Winter snow and spring temperature have differential effects on vegetation phenology and productivity across Arctic plant communities.

Tundra dominates two-thirds of the unglaciated, terrestrial Arctic. Although this region has experienced rapid and widespread changes in vegetation phenology and productivity over the last several decades, the specific climatic drivers responsible for this change remain poorly understood. Here we quantified the effect of winter snowpack and early spring temperature conditions on growing season vegetation phenology (timing of the start, peak, and end of the growing season) and productivity of the dominant tundra vegetation communities of Arctic Alaska. We used daily remotely sensed normalized difference vegetation index (NDVI), and daily snowpack and temperature variables produced by SnowModel and MicroMet, coupled physically based snow and meteorological modeling tools, to (1) determine the most important snowpack and thermal controls on tundra vegetation phenology and productivity and (2) describe the direction of these relationships within each vegetation community. Our results show that soil temperature under the snowpack, snowmelt timing, and air temperature following snowmelt are the most important drivers of growing season timing and productivity among Arctic vegetation communities. Air temperature after snowmelt was the most important control on timing of season start and end, with warmer conditions contributing to earlier phenology in all vegetation communities. In contrast, the controls on the timing of peak season and productivity also included snowmelt timing and soil temperature under the snowpack, dictated in part by the snow insulating capacity. The results of this novel analysis suggest that while future warming effects on phenology may be consistent across communities of the tundra biome, warming may result in divergent, community-specific productivity responses if coupled with reduced snow insulating capacity lowers winter soil temperature and potential nutrient cycling in the soil.

Differential stoichiometric homeostasis and growth in two native and two invasive C3 grasses.

Global changes interact with plant invasions by differentially impacting native and invasive species. For example, invasive plants often benefit from eutrophication to a greater degree than native plants. While this is well-documented, a broad, trait-based explanation for this phenomenon is lacking. Recent research shows that stoichiometric homeostasis predicts plant species responses to eutrophication and drought, but this research has not been extended into an invasion ecology paradigm. We tested the hypotheses that stoichiometric homeostasis would differ between native and invasive plants, that expressed levels of stoichiometric homeostasis would respond to water availability, and that differences in stoichiometric homeostasis would match differences in growth. In a nutrient and water manipulation study, we found that stoichiometric homeostasis differed between native grasses (Elymus canadensis and Pascopyrum smithii) and invasive grasses (Agropyron cristatum and Bromus inermis), that differences in stoichiometric homeostasis matched differences in growth in well-watered grasses, and that expressed levels of stoichiometric homeostasis were stable across the water supply treatments. These results suggest that invasive plants maintain growth advantages over native plants in eutrophic conditions because of differential homeostatic requirements. We argue that stoichiometric homeostasis is therefore a useful functional trait to explain and predict differential native and invasive plant responses to global change.

Delayed herbivory by migratory geese increases summer-long CO2 uptake in coastal western Alaska.

The advancement of spring and the differential ability of organisms to respond to changes in plant phenology may lead to "phenological mismatches" as a result of climate change. One potential for considerable mismatch is between migratory birds and food availability in northern breeding ranges, and these mismatches may have consequences for ecosystem function. We conducted a three-year experiment to examine the consequences for CO2 exchange of advanced spring green-up and altered timing of grazing by migratory Pacific black brant in a coastal wetland in western Alaska. Experimental treatments represent the variation in green-up and timing of peak grazing intensity that currently exists in the system. Delayed grazing resulted in greater net ecosystem exchange (NEE) and gross primary productivity (GPP), while early grazing reduced CO2 uptake with the potential of causing net ecosystem carbon (C) loss in late spring and early summer. Conversely, advancing the growing season only influenced ecosystem respiration (ER), resulting in a small increase in ER with no concomitant impact on GPP or NEE. The experimental treatment that represents the most likely future, with green-up advancing more rapidly than arrival of migratory geese, results in NEE changing by 1.2 µmol m-2  s-1 toward a greater CO2 sink in spring and summer. Increased sink strength, however, may be mitigated by early arrival of migratory geese, which would reduce CO2 uptake. Importantly, while the direct effect of climate warming on phenology of green-up has a minimal influence on NEE, the indirect effect of climate warming manifest through changes in the timing of peak grazing can have a significant impact on C balance in northern coastal wetlands. Furthermore, processes influencing the timing of goose migration in the winter range can significantly influence ecosystem function in summer habitats.

