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Chronic anthropogenic disturbance (CAD) and climate change represent two of the major threats to biodiversity globally, but their combined effects are not well understood. Here we investigate the individual and interactive effects of increasing CAD and decreasing rainfall on the composition and taxonomic (TD), functional (FD) and phylogenetic diversity (PD) of plants possessing extrafloral nectaries (EFNs) in semi-arid Brazilian Caatinga. EFNs attract ants that protect plants against insect herbivore attack and are extremely prevalent in the Caatinga flora. EFN-bearing plants were censused along gradients of disturbance and rainfall in Catimbau National Park in north-eastern Brazil. We recorded a total of 2243 individuals belonging to 21 species. Taxonomic and functional composition varied along the rainfall gradient, but not along the disturbance gradient. There was a significant interaction between increasing disturbance and decreasing rainfall, with CAD leading to decreased TD, FD and PD in the most arid areas, and to increased TD, FD and PD in the wettest areas. We found a strong phylogenetic signal in the EFN traits we analysed, which explains the strong matching between patterns of FD and PD along the environmental gradients. The interactive effects of disturbance and rainfall revealed by our study indicate that the decreased rainfall forecast for Caatinga under climate change will increase the sensitivity of EFN-bearing plants to anthropogenic disturbance. This has important implications for the availability of a key food resource, which would likely have cascading effects on higher trophic levels.

Recent evidence has revealed the emergence of a megadrought in southwestern North America since 2000. Megadroughts extend for at least 2 decades, making it challenging to identify such events until they are well established. Here, we examined tree-ring growth and stable isotope ratios in Pinus ponderosa at its driest niche edge to investigate whether trees growing near their aridity limit were sensitive to the megadrought climatic pre-conditions, and were capable of informing predictive efforts. During the decade before the megadrought, trees in four populations revealed increases in the cellulose δ13C content of earlywood, latewood, and false latewood, which, based on past studies are correlated with increased intrinsic water-use efficiency. However, radial growth and cellulose δ18O were not sensitive to pre-megadrought conditions. During the 2 decades preceding the megadrought, at all four sites, the changes in δ13C were caused by the high sensitivity of needle carbon and water exchange to drought trends in key winter months, and for three of the four sites during crucial summer months. Such pre-megadrought physiological sensitivity appears to be unique for trees near their arid range limit, as similar patterns were not observed in trees in ten reference sites located along a latitudinal gradient in the same megadrought domain, despite similar drying trends. Our results reveal the utility of tree-ring δ13C to reconstruct spatiotemporal patterns during the organizational phase of a megadrought, demonstrating that trees near the arid boundaries of a species' distribution might be useful in the early detection of long-lasting droughts.

Plants make leaf-level trade-offs between photosynthetic carbon assimilation and water loss, and the optimal balance between the two is dependent, in part, on water availability. “Conservative” water-use strategies, in which minimizing water loss is prioritized over assimilating carbon, tend to be favored in arid environments, while “aggressive” water-use strategies, in which carbon assimilation is prioritized over water conservation, are often favored in mesic environments. When derived from foliar carbon isotope ratios, intrinsic water-use efficiency (iWUE) serves as a seasonally integrated indicator of the balance of carbon assimilation to water loss at the leaf level. Here, we used a multi-decadal record of annual iWUE, growth, and flowering from a single population of Encelia farinosa in the Mojave Desert to evaluate the effect of iWUE on plant performance across interannual fluctuations in water availability. We identified substantial variability in iWUE among individuals and found that iWUE interacted with water availability to significantly influence growth and flowering. However, the relationships between iWUE, water availability, and plant performance did not universally suggest that “conservative” water-use strategies were advantageous in dry years or that “aggressive” strategies were advantageous in wet years. iWUE was positively related to the odds of growth regardless of water availability and to the odds of flowering in dry years, but negatively related to growth rates in dry years. In addition, we found that leaf nitrogen content affected interannual plant performance and that an individual’s iWUE plasticity in response to fluctuations in aridity was negatively related to early life drought survival and growth.

Plant parasitic nematodes are among the greatest consumers of primary production in terrestrial ecosystems. Their feeding strategies can be divided into endoparasites and ectoparasites that differ substantially, not only in their damage potential to host tissue and primary production, but also in their susceptibility to environmental changes. Climate change is predicted to increase variability of precipitation in many systems, yet the effects on belowground biodiversity and associated impacts on primary productivity remain poorly understood. To examine the impact of altered precipitation on endo- and ectoparasitic soil nematodes, we conducted a 2-year precipitation manipulation study across an arid, a semiarid, and a mesic grassland. Plant parasite feeding type abundance, functional guilds, and herbivory index in response to precipitation were evaluated. Responses of endo- and ectoparasites to increased precipitation varied by grassland type. There was little response of ectoparasites to increased precipitation although their population declined at the mesic site with increased precipitation. The abundance of endoparasites remained unchanged with increasing precipitation at the arid site, increased at the semiarid, and decreased at the mesic site. The herbivory index followed closely the trends seen in the endoparasites response by stagnating at the arid site, increasing at the semiarid, and decreasing at the mesic site. Our findings suggest that altered precipitation has differing effects on plant parasite feeding strategies as well as functional guilds. This may have important implications for grassland productivity, as plant parasite pressure may exacerbate the effects of climate change on host plants.

Recent analyses on the length of drought recovery in forests have shown multi-year legacies, particularly in semi-arid, coniferous ecosystems. Such legacies are usually attributed to ecophysiological memory, although drought frequency itself, and its effect on overlapping recovery times, could also contribute. Here, we describe a multi-decadal study of drought legacies using tree-ring carbon-isotope ratios (δ13C) and ring-width index (RWI) in Pinus ponderosa at 13 montane sites traversing a winter–summer precipitation gradient in the Southwestern U.S. Sites and trees were selected to avoid collection biases that exist in archived tree-ring databanks. The spatial hydroclimate gradient and winter–summer seasonal patterns were well predicted by seasonal and inter-annual correlations between δ13C and atmospheric vapor pressure deficit (VPD). Using VPD, we found that the probability of extreme drought has increased up to 70% in this region during the past two decades. When the recent increase in drought frequency was not considered, multi-year legacies in both δ13C and RWI were observed at most sites. When the increase in drought frequency was detrended from tree-ring chronologies, some sites exhibited short legacies (1–2 years) in both δ13C and RWI, and there was a sight trend for longer legacies in RWI. However, when considered broadly across the region and multiple decades, no significant legacies were observed, which contrasts with past studies. Our results reveal that a contribution to observed multi-year legacies is related to shifts in the climate system itself, an exogenous factor, that must be considered along with physiological memory.

Freshwater springs are important ecosystems. In the arid regions of North America, groundwater extraction has caused the desiccation of springs and the extinction of taxa. To better describe the biodiversity of freshwater springs in the hope of establishing a sensitive approach for monitoring the predicted change in spring systems, we used high-resolution genetic methods to estimate the alpha and beta diversity of 19 springs and two reservoirs within the Ash Meadows National Wildlife Refuge in southwestern Nevada. We discovered a large number of distinct taxa based on eukaryote ribosomal gene sequences and show water temperature, spring size, and the presence or absence of non-native predators predicts alpha diversity, and temperature predicts beta diversity. Our study highlights how DNA data support inferences of environmental factors influencing community diversity and demonstrates the method may be an important tool for monitoring ecological communities.

