Resource pulses in arid environments – patterns of rain, patterns of life

Global warming means that precipitation patterns will change nearly everywhere in the world, but nowhere more than in arid and semiarid ecosystems. Here, precipitation is the dominant driver of biological processes across a broad range of spatial and temporal scales. However, characterizations of water-limited ecosystems on the basis of long-term, mean precipitation are often inadequate, especially in changing environments. Research is therefore now focusing on precipitation patterns, and how changes in these patterns could modify the effect of mean precipitation on the dynamics of soil processes and plant and animal populations. Three key questions were addressed at the ‘Precipitation Pulse Use in Arid and Semi-Arid Ecosystems’ workshop (http://www.ag.arizona.edu/research/schwinn/workshop.html): How does the timing and size of a rainfall event determine its impact on organisms, communities and ecosystems? How are nutrient dynamics coupled to soil moisture pulses? What are the consequences of pulsed resource renewal on species interactions, diversity and primary productivity? ‘Pulses’ are defined as ephemeral supplies of resources available for uptake by organisms – precipitation is perhaps the most obvious, but other resources are also pulsed, such as mineral nutrients and plant biomass (as a resource to primary consumers). Precipitation pulse patterns can be defined by the distribution of pulse sizes and of the times between pulses. Although it is well established that interannual variability in precipitation is inversely related to total precipitation, much less is known about the distributions of pulse sizes and times between pulses. Bill Lauenroth (Colorado State University, CO, USA) reported that while the frequency of pulse sizes is negatively related to size and the time between pulses is negatively related to the length of the interval, both variables show large geographic variability in the arid- and semiarid portions of the western US that cannot be explained by macroclimatic variables such as annual precipitation or temperature. Moreover, characteristics of precipitation pulses interact with a number of both abiotic factors (e.g. soil surface and texture characteristics, and evaporative demand) and biotic facors (e.g. vegetation type). This creates spatially and temporally distributed resources, such as the vertical and horizontal distribution of soil moisture and nutrients (David Breshears, Los Alamos National Laboratory, NM, USA). The use of these resource distributions by plants is constrained by both physiology (e.g. photosynthetic pathway, drought resistance, growth rates, functional activity) and morphology (e.g. rooting depth). Thus, plants in water-limited environments are adapted and specialized to use only a subset of all available soil moisture (Jim Ehleringer, University of Utah, UT, USA; Osvaldo Sala, University of Buenos Aires, Argentina). As an example, short-lived plants have morphological and physiological characteristics that enable them to maximize their use of highly dynamic shallow soil moisture – small root : shoot ratios, shallow root systems, and high leaf conductance with high stomatal control. By contrast, long-lived plants tend to have adaptations better suited for using deeper, more reliable sources of soil moisture – large root to shoot ratios, deep root systems and lower leaf conductance with low stomatal control (Susan Schwinning, Biosphere 2 Center, AZ, USA). Many desert plants exhibit adaptations, such as dimorphic root distributions, that enable them to switch between water sources by increasing extraction from deeper soil layers as water is depleted from shallow soil layers. For example, the dominance of Larrea tridentata through large areas of hot deserts in North and South America may be partially explained by its capacity to acquire water from all soil layers (Jochen Shenk, University of Southern California, CA, USA). Spatial and temporal patterns of water uptake are as yet unknown for the vast majority of arid-land plants. Furthermore, the phenomenon of hydraulic redistribution complicates the characterization of water use patterns and strategies (Josh Leffler, Utah State University, UT, USA). Precipitation pulses can affect community composition by altering demographic processes (e.g. recruitment, survivorship) or affecting patterns of competition that alter growth and the subsequent ability of organisms to capture resources. Pulses are broadly recognized as critical to the process of recruitment, although linkages to community structure are not always clear, nor do they always appear to be important (Jake Weltzin, University of Tennessee, TN, USA). For example, the effects of pulses on recruitment may be obviated by other factors, such as propagule supply, neighborhood responses, or external perturbations (e.g. grazing, fire and drought). In addition, though resource supply is pulsed, recruitment itself may not be. Kerstin Wiegand (University of Giessen, Germany) showed an example in which the persistence of a long-lived desert species depended as much on frequent low recruitment rates as on rare high recruitment rates. Once plants are established, resource pulses may affect the intensity of species interactions, with ramifications for community composition. Deborah Goldberg (University of Michigan, MI, USA) outlined the two-phase resource-dynamics hypothesis, wherein resource acquisition and growth occur during pulses, and mortality occurs during interpulse periods when resources are low. An important prediction that follows from this hypothesis is that, as long as pulse-growth does not affect interpulse-mortality, a more pulsed environment should decouple growth and mortality more strongly, and therefore diminish the role of competition in structuring the community. Another prediction is that pulsed resources can change competitive hierarchies between species. As Goldberg and Anna Sher (University of New Mexico, NM, USA) described, these predictions are partly supported by empirical research, but in general there are few data to evaluate the predictions, and additional research would help determine the role of pulses in shaping species interactions and community structure. Peter Chesson (University of California, Davis, CA, USA) described how differentiation in the response of species to pulses could affect the timing and intensity of intra- and interspecific interactions, with implications for species coexistence. A pulsed resource supply promotes coexistence if species differences in physiological activity