Arctic plant ecophysiology and water source utilization in response to altered snow: isotopic (δ18O and δ2H) evidence for meltwater subsidies to deciduous shrubs.

Warming-linked woody shrub expansion in the Arctic has critical consequences for ecosystem processes and climate feedbacks. The snow-shrub interaction model has been widely implicated in observed Arctic shrub increases, yet equivocal experimental results regarding nutrient-related components of this model have highlighted the need for a consideration of the increased meltwater predicted in expanding shrub stands. We used a 22-year snow manipulation experiment to simultaneously address the unexplored role of snow meltwater in arctic plant ecophysiology and nutrient-related components of the snow-shrub hypothesis. We coupled measurements of leaf-level gas exchange and leaf tissue chemistry (%N and δ13C) with an analysis of stable isotopes (δ18O and δ2H) in soil water, precipitation, and stem water. In deeper snow areas photosynthesis, conductance, and leaf N increased and δ13C values decreased in the deciduous shrubs, Betula nana and Salix pulchra, and the graminoid, Eriophorum vaginatum, with the strongest treatment effects observed in deciduous shrubs, consistent with predictions of the snow-shrub hypothesis. We also found that deciduous shrubs, especially S. pulchra, obtained much of their water from snow melt early in the growing season (40-50%), more than either E. vaginatum or the evergreen shrub, Rhododendron tomentosum (Ledum palustre). This result provides the basis for adding a meltwater-focused feedback loop to the snow-shrub interaction model of shrub expansion in the Arctic. Our results highlight the critical role of winter snow in the ecophysiology of Arctic plants, particularly deciduous shrubs, and underline the importance of understanding how global warming will affect the Arctic winter snowpack.

Coupled long-term summer warming and deeper snow alters species composition and stimulates gross primary productivity in tussock tundra.

Climate change is expected to increase summer temperature and winter precipitation throughout the Arctic. The long-term implications of these changes for plant species composition, plant function, and ecosystem processes are difficult to predict. We report on the influence of enhanced snow depth and warmer summer temperature following 20 years of an ITEX experimental manipulation at Toolik Lake, Alaska. Winter snow depth was increased using snow fences and warming was accomplished during summer using passive open-top chambers. One of the most important consequences of these experimental treatments was an increase in active layer depth and rate of thaw, which has led to deeper drainage and lower soil moisture content. Vegetation concomitantly shifted from a relatively wet system with high cover of the sedge Eriophorum vaginatum to a drier system, dominated by deciduous shrubs including Betula nana and Salix pulchra. At the individual plant level, we observed higher leaf nitrogen concentration associated with warmer temperatures and increased snow in S. pulchra and B. nana, but high leaf nitrogen concentration did not lead to higher rates of net photosynthesis. At the ecosystem level, we observed higher GPP and NEE in response to summer warming. Our results suggest that deeper snow has a cascading set of biophysical consequences that include a deeper active layer that leads to altered species composition, greater leaf nitrogen concentration, and higher ecosystem-level carbon uptake.

A new perspective on trait differences between native and invasive exotic plants.

Functional differences between native and exotic species potentially constitute one factor responsible for plant invasion. Differences in trait values between native and exotic invasive species, however, should not be considered fixed and may depend on the context of the comparison. Furthermore, the magnitude of difference between native and exotic species necessary to trigger invasion is unknown. We propose a criterion that differences in trait values between a native and exotic invasive species must be greater than differences between co-occurring natives for this difference to be ecologically meaningful and a contributing factor to plant invasion. We used a meta-analysis to quantify the difference between native and exotic invasive species for various traits examined in previous studies and compared this value to differences among native species reported in the same studies. The effect size between native and exotic invasive species was similar to the effect size between co-occurring natives except for studies conducted in the field; in most instances, our criterion was not met although overall differences between native and exotic invasive species were slightly larger than differences between natives. Consequently, trait differences may be important in certain contexts, but other mechanisms of invasion are likely more important in most cases. We suggest that using trait values as predictors of invasion will be challenging.