Changing precipitation regimes can profoundly affect plant growth in terrestrial ecosystems, especially in arid and semi-arid regions. However, how changing precipitation, especially extreme precipitation events, alters plant diversity and community composition is still poorly understood. A 3-year field manipulative experiment with seven precipitation treatments, including - 60%, - 40%, - 20%, 0% (as a control), + 20%, + 40%, and + 60% of ambient growing-season precipitation, was conducted in a semi-arid steppe in the Mongolian Plateau. Results showed total plant community cover and forb cover were enhanced with increased precipitation and reduced under decreased precipitation, whereas grass cover was suppressed under the - 60% treatment only. Plant community and grass species richness were reduced by the - 60% treatment only. Moreover, our results demonstrated that total plant community cover was more sensitive to decreased than increased precipitation under normal and extreme precipitation change, and species richness was more sensitive to decreased than increased precipitation under extreme precipitation change. The community composition and low field water holding capacity may drive this asymmetric response. Accumulated changes in community cover may eventually lead to changes in species richness. However, compared to control, Shannon-Weiner index (H) did not respond to any precipitation treatment, and Pielou's evenness index (E) was reduced under the + 60% treatment across the 3 year, but not in each year. Thus, the findings suggest that plant biodiversity in the semi-arid steppe may have a strong resistance to precipitation pattern changes through adjusting its composition in a short term.

Molecular phylogenies are increasingly used to understand how biotic interactions and environment shape phylogenetic community structure (PCS). However, we do not understand the effects of plant-plant interactions and environment on PCS and phylogenetic diversity across spatial scales, particularly in rangelands. Here, we ask: (1) do plant-plant interactions and environment affect PCS and phylogenetic diversity differently across the three spatial scales of the patch, the community, and the habitat? (2) What are the impacts of dominant cushion-nurse plants on the phylogenetic structure of plant communities? We assessed the PCS of semi-arid plant communities along an elevation gradient at the patch, community and habitat scales. Then, we assessed co-occurrence patterns along two sample slopes. Our results indicated important roles for biotic interactions and environmental filtering in determining phylogenetic diversity, with biotic interactions, in particular, having a stronger tendency to increase phylogenetic diversity. This is most likely due to the asymmetrical effects of nurse plants across the three spatial scales on our two different slopes. The impact of biotic interactions caused non-random phylogenetic patterns in more severe environments. In conclusion, biotic interactions influence phylogenetic diversity by altering PCS across aspects and along elevation gradients.

The interplay and balance between positive and negative interactions are important and recurrent topics in plant ecology. If facilitation occurs because benefactors ameliorate stress, such balance may be driven by seed mass because stress tolerance and competitive ability of plants, particularly seedlings, are positively correlated with seed mass. Thus, small-seeded, stress-intolerant species may require facilitation, but not large-seeded ones. This would equalise the magnitudes of opposite-signed interactions because in small-seeded species, positive effects of facilitators and negative effects of competitors should be strong, while both effects should be weak in large-seeded species. To test this idea, we assessed the effects of interactions with four associated species on different components of the performance of ten focal species. As expected, the largest facilitative and competitive effects were recorded in small-seeded species, and positive interactions had similar magnitudes to negative ones for any given seed mass, especially when performance was integrated into lifelong fitness. Furthermore, the fact that small-seeded species seem to be strongly facilitated may explain why they are not outcompeted by large-seeded species. This is an alternative to other hypotheses explaining the coexistence of plants with different-sized seeds. The close balance between opposite-signed interactions in the presence of stress may also explain why interactions have strong effects on individuals (that interact with only a few species), but seemingly weaker effects on populations, where interactions between many species would cancel out.

The spatial structure of plant communities in semi-arid regions is mostly derived by plant–plant interactions and environmental heterogeneity. In this study, we investigated the intra- and interspecific interactions and their contribution to growth inhibition in the patches of Pistacia trees and Amygdalus shrubs in semi-arid woodland communities through the implementation of photogrammetric data provided by unmanned aerial vehicle (UAV). This study was conducted in a part of Wild Pistachio Natural Reserve covered by Pistacia-Amygdalus stands in Zagros Mountains, western Iran. We used univariate and bivariate forms of pair- and mark correlation functions and Analytical Global Envelopes under inhomogeneous Poisson process which allow detection of the interactions of the species within the 45-ha study area. Our results indicated that the UAV-derived photogrammetric data proved to be efficient in identification of the plant individuals (F-score ≈ 0.92 for both species). Additionally, strong coefficients of determination (R2 = 0.98 and 0.94 for Pistacia and Amygdalus, respectively) supported prediction of crown area. We observed the aggregation of the species individuals in clusters of conspecifics and heterospecifics at small spatial scales, most likely as a result of aggregation in favourable parts of the study area. The aggregation of the species within patches had a marked effect on their size (i.e., crown area, height) inferred as growth inhibition, probably due to intra- and interspecific competition. Our findings demonstrated that promising UAV photogrammetric data can be effectively utilized by ecologists for investigation of plant associations, hence increasing the potentiality of remote sensing in spatial ecology of vegetation patches in semi-arid environments.

Drought-induced tree mortality has been recently increasing and is expected to increase further under warming climate. Conversely, tree species that survive under arid conditions might provide vital information on successful drought resistance strategies. Although Acacia (Vachellia) species dominate many of the globe's deserts, little is known about their growth dynamics and water-use in situ. Stem diameter dynamics, leaf phenology, and sap flow were monitored during 3 consecutive years in five Acacia raddiana trees and five Acacia tortilis trees in the Arid Arava Valley, southern Israel (annual precipitation 20-70 mm, restricted to October-May). We hypothesized that stem growth and other tree activities are synchronized with, and limited to single rainfall or flashflood events. Unexpectedly, cambial growth of both Acacia species was arrested during the wet season, and occurred during most of the dry season, coinciding with maximum daily temperatures as high as 45 °C and vapor pressure deficit of up to 9 kPa. Summer growth was correlated with peak sap flow in June, with almost year-round activity and foliage cover. To the best of our knowledge, these are the harshest drought conditions ever documented permitting cambial growth. These findings point to the possibility that summer cambial growth in Acacia under hyper-arid conditions relies on concurrent leaf gas exchange, which is in turn permitted by access to deep soil water. Soil water can support low-density tree populations despite heat and drought, as long as recharge is kept above a minimum threshold.

Germination, establishment, phenology, and death among three drought-deciduous shrubs were influenced by ENSO/PDO and precipitation, based on 37 years of annual surveys. Encelia farinosa forms near monospecific stands on slopes, whereas E. frutescens and Ambrosia salsola dominate wash habitats. All shrubs exhibited phenological coherence. While germination, establishment, and mortality patterns were similar among wash species, these dynamics contrasted with E. farinosa on slopes. Germination was associated with El Niño years. Slope plant establishment was dependent on precipitation in the subsequent year, but not evidently so in wash species. Major mortality events were episodic, with Encelia mortality just as likely to occur in years with below or above average precipitation. In both Encelia species, mortality was associated with transitions to a cold PDO phase. In E. frutescens this response was more rapid, whereas in E. farinosa mortality lagged 1 year, resulting in contrasting slope-wash mortality patterns. 50% of newly established shrubs died within 5, 5, and 18 years for E. frutescens, E. farinosa, and A. salsola, respectively. The 90% mortality ages were 26 years for E. frutescens, 24 years for E. farinosa, and 51 years for A. salsola. While maximum life expectancies are unknown, estimated maximum life expectancies were 56, 66, and 86 years for E. frutescens, E. farinosa, and A. salsola, respectively. Overall, as the climate has become more arid over the past four decades, the populations in both slope and wash habitats have exhibited similar responses: reduced shrub abundances and reduced total supportable leaf areas.

Hibernation is a period of water deficit for some small mammals, and humidity strongly influences hibernation patterns. Dry conditions reduce length of torpor bouts, stimulate arousals, and decrease overwinter survival. To mitigate these effects, many small mammals hibernate in near saturated (100% RH) conditions. However, big brown bats (Eptesicus fuscus) hibernate in a wider variety of conditions and tolerate lower humidity than most other bats. To assess arid tolerance in this species, we compared torpid metabolic rates (TMR) and rates of total evaporative water loss (TEWL) between two populations of E. fuscus with differing winter ecologies: one that hibernates in humid karst caves and one that hibernates in relatively dry rock crevices. We used flow-through respirometry to measure TMR and TEWL of bats in humid and dry conditions. Torpid metabolic rates did not differ between populations or with humidity treatments. Rates of TEWL were similar between populations in humid conditions, but higher for cave-hibernating bats than crevice-hibernating bats in dry conditions. Our results suggest that E. fuscus hibernating in arid environments have mechanisms to decrease evaporative water loss that are not evident at more humid sites. Drought tolerance may facilitate the sedentary nature of the species, allowing them to tolerate more variable microclimates during hibernation and thus increasing the availability of overwintering habitat. The ability to survive arid conditions may also lessen the susceptibility of E. fuscus to diseases that affect water balance.

