Altered Precipitation Research Articles

Eco-Physiological Responses of Dominant Species to Watering in a Natural Grassland Community on the Semi-Arid Loess Plateau of China.

Altered precipitation regimes significantly affect ecosystem structure and function in arid and semi-arid regions. In order to investigate effects of precipitation changes on natural grassland community in the semi-arid Loess Plateau, the current research examined eco-physiological characteristics of two co-dominant species (i.e., Bothriochloa ischaemum and Lespedeza davurica) and community composition following two watering instances (i.e., precipitation pulses, July and August, 2011, respectively) in a natural grassland community. Results showed that the photosynthetic rate, transpiration rate, stomatal conductance and intercellular CO2 concentration rapidly increased on the first to third day following watering in both species, and both months. Under watering treatments, the maximum net photosynthetic rates appeared on the second to third day after watering, which increased 30–80% in B. ischaemum and 40–50% in L. davurica compared with non-watering treatments, respectively. Leaf water use efficiency kept stable or initially decreased in both species under watering treatments. Watering in July produced more promoting effects on grass photosynthesis than in August, particularly in B. ischaemum. Community above-ground biomass at the end of the growing season increased after watering, although no significant changes in species diversity were observed. Our results indicated that timing and magnitude of watering could significantly affect plant eco-physiological processes, and there were species-specific responses in B. ischaemum and L. davurica. Pulsed watering increased community productivity, while did not significantly alter community composition after one growing season. The outcomes of this study highlight eco-physiological traits in dominant species may playing important roles in reshaping community composition under altered precipitation regimes.

Nitrogen trace gas fluxes from a semiarid subtropical savanna under woody legume encroachment

AbstractSavanna ecosystems are a major source of nitrogen (N) trace gases that influence air quality and climate. These systems are experiencing widespread encroachment by woody plants, frequently associated with large increases in soil N, with no consensus on implications for trace gas emissions. We investigated the impact of encroachment by N‐fixing tree Prosopis glandulosa on total reactive N gas flux (Nt = NO + N2O + NOy + NH3) from south Texas savanna soils over 2 years. Contrary to expectations, upland Prosopis groves did not have greater Nt fluxes than adjacent unencroached grasslands. However, abiotic conditions (temperature, rainfall, and topography) were strong drivers. Emissions from moist, low‐lying Prosopis playas were up to 3 times higher than from Prosopis uplands. Though NO dominated emissions, NH3 and NOy (non‐NO oxidized N) comprised 12–16% of the total summer N flux (up to 7.9 µg N m−2 h−1). Flux responses to soil wetting were temperature dependent for NO, NH3, and NOy: a 15 mm rainfall event increased flux 3‐fold to 22‐fold after 24 h in summer but had no effect in winter. Repeated soil wetting reduced N flux responses, indicating substrate depletion as a likely control. Rapid (<1 min) increases in NO emissions following wetting of dry soils suggested that abiotic chemodenitrification contributes to pulse emissions. We conclude that temperature and wetting dynamics, rather than encroachment, are primary drivers of N flux from these upland savannas, with implications for future emission patterns under altered precipitation regimes.

Responses of Spartina alterniflora to Multiple Stressors: Changing Precipitation Patterns, Accelerated Sea Level Rise, and Nutrient Enrichment

Coastal wetlands, well recognized for their ecosystem services, have faced many threats throughout the USA and elsewhere. While managers require good information on the net impact of these combined stressors on wetlands, little such information exists. We conducted a 4-month mesocosm study to analyze the multiple stressor effects of precipitation changes, sea level rise, and eutrophication on the salt marsh plant Spartina alterniflora. Pots containing plants in an organic soil matrix were positioned in tanks and received Narragansett Bay (RI, USA) water. The study simulated three precipitation levels (ambient daily rain, biweekly storm, and drought), three levels of tidal inundations (high (15 cm below mean high water (MHW)), mean (MHW), and low (15 cm above MHW)), and two nutrient enrichment levels (unenriched and nutrient-enriched bay water). Our results demonstrate that storm and drought stressors led to significantly less above- and belowground biomass than those in ambient rain conditions. Plants that were flooded at high inundation had less belowground biomass, fine roots, and shoots. Nutrients had no detectable effect on aboveground biomass, but the enriched pots had higher stem counts and more fine roots than unenriched pots, in addition to greater CO2 emission rates; however, the unenriched pots had significantly more coarse roots and rhizomes, which help to build peat in organogenic marshes. These results suggest that multiple stressors of altered precipitation, sea level rise, and nutrient enrichment would lead to reduced marsh sustainability.