result in temporal resource partitioning (i.e. pulses that occur during a period of activity for a particular species and of inactivity for another may act as a semiexclusive resource for the active species). Similarly, differential rates of resource utilization or exploitation may facilitate coexistence, depending on physiological tradeoffs (e.g. between growth and survival) and patterns of resource availability. The second way in which pulses can promote coexistence is if species differ in terms of their saturation or threshold response to resource availability. Conversely, if species are not differentiated in either of the two ways (i.e. all pulses are available to all species), then pulses should have no effects on species coexistence. Jeremy Lundholm (University of Guelph, Canada) pointed out that in some habitats (such as rock outcrops) precipitation pulses act less as a resource and more as a disturbance factor, which can affect coexistence through differential effects on species mortality due to flooding or interpulse droughts. Although the actual mechanisms that structure communities in arid and semiarid ecosystems remain unidentified, it is clear that individual species and functional types respond differently to resource distributions in space and time. For example, Jim Reynolds (Duke University, NC, USA) investigated the relationship between climate and vegetation based on historical records and process-based models, and found that the abundance of particular plant functional types was driven by landscape position, within-season distribution of precipitation, antecedent soil moisture, and the presence of neighbors, but was unrelated to the proportional distribution of precipitation between summer and winter. Though most research focuses on the effects of precipitation on vegetation, the effects of pulsed resources on plant population and community dynamics can potentially result in non-linear population dynamics in primary consumers such as rodents (Morgan Ernest, University of New Mexico, NM, USA). Pulses have important ramifications for ecosystem-level processes such as carbon and nutrient cycling through changes in photosynthesis, decomposition, soil microbial activity, and interactions with tissue quality. However, relatively few studies have evaluated the importance of the pulsed nature of precipitation events on ecosystem function, and models developed for mesic environments are often inadequate to describe ecosystem processes in more arid environments. Amilcare Porporato (Polytechnic of Turin, Italy) demonstrated that if a common model of ecosystem C and N turnover is linked to stochastic soil moisture variation through effects on ammonification, nitrification, volatilization, leaching and plant uptake, C and N pool sizes can vary on time scales from days to centuries and may even exhibit periodic fluctuations. Different components of ecosystems may have different response times to resource pulses. Travis Huxman (University of Arizona, AZ, USA) showed that because microbes accelerated physiological processes faster than plants after rain events, ecosystem respiration and photosynthesis differed in their temporal contribution to whole ecosystem gas exchange. Moreover, whole-ecosystem responses were controlled by a number of factors, including soil type, species composition, and antecedent moisture conditions. Given the potentially complex effects of stochastic precipitation patterns on ecosystem fluxes, it is perhaps not surprising that cumulative measures of precipitation are not always correlated with ecosystem processes. For example, a synthesis of data on litter decomposition of shrub and grass species in natural water-limited ecosystems (less than 600 mm mean annual precipitation) showed no relationship between rates of mass loss for one year and mean annual precipitation (Amy Austin, University of Buenos Aires, Argentina). This suggests that other environmental variables, such as systematic differences in litter quality between shrubs and grasses, or photo-degradation, contribute to the dynamics of litter decomposition in water-limited ecosystems. An example showing that precipitation patterns may matter more than precipitation totals was presented by Indy Burke (Colorado State University, CO, USA). She showed that increasing the interval between precipitation events, while holding the total water supply constant, reduces seasonal nitrogen availability, net primary production, and potential ecosystem carbon storage. The effects of resource pulses on ecosystem fluxes may also depend strongly on timing with respect to the growing season. For example, in contrast to the general assumption that water and nutrient availability are coupled in semiarid systems, John Stark (Utah State University, UT, USA) described how pulses of soil moisture during the late growing season resulted in net nitrogen immobilization. This response may actually be engendered by shifts in substrate quality and microbial activity associated with plant senescence and changes in the quality of the labile soil organic matter pool. The workshop highlighted the potential importance of resource pulses on the structure and function of populations, communities and ecosystems, but also the lack of data and understanding required to develop a comprehensive synthesis of the field. Key future aims will be to: Define the phenomenon of resource pulses in arid environments and discuss their potential importance from ecological and evolutionary perspectives. Describe how pulses affect physiological responses at scales from leaves to ecosystems. Outline the effects of pulses on species interactions, competitive hierarchies, and species coexistence. Review how plant life forms and functional types may respond to pulses. Investigate linkages between precipitation, nutrient cycling and other soil processes. Continuing empirical and theoretical research will be facilitated by the presence of research networks such as PrecipNet (http://www.zzyx.ucsc.edu/ES/PrecipNet.htm). Michael Loik (University of California, Santa Cruz, CA, USA) described how PrecipNet analysis and synthesis activities promote development of linkages between the physical and biological aspects of pulsed resources, and their impact on human societies. The meeting was organized by Jim Ehleringer, Michael Loik, Osvaldo Sala and Susan Schwinning, and was supported in part by the NSF (Grant # DEB0222313). JFW was supported in part by USDA-CSREES (Grant # 00-35101-9308). This report was improved by comments from Susan Schwinning and Michael Loik.

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