Several previous studies have investigated the use of the stable hydrogen and oxygen isotope compositions in plant materials as indicators of palaeoclimate. However, accurate interpretation relies on a detailed understanding of both physiological and environmental drivers of the variations in isotopic enrichments that occur in leaf water and associated organic compounds. To progress this aim we measured δ18O and δ2H values in eucalypt leaf and stem water and δ18O values in leaf cellulose, along with the isotopic compositions of water vapour, across a north-eastern Australian aridity gradient. Here we compare observed leaf water enrichment, along with previously published enrichment data from a similar north Australian transect, to Craig–Gordon-modelled predictions of leaf water isotopic enrichment. Our investigation of model parameters shows that observed 18O enrichment across the aridity gradients is dominated by the relationship between atmospheric and internal leaf water vapour pressure while 2H enrichment is driven mainly by variation in the water vapour—source water isotopic disequilibrium. During exceptionally dry and hot conditions (RH 37 °C) we observed strong deviations from Craig–Gordon predicted isotope enrichments caused by partial stomatal closure. The atmospheric–leaf vapour pressure relationship is also a strong predictor of the observed leaf cellulose δ18O values across one aridity gradient. Our finding supports a wider applicability of leaf cellulose δ18O composition as a climate proxy for atmospheric humidity conditions during the leaf growing season than previously documented.

Grassland ecosystems are comprised of plants that occupy a wide array of phenological niches and vary considerably in their ability to resist the stress of seasonal soil-water deficits. Yet, the link between plant drought resistance and phenology remains unclear in perennial grassland ecosystems. To evaluate the role of soil water availability and plant drought tolerance in driving phenology, we measured leaf hydraulic conductance (Ksat), resistance to hydraulic failure (P50), leaf gas exchange, plant and soil water stable isotope ratios (δ18O), and several phenology metrics on ten perennial herbaceous species in mixed-grass prairie. The interaction between P50 and δ18O of xylem water explained 67% of differences in phenology, with lower P50 values associated with later season activity, but only among shallow-rooted species. In addition, stomatal control and high water-use efficiency also contributed to the late flowering and late senescence strategies of plants that had low P50 values and relied upon shallow soil water. Alternatively, plants with deeper roots did not possess drought-tolerant leaves, but had high hydraulic efficiency, contributing to their ability to efficiently move water longer distances while maintaining leaf water potential at relatively high values. The suites of traits that characterize these contrasting strategies provide a mechanistic link between phenology and plant-water relations; thus, these traits could help predict grassland community responses to changes in water availability, both temporally and vertically within the soil profile.

Semi-arid ecosystems are strongly water-limited and typically quite responsive to changes in precipitation amount and event size. In the C4-dominated shortgrass steppe of the Central US, previous experiments suggest that large rain events more effectively stimulate plant growth and aboveground net primary production (ANPP) than an equal amount of precipitation from smaller events. Responses to naturally occurring large events have generally been consistent with experimental results, with the exception of large events occurring later in the growing season (e.g., August). These have been reported as less effective at increasing net C uptake, despite temperatures optimal for C4 plant growth. Since atmospheric warming is increasing the frequency of statistically extreme rain events (deluges) throughout the growing season, how late-season deluges affect semi-arid ecosystems remains to be resolved. We applied deluges in August of three sizes (1.0-2.5 times average August precipitation) to assess the potential for late-season deluges to stimulate plant growth and ANPP. These late-season deluges led to significant "green-up" of this grassland, with new leaf production, and an increase in flowering of the dominant grass species. Further, these responses increased as deluge size increased, suggesting that larger or multiple deluges may lead to even greater growth responses. However, despite strong plant-level responses, no increase in ANPP was measured. Our results confirm that aboveground plant growth in the C4-dominated shortgrass steppe does respond to late-season deluges; however, if there is an increase in plant biomass, net accumulation aboveground is minimal at this time of year.

Extreme high temperatures are occurring more frequently with ongoing anthropogenic climate warming, but the experimental tests of the effects of high temperatures on terrestrial vertebrates in natural conditions are rare. In this study, we investigated the effects of extreme high temperatures on female reproduction and offspring traits of multi-ocellated racerunners (Eremias multiocellata) kept in field enclosures in the desert steppe of Inner Mongolia. Our studies indicate that high temperatures significantly affect the gestation period and reproductive output of females and the offspring sex ratio, but have little impact on offspring body size and mass. More interestingly, we found that the effect of extreme high temperatures on female reproductive output was not consistent between two consecutive years that differed in precipitation. Low precipitation may aggravate the impact of climate warming on lizards and negatively affect the survival of lizards in the desert steppe. Our results provide evidence that temperature interacts with precipitation to determine the life history of lizards, and they suggest that a drier and hotter environment, such as the future climate in arid mid-latitude areas, will likely impose severe pressure on lizard populations, which are an important component of the food web in desert areas around the world.

Variation in seed size and dormancy can take the form of seed heteromorphism, i.e., production of different kinds of seeds by a single individual. In this paper, I tested for the effect of seed position within a spikelet on its germination over time, and the contribution of this effect to population differentiation along an aridity gradient in an annual grass, Triticum dicoccoides. The results show that the upper grain in a spikelet is larger than the bottom grain, and either germinates in the season following dispersal, or dies. In contrast, a substantial fraction of the bottom grains do not germinate in the first season, but remain dormant in the soil seed bank for 1 and, very rarely, 2 years. This pattern was observed in seeds of all origins, but the bottom grains from the most arid location had the lowest, and from the least arid location, the highest germination fraction in the 1st year and vice versa in the 2nd year. This difference in germination fraction was observed under controlled irrigation conditions but not in the field experiment. These mixed results suggest that seed dimorphism is a life history trait with a complicated evolutionary history and wide adaptive implications. Seed dimorphism in T. dicoccoides could initially be an adaptation for reducing competition in productive (i.e., high precipitation) environments. In addition to this, seed dimorphism under increasing aridity could become a bet-hedging trait allowing a population to survive periods of insufficient rainfall through dormancy.

Drylands are predicted to become more arid and saline due to increasing global temperature and drought. Although species from the Caatinga, a Brazilian tropical dry forest, are tolerant to these conditions, the capacity for germination to withstand extreme soil temperature and water deficit associated with climate change remains to be quantified. We aimed to evaluate how germination will be affected under future climate change scenarios of limited water and increased temperature. Seeds of three species were germinated at different temperatures and osmotic potentials. Thermal time and hydrotime model parameters were established and thresholds for germination calculated. Germination performance in 2055 was predicted, by combining temperature and osmotic/salt stress thresholds, considering soil temperature and moisture following rainfall events. The most pessimistic climate scenario predicts an increase of 3.9 °C in soil temperature and 30% decrease in rainfall. Under this scenario, soil temperature is never lower than the minimum and seldomly higher than maximum temperature thresholds for germination. As long as the soil moisture (0.139 cm3 cm3) requirements are met, germination can be achieved in 1 day. According to the base water potential and soil characteristics, the minimum weekly rainfall for germination is estimated to be 17.5 mm. Currently, the required minimum rainfall occurs in 14 weeks of the year but will be reduced to 4 weeks by 2055. This may not be sufficient for seedling recruitment of some species in the natural environment. Thus, in future climate scenarios, rainfall rather than temperature will be extremely limiting for seed germination. Made available in DSpace on 2020-02-21T18:10:31Z (GMT). No. of bitstreams: 1 Rainfallnotsoiltemperaturewilllimittheseedgerminationofdry2020.pdf: 3202683 bytes, checksum: 1a6594dd57f074a3945a09b035657ac6 (MD5) Previous issue date: 2020

We investigated the possible use of dew as a water source for three desert plant species native to the Negev Desert: an annual Salsola inermis, and two perennials Artemisia sieberi and Haloxylon scoparium, with different rooting depths of 15, 30 and 90 cm, respectively. We quantified dew-water inputs and used stable isotope analyses to determine the proportion of dew as compared to the proportion of soil water each species utilized. Dew was isotopically enriched (δD values ranged from -25 to 5 ‰), relative to rainfall with δD values that ranged from -40 to -20 ‰ and relative to soil water with δD values that ranged from -65 to -35 ‰. Using a two-source isotope mixing model, we found that S. inermis, A. sieberi and H. scoparium used, on average, 56, 63 and 46 % of their water from dewfall, respectively. Our results suggest that dew-water utilization by Negev Desert plants is highly significant ecologically and thus may be more common than previously thought. In light of future predicted climate change, it may be increasingly important for plants of the Negev Desert to make use of dew as a water resource as it may play an important role in their ability to cope with the associated hydrological constraints predicted for the Negev region.