Simulated Herbicide Drift Effects on Seed Germination, Seedling Emergence, and Seedling Growth of Native Plants of the Northern Great Plains.

Small concentrations of herbicide, such as those found in drift, can affect nontarget plants at different life-history stages including seed germination and seedling emergence as well as seedling growth. Fragmented landscapes, such as those in the northern Great Plains, lead to increased proximity of ecological restoration sites to agricultural lands using herbicides. Germination, emergence, and growth are crucial life-history stages leading to ecological restoration success, but these stages are sensitive to impacts from external factors such as herbicide exposure. A lab germination experiment and a greenhouse emergence experiment were performed to examine the effect of herbicides (2,4-dichlorophenoxyacetic acid [2,4-D], atrazine, and trifluralin) on species used in ecological restorations in the northern Great Plains. Seed germination, seedling emergence, and seedling growth of many study species decreased with exposure to herbicides at different concentrations representative of herbicide drift. At concentrations as low as 0.1% recommended application rate 2,4-D elicited broad effects on final seed germination percentage and germination rate. Atrazine affected seedling emergence and growth for a number of study species at concentrations as low as 10% recommended application rate. Trifluralin affected germination, emergence, and growth of the fewest number of study species. The information gained from these experiments can be used to inform restoration practitioners of best practices and recommended species to use when implementing ecological restoration adjacent to agricultural lands. Environ Toxicol Chem 2024;43:2387-2397. © 2024 The Author(s). Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Herbivore diversity effects on Arctic tundra ecosystems: a systematic review.

BACKGROUND: Northern ecosystems are strongly influenced by herbivores that differ in their impacts on the ecosystem. Yet the role of herbivore diversity in shaping the structure and functioning of tundra ecosystems has been overlooked. With climate and land-use changes causing rapid shifts in Arctic species assemblages, a better understanding of the consequences of herbivore diversity changes for tundra ecosystem functioning is urgently needed. This systematic review synthesizes available evidence on the effects of herbivore diversity on different processes, functions, and properties of tundra ecosystems. METHODS: Following a published protocol, our systematic review combined primary field studies retrieved from bibliographic databases, search engines and specialist websites that compared tundra ecosystem responses to different levels of vertebrate and invertebrate herbivore diversity. We used the number of functional groups of herbivores (i.e., functional group richness) as a measure of the diversity of the herbivore assemblage. We screened titles, abstracts, and full texts of studies using pre-defined eligibility criteria. We critically appraised the validity of the studies, tested the influence of different moderators, and conducted sensitivity analyses. Quantitative synthesis (i.e., calculation of effect sizes) was performed for ecosystem responses reported by at least five articles and meta-regressions including the effects of potential modifiers for those reported by at least 10 articles. REVIEW FINDINGS: The literature searches retrieved 5944 articles. After screening titles, abstracts, and full texts, 201 articles including 3713 studies (i.e., individual comparisons) were deemed relevant for the systematic review, with 2844 of these studies included in quantitative syntheses. The available evidence base on the effects of herbivore diversity on tundra ecosystems is concentrated around well-established research locations and focuses mainly on the impacts of vertebrate herbivores on vegetation. Overall, greater herbivore diversity led to increased abundance of feeding marks by herbivores and soil temperature, and to reduced total abundance of plants, graminoids, forbs, and litter, plant leaf size, plant height, and moss depth, but the effects of herbivore diversity were difficult to tease apart from those of excluding vertebrate herbivores. The effects of different functional groups of herbivores on graminoid and lichen abundance compensated each other, leading to no net effects when herbivore effects were combined. In turn, smaller herbivores and large-bodied herbivores only reduced plant height when occurring together but not when occurring separately. Greater herbivore diversity increased plant diversity in graminoid tundra but not in other habitat types. CONCLUSIONS: This systematic review underscores the importance of herbivore diversity in shaping the structure and function of Arctic ecosystems, with different functional groups of herbivores exerting additive or compensatory effects that can be modulated by environmental conditions. Still, many challenges remain to fully understand the complex impacts of herbivore diversity on tundra ecosystems. Future studies should explicitly address the role of herbivore diversity beyond presence-absence, targeting a broader range of ecosystem responses and explicitly including invertebrate herbivores. A better understanding of the role of herbivore diversity will enhance our ability to predict whether and where shifts in herbivore assemblages might mitigate or further amplify the impacts of environmental change on Arctic ecosystems.