Species identity of biocrust-forming lichens drives the response of soil nitrogen cycle to altered precipitation frequency and nitrogen amendment

Biological soil crusts (biocrusts) are fundamental components of drylands worldwide, and are of great importance for the regulation of ecosystem functioning. However, little is known on the role of species identify of biocrust-forming lichens in mediating the response of nitrogen (N) cycling to concurring global environmental change. Here, we conducted a microcosm study to evaluate how the species identity of biocrust-forming lichens (Diploschistes thunbergianus, Psora crystallifera and Xanthoparmelia reptans) regulate key processes of N cycling in response to simulated changes in rainfall frequency and N addition. We explicitly considered both direct and indirect effects (i.e. driven via microbial diversity and abundance) of global changes on N availability and losses using structural equation models. Our results showed that species of biocrust-forming lichens differentially mediated effects of N amendment and altered rainfall frequencies on belowground nitrate availability and N2O flux rate. For instance, soils under P. crystallifera species showed the highest increase in nitrate content in response to N amendment under low rainfall frequency. Moreover, soils under D. thunbergianus showed the highest N2O flux under high rainfall frequency without N addition. Interestingly, soils under X. reptans showed lowest and highest resistance in nitrate availability and N2O flux, respectively, in response to N addition regardless of different rainfall frequencies. Strikingly, we only found an indirect impact of either rainfall frequency or N amendment on the nitrate availability (but not N2O flux) driven via the ammonia-oxidizing community under X. reptans. Our results provide evidence that the species identity of biocrust-forming lichens modulates the response of N cycling to global change drivers. These findings have implications for predicting the potential consequence of altered rainfall patterns and environmental N inputs in dryland ecosystems.

Unchanged carbon balance driven by equivalent responses of production and respiration to climate change in a mixed-grass prairie.

Responses of grassland carbon (C) cycling to climate change and land use remain a major uncertainty in model prediction of future climate. To explore the impacts of global change on ecosystem C fluxes and the consequent changes in C storage, we have conducted a field experiment with warming (+3°C), altered precipitation (doubled and halved), and annual clipping at the end of growing seasons in a mixed-grass prairie in Oklahoma, USA, from 2009 to 2013. Results showed that although ecosystem respiration (ER) and gross primary production (GPP) negatively responded to warming, net ecosystem exchange of CO2 (NEE) did not significantly change under warming. Doubled precipitation stimulated and halved precipitation suppressed ER and GPP equivalently, with the net outcome being unchanged in NEE. These results indicate that warming and altered precipitation do not necessarily have profound impacts on ecosystem C storage. In addition, we found that clipping enhanced NEE due to a stronger positive response of GPP compared to ER, indicating that clipping could potentially be an effective land practice that could increase C storage. No significant interactions between warming, altered precipitation, and clipping were observed. Meanwhile, we found that belowground net primary production (BNPP) in general was sensitive to climate change and land use though no significant changes were found in NPP across treatments. Moreover, negative correlations of the ER/GPP ratio with soil temperature and moisture did not differ across treatments, highlighting the roles of abiotic factors in mediating ecosystem C fluxes in this grassland. Importantly, our results suggest that belowground C cycling (e.g., BNPP) could respond to climate change with no alterations in ecosystem C storage in the same period.

Precipitation regulates plant gas exchange and its long-term response to climate change in a temperate grassland

Aims Climate change largely impacts ecosystem carbon and water cycles by changing plant gas exchange, which may further cause positive or negative feedback to global climate change. However, long-term global change manipulative experiments are seldom conducted to reveal plant ecophysiological responses to climatic warming and altered precipitation regimes. Methods An 8-year field experiment with both warming and increased precipitation was conducted in a temperate grassland in northern China. We measured leaf gas exchange rates (including plant photosynthesis, transpiration and instantaneous water use efficiency [WUE]) of two dominant plant species (Stipa sareptana var. krylovii and Agropyron cristatum) from 2005 to 2012 (except 2006 and 2010) and those of other six species from 2011 to 2012. Important Findings Increased precipitation significantly stimulated plant photosynthetic rates (A) by 29.5% and 19.9% and transpiration rates (E) by 42.2% and 51.2% for both dominant species S. sareptana var. krylovii and A. cristatum, respectively, across the 8 years. Similarly, A and E of the six plant functional types were all stimulated by increased precipitation in 2011 and 2012. As the balance of A and E, the instantaneous WUEs of different plant species had species-specific responses to increased precipitation. In contrast, neither warming nor its interaction with increased precipitation significantly affected plant leaf gas exchange rates. Furthermore, A and E of the two dominant species and their response magnitudes to water treatments positively correlated with rainfall amount in July across years. We did not find any significant difference between the short-term versus long-term responses of plant photosynthesis, suggesting the flexibility of leaf gas exchange under climate change. The results suggest that changing precipitation rather than global warming plays a prominent role in determining production of this grassland in the context of climate change.

Altered rainfall patterns increase forb abundance and richness in native tallgrass prairie.