Dryland ecosystems represent >40 % of the terrestrial landscape and support over two billion people; consequently, it is vital to understand how drylands will respond to climatic change. However, while arid and semiarid ecosystems commonly experience extremely hot and dry conditions, our understanding of how further temperature increases or altered precipitation will affect dryland plant communities remains poor. To address this question, we assessed plant physiology and growth at a long-term (7-year) climate experiment on the Colorado Plateau, USA, where the community is a mix of shallow-rooted C3 and C4 grasses and deep-rooted C4 shrubs. The experiment maintained elevated-temperature treatments (+2 or +4 °C) in combination with altered summer monsoonal precipitation (+small frequent precipitation events or +large infrequent events). Increased temperature negatively affected photosynthesis and growth of the C3 and C4 grasses, but effects varied in their timing: +4 °C treatments negatively affected the C3 grass early in the growing season of both years, while the negative effects of temperature on the C4 grass were seen in the +2 and +4 °C treatments, but only during the late growing season of the drier year. Increased summer precipitation did not affect photosynthesis or biomass for any species, either in the year the precipitation was applied or the following year. Although previous research suggests dryland plants, and C4 grasses in particular, may respond positively to elevated temperature, our findings from a cool desert show marked declines in C3 and C4 photosynthesis and growth, with temperature effects dependent on the degree of warming and growing-season precipitation.

Although climate models forecast warmer temperatures with a high degree of certainty, precipitation is the primary driver of aboveground net primary production (ANPP) in most grasslands. Conversely, variations in temperature seldom are related to patterns of ANPP. Thus forecasting responses to warming is a challenge, and raises the question: how sensitive will grassland ANPP be to warming? We evaluated climate and multi-year ANPP data (67 years) from eight western US grasslands arrayed along mean annual temperature (MAT; ~7-14 °C) and mean annual precipitation (MAP; ~250-500 mm) gradients. We used regression and analysis of covariance to assess relationships between ANPP and temperature, as well as precipitation (annual and growing season) to evaluate temperature sensitivity of ANPP. We also related ANPP to the standardized precipitation evaporation index (SPEI), which combines precipitation and evapotranspiration to better represent moisture available for plant growth. Regression models indicated that variation in growing season temperature was negatively related to total and graminoid ANPP, but precipitation was a stronger predictor than temperature. Growing season temperature was also a significant parameter in more complex models, but again precipitation was consistently a stronger predictor of ANPP. Surprisingly, neither annual nor growing season SPEI were as strongly related to ANPP as precipitation. We conclude that forecasted warming likely will affect ANPP in these grasslands, but that predicting temperature effects from natural climatic gradients is difficult. This is because, unlike precipitation, warming effects can be positive or negative and moderated by shifts in the C3/C4 ratios of plant communities.

Climate change is expected to reduce annual precipitation by 20 % and increase its standard deviation by 20 % in the eastern Mediterranean. We have examined how these changes may affect herbaceous aboveground net primary production (ANPP) and its inter-annual coefficient of variation (CV) in natural rangelands along a desert—Mediterranean precipitation gradient, at five sites representing arid, semi-arid, and Mediterranean-type ecosystems, respectively, all showing positive linear relationships between herbaceous ANPP and annual precipitation. Scenarios of reduced annual precipitation and increased inter-annual precipitation variability were defined by manipulating mean annual precipitation (MAP) and its standard deviation. We simulated precipitation and calculated ANPP using current ANPP–precipitation relationships. Our model predicts that reduced precipitation will strongly reduce ANPP in arid and semi-arid sites. Moreover, the effect of reduced precipitation on the CV of ANPP along the entire gradient may be modified by changes in inter-annual variability in MAP. Reduced precipitation combined with increased precipitation variability was the scenario most relevant to the wet end of the gradient, due to the increased likelihood for both dry and rainy years. In contrast, the scenario most relevant to the arid end of the gradient combined reduced precipitation with decreased precipitation variability, due to the strong effect on mean ANPP. All scenarios increased variability of ANPP along the entire gradient. However, the higher sensitivity of vegetation at arid and semi-arid sites (i.e., lower forage production) to future changes in the precipitation regime emphasizes the need to adapt grazing management in these ecosystems to secure their long-term viability as sustainable rangelands.

Stomata simultaneously regulate plant carbon gain and water loss, and patterns of stomatal conductance (g s) provide insight into water use strategies. In arid systems, g s varies seasonally based on factors such as water availability and temperature. Moreover, the presence and species identity of neighboring plants likely affects g s of the focal plant by altering available soil water and microclimate conditions. We investigated stomatal behavior in Larrea tridentata, a drought-tolerant, evergreen shrub occurring throughout the arid southwestern United States. We measured g s in Larrea over multiple seasons in the presence of neighbors representing different woody species. The data were analyzed in the context of a commonly used phenomenological model that relates g s to vapor pressure deficit (D) to understand spatial and temporal differences in stomatal behavior. We found that g s in Larrea was affected by neighborhood association, and these effects varied seasonally. The greatest effect of neighborhood association on g s occurred during the winter period, where Larrea growing alone (without neighbors) had higher g s compared to Larrea growing with neighbors. Larrea’s stomatal sensitivity to D and reference conductance (i.e., g s at D = 1 kPa) also differed significantly among different neighbor associations. Random effects indicated reference g s varied over short time scales (daily), while stomatal sensitivity showed little daily or seasonal variation, but was notably affected by neighbor associations such that neighboring species, especially trees, reduced Larrea’s sensitivity to D. Overall, seasonal dynamics and neighborhood conditions appear critical to understanding temporal and spatial variation in Larrea’s physiological behavior.

Investment in immune defences is predicted to covary with a variety of ecologically and evolutionarily relevant axes, with pace of life and environmental antigen exposure being two examples. These axes may themselves covary directly or inversely, and such relationships can lead to conflicting predictions regarding immune investment. If pace of life shapes immune investment then, following life history theory, slow-living, arid zone and tropical species should invest more in immunity than fast-living temperate species. Alternatively, if antigen exposure drives immune investment, then species in antigen-rich tropical and temperate environments are predicted to exhibit higher immune indices than species from antigen-poor arid locations. To test these contrasting predictions we investigated how variation in pace of life and antigen exposure influence immune investment in related lark species (Alaudidae) with differing life histories and predicted risks of exposure to environmental microbes and parasites. We used clutch size and total number of eggs laid per year as indicators of pace of life, and aridity, and the climatic variables that influence aridity, as correlates of antigen abundance. We quantified immune investment by measuring four indices of innate immunity. Pace of life explained little of the variation in immune investment, and only one immune measure correlated significantly with pace of life, but not in the predicted direction. Conversely, aridity, our proxy for environmental antigen exposure, was predictive of immune investment, and larks in more mesic environments had higher immune indices than those living in arid, low-risk locations. Our study suggests that abiotic environmental variables with strong ties to environmental antigen exposure can be important correlates of immunological variation.