Temperature and functional traits influence differences in nitrogen uptake capacity between native and invasive grasses.

Performance differences between native and exotic invasive plants are often considered static, but invasive grasses may achieve growth advantages in western North America shrublands and steppe under only optimal growing conditions. We examine differences in N uptake and several morphological variables that influence uptake at temperatures between 5 and 25 °C. We contrast two native perennial grasses in western North America: Elymus elymoides and Pseudoroegneria spicata; two invasive annual grasses: Bromus tectorum and Taeniatherum caput-medusae; and one highly selected non-native perennial grass: Agropyron cristatum. The influence of temperature on N uptake is poorly characterized, yet these invasive annual grasses are known to germinate in warm soils in the autumn, and both experience cool soils during the short growing season following snowmelt in the spring. To further explore the influence of temperature on the correlation between morphological variables and N uptake, our data are applied to a previously published path model and one proposed here. Differences in N uptake between native and invasive grasses were small at the lowest temperature, but were large at the highest temperature. At lower temperatures, uptake of N by annuals and perennials was correlated with leaf N and mass. At higher temperatures, uptake by annuals was correlated only with these leaf traits, but uptake by perennials was correlated with these leaf traits as well as root N and mass. Consequently, our results imply that annual grasses face fewer morphological constraints on N uptake than perennial grasses, and annual grasses may gain further advantage in warmer temperature conditions or during more frequent warm periods.

The Missing Angle: Ecosystem Consequences of Phenological Mismatch.

Climate change leads to unequal shifts in the phenology of interacting species, such as consumers and their resources, leading to potential phenological mismatches. While studies have investigated how phenological mismatch affects wild populations, we still lack studies and a framework for investigating how phenological mismatch affects ecosystems, particularly nutrient cycling.

Migratory goose arrival time plays a larger role in influencing forage quality than advancing springs in an Arctic coastal wetland.

With warmer springs, herbivores migrating to Arctic breeding grounds may experience phenological mismatches between their energy demands and the availability of high quality forage. Yet, how the timing of the start of the season and herbivore arrival influences forage quality is often unknown. In coastal western Alaska, approximately one million migratory geese arrive each spring to breed, where foliar %N and C:N ratios are linked to gosling survival and population growth. We conducted a three-year experiment where we manipulated the start of the growing season using warming chambers and grazing times using captive Pacific black brant (Branta bernicla nigricans) to examine how the timing of these events influences the quality of an important forage species. Our results suggest that grazing timing plays a much greater role than an advanced growing season in determining forage quality. All top models included grazing timing, and suggested that compared to typical grazing timing, early grazing significantly reduced foliar %C by 6% and C:N ratios by 16%, while late goose grazing significantly reduced foliar %N by 15% and increased foliar C:N ratios by 21%. While second-ranking top models included the effect of season, the advanced growing season effect was not significant and only reduced %N by 4%, increased %C by <1%, and increased C:N ratios by 5% compared to an ambient growing season. In summary, in years where geese arrive early, they will consume higher quality forage when they arrive and throughout the season, while in years that geese arrive late they will consume lower quality forage when they arrive and for the remainder of the season. When the growing season starts has only a minor influence on this pattern. Our findings suggest that cues determining migration and arrival times to breeding areas are important factors influencing forage quality for geese in western Alaska.

Root turnover and relocation in the soil profile in response to seasonal soil water variation in a natural stand of Utah juniper (Juniperus osteosperma).

