses only trigger a small number of relatively minor ecological events, and larger pulses trigger a more inclusive set and some larger ecological events. This framework hinges on the observation that many biological state changes, where organisms transition from a state of lower to higher physiological activity, require a minimal triggering event size. Response thresholds are often determined by the ability of organisms to utilize soil moisture pulses of different infiltration depth or duration. For example, brief, shallow pulses can only affect surface dwelling organisms with fast response times and high tolerance for low resource levels, such as some species of the soil micro-fauna and -flora, while it takes more water and deeper infiltration to affect the physiology, growth or reproduction of higher plants. This review first discusses how precipitation, climate and site factors translate into soil moisture pulses of varying magnitude and duration. Next, the idea of the response hierarchy for ecosystem processes is developed, followed by an exploration of the possible evolutionary background for the existence of response thresholds to resource pulses. The review concludes with an outlook on global change: does the hierarchical view of precipitation effects in ecosystems provide new perspectives on the future of arid/semiarid lands?  Precipitation - Environmental variability - Storage effect - Relative nonlinearityArid environments are characterized by limited and variable rainfall that supplies resources in pulses. Resource pulsing is a special form of environmental variation, and the general theory of coexistence in variable environments suggests specific mechanisms by which rainfall variability might contribute to the maintenance of high species diversity in arid ecosystems. In this review, we discuss physiological, morphological, and life-history traits that facilitate plant survival and growth in strongly water-limited variable environments, outlining how species differences in these traits may promote diversity. Our analysis emphasizes that the variability of pulsed environments does not reduce the importance of species interactions in structuring communities, but instead provides axes of ecological differentiation between species that facilitate their coexistence. Pulses of rainfall also influence higher trophic levels and entire food webs. Better understanding of how rainfall affects the diversity, species composition, and dynamics of arid environments can contribute to solving environmental problems stemming from land use and global climate change.F?,Schwinning, S. Starr, B.I. Eheleringer, J.R.2004}Summer and winter drought in a cold desert ecosystem (Colorado Plateau) - II: effects on plant carbon assimilation and growthJournal of Arid Environmentsin pressF?,Schwinning, S. Starr, B.I. Eheleringer, J.R.2004ySummer and winter drought in a cold desert ecosystem (Colorado Plateau) - I: effects on soil water and plant water uptakeJournal of Arid Environmentsin press A?Huxman, Travis E. Snyder, Keirith A. Tissue, David A. Leffler, Joshua Ogle, Kiona Pockman, William T. Sandquist, Darren R. Potts, Daniel L. Schwinning, Susan2004FPrecipitation pulses and carbon fluxes in semiarid and arid ecosystems 254 - 268 Oecologia141EDesert plants - Precipitation - Carbon - Photosynthesis - RespirationIn the arid and semiarid regions of North America, discrete precipitation pulses are important triggers for biological activity. The timing and magnitude of these pulses may differentially affect the activity of plants and microbes, combining to influence the C balance of desert ecosystems. Here, we evaluate how a pulse of water influences physiological activity in plants, soils and ecosystems, and how characteristics, such as precipitation pulse size and frequency are important controllers of biological and physical processes in arid land ecosystems. We show that pulse size regulates C balance by determining the temporal duration of activity for different components of the biota. Microbial respiration responds to very small events, but the relationship between pulse size and duration of activity likely saturates at moderate event sizes. Photosynthetic activity of vascular plants generally increases following relatively larger pulses or a series of small pulses. In this case, the duration of physiological activity is an increasing function of pulse size up to events that are infrequent in these hydroclimatological regions. This differential responsiveness of photosynthesis and respiration results in arid ecosystems acting as immediate C sources to the atmosphere following rainfall, with subsequent periods of C accumulation should pulse size be sufficient to initiate vascular plant activity. Using the average pulse size distributions in the North American deserts, a simple modeling exercise shows that net ecosystem exchange of CO2 is sensitive to changes in the event size distribution representative of wet and dry years. An important regulator of the pulse response is initial soil and canopy conditions and the physical structuring of bare soil and beneath canopy patches on the landscape. Initial condition influences responses to pulses of varying magnitude, while bare soil/beneath canopy patches interact to introduce nonlinearity in the relationship between pulse size and soil water response. Building on this conceptual framework and developing a greater understanding of the complexities of these eco-hydrologic systems may enhance our ability to describe the ecology of desert ecosystems and their sensitivity to global change.