We examined the distribution of butterflies over the mostly arid and semi-arid continent of Australia and analyzed the proportion of migrant species and species diversity with respect to an array of climatic and geographic variables. On a continent-wide scale, latitude explained virtually no variance in either proportion of migrants (r2=0.01) or species diversity (r2=0.03) in Australian butterflies. These results are in marked contrast to those for temperate-zone birds from three continents where latitude explained between 82 and 98% of the variance in frequency of migrants and also accounted for much of the variance in bird species diversity. In eastern Australia where rainfall regimes are similar to those in temperate Europe and North and South America, latitude explains 78% of the variance in frequency of butterfly migrants. In both eastern and central Australia, latitude also accounts for relatively high proportions of the variance in species diversity. Rainfall patterns and especially soil moisture are negatively associated with migration frequency in Australian butterfly faunas, both alone and in combination with other climate variables. Where moisture levels are relatively high, as in eastern Australia, measures of temperature are associated with migration frequency, a result consistent with findings for temperate-zone birds, suggesting latitude is a surrogate for temperature. The ultimate causes of migration in temperate-zone birds and Australian butterflies are the uneven temporal, and in Australia also spatial, distribution of resources. Uneven distribution is brought about primarily by temperature in temperate regions and by erratic rainfall over much of arid Australia. As a key determinant of productivity, especially in the tropics and subtropics, aridity is likely to be an important determinant of the global distributions of migrants.

Leaf carbon isotope ratios (δ13C) and photosynthetic gas exchange were measured on plants growing in hanging garden communities in southern Utah, USA. Hanging gardens are unusual, mesic cliff communities occurring where water seeps from the sandstone bedrock in an otherwise extremely arid region; there is very limited overlap in species distributions inside and outside these gardens. Solar exposure in hanging gardens varied with orientation and one of the gardens (Ribbon Garden) was shaded throughout the day. The leaf δ13C values of plants in hanging gardens were significantly more negative than for plants from either nearby ephemeral wash or riparian communities. In Ribbon Garden, the observed δ13C values were as low as -34.8‰, placing them among the most negative values reported for any terrestrial plant species growing in a natural environment. Hanging garden plants were exposed to normal atmospheric CO2 with an average δ13C value of -7.9‰ and so the low leaf δ13C values could not be attributed to exposure to a CO2 source with low 13C content. There was a seasonal change toward more negative leaf δ13C values at the end of the growing season. The observed leaf δ13C values were consistent with photosynthetic gas exchange measurements that indicated unusually high leaf intercellular CO2 concentrations associated with the relatively low light levels in hanging gardens. Thus, extremely negative leaf δ13C values would be expected if significant amounts of the seasonal carbon gain occur at light levels low enough to be near the light compensation point. Maximum observed photosynthetic rates varied with light levels at each of the gardens, with maximum rates averaging 20.3, 14.6, and 3.1 μmol m-2 s-1 at Double Garden, Lost Garden, and Ribbon Garden, respectively. Leaf nitrogen contents averaged 18.5 mg g-1 in species from the more shaded hanging gardens (Lost and Ribbon). When expressed on a leaf area basis, nitrogen contents averaged 117 mmol N m-2 at Lost Garden and 65 mmol N m-2 at Ribbon Garden (shadiest of the two gardens). Leaf nitrogen isotope ratios averaged -2.3‰ (range of -0.7 to -6.1‰), suggesting that most of the nitrogen was derived from a biological fixation source which is most likely the Nostoc growing on the sandstone walls at the seep. These values contrast with leaf nitrogen isotope ratios of 5-9‰ which have been previously reported for arid zone plants in nearby ecosystems.

We used the doubly labeled water method to measure the field metabolic rates (FMRs, in kJ kg−1 day−1) and water flux rates (WIRs, in ml H2O kg−1 day−1) of adult desert tortoises (Gopherus agassizii) in three parts of the Mojave Desert in California over a 3.5-year period, in order to develop insights into the physiological responses of this threatened species to climate variation among sites and years. FMR, WIR, and the water economy index (WEI, in ml H2O kJ−1, an indicator of drinking of free water) differed extensively among seasons, among study sites, between sexes, and among years. In high-rainfall years, males had higher FMRs than females. Average daily rates of energy and water use by desert tortoises were extraordinarily variable: 28-fold differences in FMR and 237-fold differences in WIR were measured. Some of this variation was due to seasonal conditions, with rates being low during cold winter months and higher in the warm seasons. However, much of the variation was due to responses to year-to-year variation in rainfall. Annual spring peaks in FMR and WIR were higher in wet years than in drought years. Site differences in seasonal patterns were apparently due to geographic differences in rainfall patterns (more summer rain at eastern Mojave sites). In spring 1992, during an El Nino (ENSO) event, the WEI was greater than the maximal value obtainable from consuming succulent vegetation, indicating copious drinking of rainwater at that time. The physiological and behavioral flexibility of desert tortoises, evident in individuals living at all three study sites, appears central to their ability to survive droughts and benefit from periods of resource abundance. The strong effects of the El Nino (ENSO) weather pattern on tortoise physiology, reproduction, and survival elucidated in this and other studies suggest that local manifestations of global climate events could have a long-term influence on the tortoise populations in the Mojave Desert.

Reflective leaf pubescence of the desert shrub Encelia farinosa (brittlebrush) reduces leaf temperature and plant water loss, and is considered adaptive in xeric environments. Yet, little is known about intraspecific variation in this trait. Among three populations in the northern range of E. farinosa, which span a very broad precipitation gradient, both leaf absorptance variation and differences in the timing of drought-induced leaf loss were broadly associated with climatic variability. Where mean annual rainfall was greatest, drought-induced leaf loss was earliest, but these plants also had higher population-level mean leaf absorptance values. Higher absorptance increases the relative dependence on latent heat transfer (transpirational cooling), but it also provides greater instantaneous carbon assimilation. Plants at the driest site reached lower leaf absorptance values and maintained leaves longer into the drought period. Lower leaf absorptance reduces water consumption, and extended leaf longevity may buffer against the unpredictability of growing conditions experienced in the driest site. These observations are consistent with a trade-off scenario in which plants from wetter regions might trade off water conservation for higher instantaneous carbon gain, whereas plants from drier regions reduce water consumption and extend leaf longevity to maintain photosynthetic activity in the face of unpredictable growing conditions.

Spatial and temporal environmental variability can lead to variation in selection pressures across a landscape. Strategies for coping with environmental heterogeneity range from specialized phenotypic responses to a narrow range of conditions to generalist strategies that function under a range of conditions. Here, we ask how mean climate and climate variation at individual sites and across a species' range affect the specialist-generalist spectrum of germination strategies exhibited by 10 arid land forbs. We investigated these relationships using climate data for the western United States, occurrence records from herbaria, and germination trials with field-collected seeds, and predicted that generalist strategies would be most common in species that experience a high degree of climate variation or occur over a wide range of conditions. We used two metrics to describe variation in germination strategies: (a) selectivity (did seeds require specific cues to germinate?) and (b) population-level variation (did populations differ in their responses to germination cues?) in germination displayed by each species. Species exhibited distinct germination strategies, with some species demonstrating as much among-population variation as we observed among species. Modeling efforts suggested that generalist strategies evolve in response to higher spatial variation in actual evapotranspiration at a local scale and in available water in the spring and annual precipitation at a range-wide scale. Describing the conditions that lead to variation in early life-history traits is important for understanding the evolution of diversity in natural systems, as well as the possible responses of individual species to global climate change.