Juniper species are noted for long-lived foliage, low and persistent gas exchange activity and drought tolerance. Because leaves and roots of the same species are thought to be similar in structure and life history, we hypothesized that Juniperus osteosperma (Torr.) Little (Utah juniper) fine roots would reflect the persistent aboveground foliage characteristic of this species. We monitored fine roots, less than 1 mm in diameter, by minirhizotron imaging to a depth of 150 cm over two growing seasons from April 2002 to December 2003. We measured fine root numbers, lengths and diameters, and noted the time of birth and death of root segments. We correlated our root data with soil water potential measured by thermocouple psychrometry and ecosystem evapotranspiration measured by ecosystem eddy flux. Median fine root lifespan, determined by the Kaplan-Meier product-limit method, was about one year, much less than foliage lifespan estimates of more than five years. Yet, roots of juniper live much longer than those of other Great Basin species. The median survivorship of shallow and deep roots was 144 and 448 days, respectively. Production of new roots was observed during periods of favorable soil water potential and there was a seasonal progression of increased new roots and root length during the warm season toward lower soil depths with root loss in the upper soil layers. This was also reflected in water extraction which progressed to greater soil depths later in the warm season. Aboveground, rates of ecosystem evapotranspiration decreased with decreasing soil water potentials in a similar manner in both 2002 and 2003, reflecting the relocation of roots to available water at depth. Juniper exhibited a flexible root depth distribution throughout the 20 months of this study, indicating the potential to respond to shifting soil water resources despite long fine root lifespans.

Variation in carbon isotope composition among years in the riparian tree Populus fremontii.

Water availability is an important factor limiting the productivity of desert plants but little is known about the impact of water-limiting conditions on the physiology of plants in mesic environments. Riparian ecosystems of the western US receive significantly more water than the surrounding desert environments but experience dramatic interannual fluctuations in water availability because both stream flow and precipitation are highly variable over time. This variability results in different growing conditions each year which may influence the physiology of riparian species such as Fremont cottonwood (Populus fremontii), the dominant, native canopy tree species in lowland southwestern US river systems. We wished to determine if the physiology of this species varies among years, what climatic parameters are related to any observed physiological variation and if individuals within a P. fremontii population differ in their physiological response to variation through time. We collected tree ring cores from a central New Mexico cottonwood population and analyzed carbon isotope composition (δ13C) in each year from 1981 to 1995. We used δ13C analysis in this study because it allowed us to obtain multi-year estimates of physiological activity. During these years, mean stream flow at our study site ranged over two orders of magnitude from 0.82 to 80.94 m3 s-1, precipitation ranged fourfold from 49 to 215 ccmm and mean temperature ranged from 20.5 to 22.6°C during the growing season. δ13C varied from a low of -26.7‰ in 1984 to a high of -24.7‰ in 1981. Low δ13C values were associated with years in which stream flow and/or precipitation were high and temperature was low. The opposite was true of years with high δ13C values. We observed a strong linear relationship between δ13C and stream flow during years when stream flow was <25 m3 s-1 but no significant relationship between these variables when stream flow was >25 m3 s-1. Additionally, there was a linear relationship between δ13C and precipitation during years when stream flow was <25 m3 s-1 but not in years when stream flow was >25 m3 s-1. These data suggest that above a threshold of total stream flow, increased flow does not influence physiology. Below this threshold, precipitation can be an important water source. The ten individuals within our study population varied significantly in mean δ13C values but responded to interannual variation in a similar manner (i.e., all individuals had low δ13C values when water was abundant). These results suggest that precipitation as well as stream flow are important factors influencing the physiology of this riparian tree.

Carbon acquisition and water use in a Northern Utah Juniperus osteosperma (Utah juniper) population.

Water use and carbon acquisition were examined in a northern Utah population of Juniperus osteosperma (Torr.) Little. Leaf-level carbon assimilation, which was greatest in the spring and autumn, was limited by soil water availability. Gas exchange, plant water potential and tissue hydrogen stable isotopic ratio (deltaD) data suggested that plants responded rapidly to summer rain events. Based on a leaf area index of 1.4, leaf-level water use and carbon acquisition scaled to canopy-level means of 0.59 mm day(-1) and 0.13 mol m(-2) ground surface day(-1), respectively. Patterns of soil water potential indicated that J. osteosperma dries the soil from the surface downward to a depth of about 1 m. Hydraulic redistribution is a significant process in soil water dynamics. Eddy covariance data indicated a mean evapotranspiration rate of 0.85 mm day(-1) from March to October 2001, during which period the juniper population at the eddy flux site was a net source of CO2 (3.9 mol m(-2) ground area). We discuss these results in relation to the rapid range expansion of juniper species during the past century.