Plant species are shifting their ranges in response to global climate change, thus intensifying the need to predict such changes accurately. As the environmental requirements controlling plant distribution act differently at each developmental stage, there is a need to acquire a demographic-specific understanding of the factors which determine these distributions. Here we investigated the germination niche of two common savanna species Acacia nigrescens and Colophospermum mopane, with the aims to disentangle the direct and indirect effects of temperature on seed germination and establishment and to explore the impact of higher temperatures on the establishment success of savanna trees. Under laboratory conditions, we used thermal gradient plates to determine the thermal germination niche of both species, and a water stress experiment was conducted on C. mopane to account for water-temperature interactions. Using these data we parameterised a soil-moisture model to determine germination and establishment success under field conditions at current and future temperatures (+4 °C). Based on this model, higher future temperatures will not limit germination directly, but they will reduce the number of germination events by reducing the time window of suitable available soil water. Conversely, warmer conditions will accelerate the rate of radicle extension and increase the frequency of seedling establishment events. An additional advantage of higher temperatures is that fewer seeds will germinate, resulting in slower seed bank depletion when successful seedling establishment events do occur.

Climate change does affect not only average rainfall and temperature but also their variation, which can reduce the predictability of suitable conditions for growth and reproduction. This situation is problematic for inhabitants of temporary waters whose reproductive success depends on rainfall and evaporation that determine the length of the aquatic phase. For organisms with long-lived dormant life stages, bet hedging models suggest that a fraction of these should stay dormant during each growing season to buffer against the probability of total reproductive failure in variable environments. Thus far, however, little empirical evidence supports this prediction in aquatic organisms. We study geographic variation in delayed hatching of dormant eggs in natural populations of two crustaceans, Branchinella longirostris and Paralimnadia badia, that occur in temporary rock pools along a 725 km latitudinal aridity gradient in Western Australia. Consistent with bet hedging theory, populations of both species were characterised by delayed hatching under common garden conditions and hatching fractions decreased towards the drier end of the gradient where the probability of reproductive success was shown to be lower. This decrease was most pronounced in the species with the longer maturation time, presumably because it is more sensitive to the higher prevalence of short inundations. Overall, these findings illustrate that regional variation in climate can be reflected in differential investment in bet hedging and hints at a higher importance of delayed hatching to persist when the climate becomes harsher. Such strategies could become exceedingly relevant as determinants of vulnerability under climate change.

Making predictions as to how heatwaves will affect forests in the future is a major challenge in ecosystem science, not the least because we have few documented examples of how they respond now. We captured the effects of drought and a record-breaking heatwave on whole-tree water use (Q) in Eucalyptus regnans during the summer drought of 2008/2009 in southeastern Australia. While air temperatures steadily increased, average maximum sap flow (J Smax) declined with progression of the drought prior to the heatwave. In the period approaching the heatwave, Q during daytime (Q d) steadily declined, while nighttime Q (Q n) increased. This pattern was particularly pronounced during nights that followed hot days (>32 °C) where Q n was frequently 20–30 % of Q d. We found clear trends in the relation of Q d to Q n that point to the increasing importance of refilling depleted stem water stores following hot days. On the day the heatwave climaxed (7 February 2009), sap flow (J S) was dramatically low, and declined as weather conditions became increasingly arid (air temperature > 42 °C, vapor pressure deficit >7 kPa). Almost immediately after the heatwave passed J S resumed its common diurnal hysteresis, albeit at slightly slower rates. In the context of prognosticated effects of future climate, our data highlight that depletion and refill of stored water in E. regnans are likely important features for the tree to endure drought- and heat-related climatic extremes. We suggest that elucidating the peculiarity of capacitance and defining its threshold for keystone tree species, such as E. regnans, can add to our understanding of how climatic extremes may affect forests.

Understanding how animals select habitat is important for understanding how to better conserve those species. As droughts become more frequent and water availability declines in many systems, understanding selection of water sources becomes even more important for conservation. Tinajas and anthropogenic catchments are critical ephemeral breeding sites for Sonoran Desert anurans. Tadpoles have been documented in both water types even though anthropogenic catchments can contain very high concentrations of ammonia. We currently do not know how amphibians are selecting breeding habitat. We tested three hypotheses of habitat selection based on resource quality, resource quality and territoriality, and proximity of water site to other water sites. Male Anaxyrus punctatus called from all sites regardless of habitat quality or male quality; however, they were found more often at sites within 2 km of other sites. This suggests that male desert anurans are selecting close breeding habitat regardless of quality for breeding, indicating ammoniated sites are likely either population sinks or ecological traps. Consequently, adding anthropogenic water sites, without managing to reduce ammonia, will provide low quality habitat that could cause long-term declines in desert anuran populations.

Degradation of semiarid ecosystems from overgrazing threatens a variety of ecosystem services. Rainfall and nitrogen commonly co-limit production in semiarid grassland ecosystems; however, few studies have reported how interactive effects of precipitation and nitrogen addition influence the recovery of grasslands degraded by overgrazing. We conducted a 6-year experiment manipulating precipitation (natural precipitation and simulated wet year precipitation) and nitrogen (0, 25 and 50 kg N ha−1) addition at two sites with different histories of livestock grazing (moderately and heavily grazed) in Inner Mongolian steppe. Our results suggest that recovery of plant community composition and recovery of production can be decoupled. Perennial grasses provide long-term stability of high-quality forage production in this system. Supplemental water combined with exclosures led, in the heavily grazed site, to the strongest recovery of perennial grasses, although widespread irrigation of rangeland is not a feasible management strategy in many semiarid and arid regions. N fertilization combined with exclosures, but without water addition, increased dominance of unpalatable annual species, which in turn retarded growth of perennial species and increased inter-annual variation in primary production at both sites. Alleviation of grazing pressure alone allowed recovery of desired perennial species via successional processes in the heavily grazed site. Our experiments suggest that recovery of primary production and desirable community composition are not necessarily correlated. The use of N fertilization for the management of overgrazed grassland needs careful and systematic evaluation, as it has potential to impede, rather than aid, recovery.

Resources are seldom distributed equally across space, but many species exhibit spatially synchronous population dynamics. Such synchrony suggests the operation of large-scale external drivers, such as rainfall or wildfire, or the influence of oasis sites that provide water, shelter, or other resources. However, testing the generality of these factors is not easy, especially in variable environments. Using a long-term dataset (13-22 years) from a large (8000 km(2)) study region in arid Central Australia, we tested firstly for regional synchrony in annual rainfall and the dynamics of six reptile species across nine widely separated sites. For species that showed synchronous spatial dynamics, we then used multivariate follow a multivariate auto-regressive state-space (MARSS) models to predict that regional rainfall would be positively associated with their populations. For asynchronous species, we used MARSS models to explore four other possible population structures: (1) populations were asynchronous, (2) differed between oasis and non-oasis sites, (3) differed between burnt and unburnt sites, or (4) differed between three sub-regions with different rainfall gradients. Only one species showed evidence of spatial population synchrony and our results provide little evidence that rainfall synchronizes reptile populations. The oasis or the wildfire hypotheses were the best-fitting models for the other five species. Thus, our six study species appear generally to be structured in space into one or two populations across the study region. Our findings suggest that for arid-dwelling reptile populations, spatial and temporal dynamics are structured by abiotic events, but individual responses to covariates at smaller spatial scales are complex and poorly understood.

The effects of global change factors on the stoichiometric composition of green and senesced plant tissues are critical determinants of ecosystem feedbacks to anthropogenic-driven global change. So far, little is known about species stoichiometric responses to these changes. We conducted a manipulative field experiment with nitrogen (N; 17.5 g m(-2) year(-1)) and water addition (180 mm per growing season) in a temperate steppe of northern China that is potentially highly vulnerable to global change. A unique and important outcome of our study is that water availability modulated plant nutritional and stoichiometric responses to increased N availability. N addition significantly reduced C:N ratios and increased N:P ratios but only under ambient water conditions. Under increased water supply, N addition had no effect on C:N ratios in green and senesced leaves and N:P ratios in senesced leaves, and significantly decreased C:P ratios in both green and senesced leaves and N:P ratios in green leaves. Stoichiometric ratios varied greatly among species. Our results suggest that N and water addition and species identity can affect stoichiometric ratios of both green and senesced tissues through direct and interactive means. Our findings highlight the importance of water availability in modulating stoichiometric responses of plants to potentially increased N availability in semi-arid grasslands.

There are surprisingly few studies documenting effects of tropical cyclones (including hurricanes and typhoons) on rainforest animals, and especially insects, considering that many tropical forests are frequently affected by cyclonic disturbance. Consequently, we sampled a beetle assemblage inhabiting 18 upland rainforest sites in a fragmented landscape in north-eastern Queensland, Australia, using a standardised sampling protocol in 2002 and again 12 months after the passage of Severe Tropical Cyclone Larry (March 2006). The spatial configuration of sites allowed us to test if the effects of a cyclone and those from fragmentation interact. From all insect samples we extracted 12,568 beetles of 382 species from ten families. Beetle species composition was significantly different pre-and post-cyclone although the magnitude of faunal change was not large with 205 species, representing 96% of all individuals, present in both sampling events. Sites with the greatest changes to structure had the greatest changes in species composition. At the site level, increases in woody debris and wood-feeding beetle (Scolytinae) counts were significantly correlated but changes in the percent of ground vegetation were not mirrored by changes in the abundance of foliage-feeding beetles (Chrysomelidae). The overall direction of beetle assemblage change was consistent with increasing aridity, presumably caused by the loss of canopy cover. Sites with the greatest canopy loss had the strongest changes in the proportion of species previously identified in the pre-cyclone study as preferring arid or moist rainforest environments. The magnitude of fragmentation effects was virtually unaltered by the passage of Cyclone Larry. We postulate that in the short-term the effects of cyclonic disturbance and forest fragmentation both reduce the extent of moist, interior habitat.

The idea that many processes in arid and semi-arid ecosystems are dormant until activated by a pulse of rainfall, and then decay from a maximum rate as the soil dries, is widely used as a conceptual and mathematical model, but has rarely been evaluated with data. This paper examines soil water, evapotranspiration (ET), and net ecosystem CO2 exchange measured for 5 years at an eddy covariance tower sited in an Acacia–Combretum savanna near Skukuza in the Kruger National Park, South Africa. The analysis characterizes ecosystem flux responses to discrete rain events and evaluates the skill of increasingly complex “pulse models”. Rainfall pulses exert strong control over ecosystem-scale water and CO2 fluxes at this site, but the simplest pulse models do a poor job of characterizing the dynamics of the response. Successful models need to include the time lag between the wetting event and the process peak, which differ for evaporation, photosynthesis and respiration. Adding further complexity, the time lag depends on the prior duration and degree of water stress. ET response is well characterized by a linear function of potential ET and a logistic function of profile-total soil water content, with remaining seasonal variation correlating with vegetation phenological dynamics (leaf area). A 1- to 3-day lag to maximal ET following wetting is a source of hysteresis in the ET response to soil water. Respiration responds to wetting within days, while photosynthesis takes a week or longer to reach its peak if the rainfall was preceded by a long dry spell. Both processes exhibit nonlinear functional responses that vary seasonally. We conclude that a more mechanistic approach than simple pulse modeling is needed to represent daily ecosystem C processes in semiarid savannas. Electronic supplementary material The online version of this article (doi:10.1007/s00442-009-1405-y) contains supplementary material, which is available to authorized users.

The role of local adaptation and factors other than climate in determining extinction probabilities of species under climate change has not been yet explicitly studied. Here we performed a field experiment with annual plants growing along a steep climatic gradient in Israel to isolate climatic effects for local trait expression. The focus trait was seed dormancy, for which many theoretical predictions exist regarding climate-driven optimal germination behaviour. We evaluated how germination is consistent with theory, indicating local adaptation to current and changing climatic conditions, and how it varies among species and between natural and standardised soil conditions. We reciprocally sowed seeds from three or four origins for each of three annual species, Biscutella didyma, Bromus fasciculatus and Hymenocarpos circinnatus, in their home and neighbouring sowing locations along an aridity gradient. Our predictions were: lower germination fraction for seeds from more arid origins, and higher germination at wetter sowing locations for all seed origins. By sowing seeds in both local and standard soil, we separated climatic effects from local conditions. At the arid sowing location, two species supported the prediction of low germination of drier seed origins, but differences between seed origins at the other sites were not substantial. There were no clear rainfall effects on germination. Germination fractions were consistently lower on local soil than on standard soil, indicating the important role of soil type and neighbour conditions for trait expression. Local environmental conditions may override effects of climate and so should be carefully addressed in future studies testing for the potential of species to adapt or plastically respond to climate change.

The global water cycle is intimately linked to vegetation structure and function. Nowhere is this more apparent than in the arid west where riparian forests serve as ribbons of productivity in otherwise mostly unproduc- tive landscapes. Dioecy is common among tree species that make up western North American riparian forests. There are intrinsic physiological differences between male and female dioecious riparian trees that may influence popu- lation structure (i.e., the ratio of male to female trees) and impact ecohydrology at large scales. In this paper, we re- view the current literature on sex ratio patterns and phys- iology of dioecious riparian tree species. Then develop a conceptual framework of the mechanisms that underlie population structure of dominant riparian tree species. Finally, we identify linkages between population structure and ecohydrological processes such as evapotranspiration and streamflow. A more thorough understanding of the mechanisms that underlie population structure of dominant riparian tree species will enable us to better predict global change impacts on vegetation and water cycling at multiple scales.

In the nurse plant syndrome, or nurse association, seedlings (beneficiaries) are associated with adult shrubs/trees (benefactors). This phenomenon has been documented in several regions of the planet. Abiotic stress amelioration (one mechanism of facilitation) is one of the causes of this association. Most of the studies addressing the nurse syndrome have been conducted on spatial scales of a few hectares and have focused on only one or a few species. Moreover, there is an almost complete lack of studies addressing the incidence and characteristics of the nurse phenomenon in the arid Andes of South America. We undertook a first approximation to the study of facilitation in these ecosystems. The study was conducted at local and regional scales and involved the assessment of the spatial distribution of juveniles (seedlings and saplings) of 51 populations of 16 shrub and 12 cactus species in relation to shrub cover at 20 localities of the Prepuna (subtropical Andes of Bolivia and Argentina, 20-26 degrees S). In terms of spatial distribution, the juveniles of most of the populations of shrubs studied were distributed both under the shrubs and in open spaces, thereby showing an apparent indifference to microhabitat. Globose and opuntioid cacti were preferentially distributed below the canopies of shrubs and were usually more associated with the dominant shrub species, which stood out as better potential nurses. The pattern was consistent throughout the region, including the more mesic and arid localities. The fact that Prepuna woody species are capable of establishing in open spaces would confer this region a greater resilience. Our findings further suggest that community dynamics in arid and semi-arid environments are more variable than previously thought.

Soil nutrients in arid systems are supplied to plants in brief pulses following precipitation inputs. While these resource dynamics have been well documented, little is known about how this temporal heterogeneity influences competitive interactions. We examined the impacts of the temporal pattern of N supply on competitive intensity and ability in an N-limited desert shrub community. At our field site, the three codominant shrubs, Atriplex confertifolia, A. parryi, and Sarcobatus vermiculatus, differ in seasonal growth patterns, with A. confertifolia and S. vermiculatus achieving higher growth rates earlier in the growing season than A. parryi. We predicted that these timing differences in maximum growth rate may interact with temporal variation in N supply to alter competitive abilities over time. Seedlings of the two Atriplex species were planted either individually in field plots or as target plants surrounded by neighbor seedlings. After one year of establishment, the same amount of (15)N was applied to plots either as early spring pulses, mid spring pulses or continuously through the second growing season. Competitive effects were observed under continuous and pulsed N supply. Averaged across all target-neighbor treatments, competitive intensity was approximately 1.8-fold greater when N was pulsed compared to when N was supplied continuously, but overall, the outcome of competitive interactions was not influenced by N pulse timing. While the timing of resource supply did not differentially influence the competitive abilities of coexisting species in this system, the temporal pattern of resource supply did alter the intensity of competitive interactions among species. While additional studies in other systems are needed to evaluate the generality of these results, this study suggests that competitive intensity may not necessarily be a direct function of productivity or resource availability as traditionally assumed. Instead, the intensity of competitive interactions in resource-poor systems may depend upon the temporal pattern of resource supply.