Fine root distribution and persistence under field conditions of three co-occurring Great Basin species of different life form.

Fine roots of an annual grass, a perennial grass and a perennial shrub were examined. Based on life histories and tissue composition, we expected the greatest root persistence for the shrub and shortest for the annual grass. Roots were observed with minirhizotrons over 2 yr for number, length and diameter changes. A Cox proportional hazard regression correlated root persistence with soil water, depth, diameter and date of production. In 2001, grass roots had similar persistence times, but shrub roots had the shortest. In 2002, the annual had the longest median root persistence, the perennial grass intermediate and the perennial shrub had the shortest. All species responded similarly to the magnitude of seasonal precipitation; root numbers increased with favorable soil moisture and disappeared with drying; fewer, thinner roots at greater soil depths were found in the drier year (2001). Root persistence increased with soil moisture, diameter and earlier appearance in the spring. Plasticity in root morphology and placement was influenced by water availability, yet persistence was surprisingly contrary to expectations.

How much variance is explained by ecologists? Additional perspectives.

A recent meta-analysis of meta-analyses by Møller and Jennions suggested that ecologists using statistical models are explaining between 2.5% and 5.42% of the variability in ecological studies. Although we agree that there is considerable variability in ecological systems that is not explained, we disagree with the approach and general conclusions of Møller and Jennions. As an alternate perspective, we explored the question: "How much ecological variation in relationships is not explained?" We did this by examining published studies in five different journals representative of the numerous sub-disciplines of ecology. We quantified the proportion of variance not explained in statistical models as the residual or random error compared to the total variation in the data set. Our results indicate that statistical models explain roughly half of the variation in variables of interest, vastly different from the 2.5%-5.42% reported by Møller and Jennions. This difference resulted largely from a different level of analysis: we considered the original study to be the appropriate level for quantifying variability while Møller and Jennions combined studies at different temporal and spatial scales and attempted to find universal single-factor relationships between ecological variables across study organisms or locations. Therefore, we believe that Møller and Jennions actually measured the universality of single factor effects across multiple ecological systems, not the amount of variability in ecological studies explained by ecologists. This study, combined with Møller and Jennions', illustrates importance of applying statistical models appropriately to assess ecological relationships.

Root responses and nitrogen acquisition by Artemisia tridentata and Agropyron desertorum following small summer rainfall events.

Resources in the Great Basin of western North America often occur in pulses, and plant species must rapidly respond to temporary increases in water and nutrients during the growing season. A field study was conducted to evaluate below ground responses of Artemisia tridentata and Agropyron desertorum, common Great Basin shrub and grass species, respectively, to simulated 5-mm (typical summer rain) and 15-mm (large summer rain) summer rainfall events. The simulated rainfall was labeled with K(15)NO(3) so that timing of plant nitrogen uptake could be monitored. In addition, soil NH(4)(+) and NO(3)(-) concentrations and physiological uptake capacities for NO(3)(-) and NH(4)(+) were determined before and after the rainfall events. Root growth in the top 15 cm of soil was monitored using a minirhizotron system. Surprisingly, there was no difference in the amount of labeled N acquired in response to the two rainfall amounts by either species during the 7-day sample period. However, there were differences between species in the timing of labeled N uptake. The N label was detected in above ground tissue of Agropyron within 1 h of the simulated rainfall events, but not until 24 h after the rainfall in Artemisia. For both Agropyron and Artemisia, root uptake capacity was similarly affected by the 5-mm and 15-mm rainfall. There was, however, a greater increase in uptake capacity for NH(4)(+) than for NO(3)(-), and the 15-mm event resulted in a longer response. No root growth occurred in either species in response to either rainfall event during this 8-day period. The results of this study indicate that these species are capable of utilizing nitrogen pulses following even small summer rainfall events during the most stressful period of the summer and further emphasize the importance of small precipitation events in arid systems.

Precipitation pulses and carbon fluxes in semiarid and arid ecosystems.

In 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.