Arid and semi-arid ecosystems of the southwestern US are undergoing changes in vegetation composition and are predicted to experience shifts in climate. To understand implications of these current and predicted changes, we conducted a precipitation manipulation experiment on the Santa Rita Experimental Range in southeastern Arizona. The objectives of our study were to determine how soil surface and seasonal timing of rainfall events mediate the dynamics of leaf-level photosynthesis and plant water status of a native and non-native grass species in response to precipitation pulse events. We followed a simulated precipitation event (pulse) that occurred prior to the onset of the North American monsoon (in June) and at the peak of the monsoon (in August) for 2002 and 2003. We measured responses of pre-dawn water potential, photosynthetic rate, and stomatal conductance of native (Heteropogon contortus) and non-native (Eragrostis lehmanniana) C(4) bunchgrasses on sandy and clay-rich soil surfaces. Soil surface did not always amplify differences in plant response to a pulse event. A June pulse event lead to an increase in plant water status and photosynthesis. Whereas the August pulse did not lead to an increase in plant water status and photosynthesis, due to favorable soil moisture conditions facilitating high plant performance during this period. E. lehmanniana did not demonstrate heightened photosynthetic performance over the native species in response to pulses across both soil surfaces. Overall accumulated leaf-level CO(2) response to a pulse event was dependent on antecedent soil moisture during the August pulse event, but not during the June pulse event. This work highlights the need to understand how desert species respond to pulse events across contrasting soil surfaces in water-limited systems that are predicted to experience changes in climate.

Ants are prominent seed dispersal agents in many ecosystems, and dispersal distances are small in comparison with vertebrate dispersal agents. However, the distance and distribution of ant-mediated dispersal in arid/semi-arid environments remains poorly explored. We used microsatellite markers and parentage assignment to quantify the distance and distribution of dispersed seeds of Acacia karina, retrieved from the middens of Iridomyrmex agilis and Melophorus turneri perthensis. From parentage assignment, we could not distinguish the maternal from each parent pair assigned to each seed, so we applied two approaches to estimate dispersal distances, one conservative (CONS), where the parent closest to the ant midden was considered to be maternal, and the second where both parents were deemed equally likely (EL) to be maternal, and used both distances. Parentage was assigned to 124 seeds from eight middens. Maximum seed dispersal distances detected were 417 m (CONS) and 423 m (EL), more than double the estimated global maximum. Mean seed dispersal distances of 40 m (±5.8 SE) (CONS) and 79 m (±6.4 SE) (EL) exceeded the published global average of 2.24 m (±7.19 SD) by at least one order of magnitude. For both approaches and both ant species, seed dispersal was predominantly (44–84 % of all seeds) within 50 m from the maternal source, with fewer dispersal events at longer distances. Ants in this semi-arid environment have demonstrated a greater capacity to disperse seeds than estimated elsewhere, which highlights their important role in this system, and suggests significant novel ecological and evolutionary consequences for myrmecochorous species in arid/semi-arid Australia.

Plant-plant interactions (competition and facilitation) in terrestrial ecosystems include: (1) short-term effects, primarily quantified with experimental removals; and (2) long-term effects, mostly due to soil weathering processes, primarily quantified with observational methods. It has been argued that these effects are likely to vary in contrasting directions with increasing drought stress in arid systems. However, few studies have used appropriate methodology to assess both types of effects and their variation across nurse species and environmental conditions, in particular in arid systems. This knowledge is crucial for predicting variation in the mediating role of facilitation with climate change and assessing the importance of nurse effects in ecological restoration. In the arid climate of central-south Tunisia, understory species' biomass, abundance and composition and soil parameters were compared in shrub-control, shrub-removed and open areas for three shrub species and in two habitats with contrasting soil moisture conditions. Long-term effects were dominant, positive for all three shrub species and associated with increasing nutrient content in shrub patches. Short-term effects, mainly related to water consumption, were weaker, mostly negative and dependent on shrub species. Additionally, long-term effects were less positive and short-term effects more negative in the dry habitat than in the wet habitat. Our study provides evidence of the primary influence of positive (facilitative) long-term effects in this arid system. However, the net effects of shrubs could be less beneficial for other species with increasing aridity under climate change, due to both a decrease in positive long-term effects and an increase in negative short-term effects.

In arid environments, episodically-pulsed resources are important components of annual water and nutrient supply for plants. This study set out to test whether seedlings have an increased capacity for using pulsed resources, which might then improve establishment when in competition with older individuals. A second aim was to determine whether there is a trade-off in competitive strategies when resources are supplied continuously at low concentrations, or as pulses with pronounced inter-pulse periods. A glasshouse experiment used a target-neighbour design of size-asymmetric competition, with juveniles of Panicum antidotale (blue panicgrass) introduced into contrasting densities of adult plants. Stable isotopes of nitrogen were used for measuring plant resource uptake from pulses, and tolerance to inter-pulse conditions was assessed as the mean residence time (MRT) of nitrogen. A higher root/shoot ratio and finer root system enhanced the capacity of juveniles to use resources when pulsed, rather than when continuously supplied. Higher resource uptake during pulses improved the establishment of juvenile Panicum in mixed cultures with older individuals. However, a trade-off was observed in plant strategies, with juveniles showing a lower MRT for nitrogen, which suggested reduced tolerance to resource deficit during inter-pulse periods. Under field conditions, higher utilization of pulsed resources would lead to the improved seedling establishment of Panicum adjacent to "nurse" plants, whereas mature plants with well-developed roots, exploiting a greater soil volume, maintain more constant resource uptake and retention during inter-pulse periods.

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.

The 'pulse-reserve' conceptual model--arguably one of the most-cited paradigms in aridland ecology--depicts a simple, direct relationship between rainfall, which triggers pulses of plant growth, and reserves of carbon and energy. While the heuristics of 'pulses', 'triggers' and 'reserves' are intuitive and thus appealing, the value of the paradigm is limited, both as a conceptual model of how pulsed water inputs are translated into primary production and as a framework for developing quantitative models. To overcome these limitations, we propose a revision of the pulse-reserve model that emphasizes the following: (1) what explicitly constitutes a biologically significant 'rainfall pulse', (2) how do rainfall pulses translate into usable 'soil moisture pulses', and (3) how are soil moisture pulses differentially utilized by various plant functional types (FTs) in terms of growth? We explore these questions using the patch arid lands simulation (PALS) model for sites in the Mojave, Sonoran, and Chihuahuan deserts of North America. Our analyses indicate that rainfall variability is best understood in terms of sequences of rainfall events that produce biologically-significant 'pulses' of soil moisture recharge, as opposed to individual rain events. In the desert regions investigated, biologically significant pulses of soil moisture occur in either winter (October-March) or summer (July-September), as determined by the period of activity of the plant FTs. Nevertheless, it is difficult to make generalizations regarding specific growth responses to moisture pulses, because of the strong effects of and interactions between precipitation, antecedent soil moisture, and plant FT responses, all of which vary among deserts and seasons. Our results further suggest that, in most soil types and in most seasons, there is little separation of soil water with depth. Thus, coexistence of plant FTs in a single patch as examined in this PALS study is likely to be fostered by factors that promote: (1) separation of water use over time (seasonal differences in growth), (2) relative differences in the utilization of water in the upper soil layers, or (3) separation in the responses of plant FTs as a function of preceding conditions, i.e., the physiological and morphological readiness of the plant for water-uptake and growth. Finally, the high seasonal and annual variability in soil water recharge and plant growth, which result from the complex interactions that occur as a result of rainfall variability, antecedent soil moisture conditions, nutrient availability, and plant FT composition and cover, call into question the use of simplified vegetation models in forecasting potential impacts of climate change in the arid zones in North America.