Models predict that precipitation variability will increase with climate change. We used a 15-year precipitation manipulation experiment to determine if altering the timing and amount of growing season rainfall will impact plant community structure in annually burned, native tallgrass prairie. The altered precipitation treatment maintained the same total growing season precipitation as the ambient precipitation treatment, but received a rainfall regime of fewer, larger rain events, and longer intervals between events each growing season. Although this change in precipitation regime significantly lowered mean soil water content, overall this plant community was remarkably resistant to altered precipitation with species composition relatively stable over time. However, we found significantly higher forb cover and richness and slightly lower grass cover on average with altered precipitation, but the forb responses were manifest only after a ten-year lag period. Thus, although community structure in this grassland is relatively resistant to this type of altered precipitation regime, forb abundance in native tallgrass prairie may increase in a future characterized by increased growing season precipitation variability.

Spatial and temporal trends in summertime climate and water quality indicators in the coastal embayments of Buzzards Bay, Massachusetts

Abstract. Degradation of coastal ecosystems by eutrophication is largely defined by nitrogen loading from land via surface water and groundwater flows. However, indicators of water quality are highly variable due to a myriad of other drivers, including temperature and precipitation. To evaluate these drivers, we examined spatial and temporal trends in a 22-year record of summer water quality data from 122 stations in 17 embayments within Buzzards Bay, MA (USA), collected through a citizen science monitoring program managed by Buzzards Bay Coalition. To identify spatial patterns across Buzzards Bay's embayments, we used a principle component and factor analysis and found that rotated factor loadings indicated little correlation between inorganic nutrients and organic matter or chlorophyll a (Chl a) concentration. Factor scores showed that embayment geomorphology in addition to nutrient loading was a strong driver of water quality, where embayments with surface water inputs showed larger biological impacts than embayments dominated by groundwater influx. A linear regression analysis of annual summertime water quality indicators over time revealed that from 1992 to 2013, most embayments (15 of 17) exhibited an increase in temperature (mean rate of 0.082 ± 0.025 (SD) °C yr−1) and Chl a (mean rate of 0.0171 ± 0.0088 log10 (Chl a; mg m−3) yr−1, equivalent to a 4.0 % increase per year). However, only seven embayments exhibited an increase in total nitrogen (TN) concentration (mean rate 0.32 ± 0.47 (SD) µM yr−1). Average summertime log10(TN) and log10(Chl a) were correlated with an indication that the yield of Chl a per unit total nitrogen increased with time suggesting the estuarine response to TN may have changed because of other stressors such as warming, altered precipitation patterns, or changing light levels. These findings affirm that nitrogen loading and physical aspects of embayments are essential in explaining the observed ecosystem response. However, climate-related stressors may also need to be considered by managers because increased temperature and precipitation may worsen water quality and partially offset benefits achieved by reducing nitrogen loading.

Climate change impacts in Central Asia and their implications for development

This paper synthesizes what is known about the physical and biophysical impacts of climate change and their consequences for societies and development under different levels of global warming in Central Asia. Projections show mean temperatures increasing by up to 6.5 °C compared to pre-industrial by the end of this century across the region. Associated physical impacts include altered precipitation regimes, more frequent heat extremes and increasing aridity. Increasing rates of glacial and snow melt could lead to greater river runoff, but also to greater seasonality of runoff in the short term and to decreasing water availability in the medium term to long term. These changes have negative implications for the water availability in the region and for conflicting water demands between agriculture and hydropower. Climate change could mostly decrease crop yields, challenging food security, but in more northern regions there could also be positive effects. Studies on climate change impacts on energy systems are scarce and yield conflicting results, but the more regional study shows decreasing prospects for hydropower. The health of the population is already sensitive to heat extremes and is projected to be exposed to more frequent and prolonged heat waves in the future, among other potential health impacts. While the evidence for a link between climate and migration is weak, the rural-to-urban migration can be especially expected to intensify. The paper concludes that Central Asia will be severely affected by climate change even if the global mean temperature increase is limited to 2 °C above pre-industrial levels, due to the potential for impacts to occur simultaneously and compound one another as well as interactions with wider development challenges, while risks will be strongly amplified if this threshold is crossed.

Effects of altered precipitation regimes on plant productivity in the arid region of northern China

Climate change scenarios have predicted significant alterations in precipitation patterns over most of the mid-latitude land areas by the end of this century, but the degree to which altered precipitation regimes influence terrestrial ecosystem function in arid regions is uncertain. Precipitation is a primary climatic factor that regulates ecosystem function in arid regions. Based on remote sensing and meteorological data from 2000 to 2013, we analysed the spatiotemporal variations in annual net primary productivity (NPP) for different land cover types in the arid region of northern China and quantified the effects of growing season precipitation (GSP) and seasonal distribution of precipitation (SDP) on NPP variability by using the ecological process Carnegie–Ames–Stanford Approach (CASA) model. Our results suggested that significant NPP increases were found in most of the vegetated areas, especially in grasslands and shrublands. Responses of NPP to precipitation variability were related to land cover types. Grassland and shrubland were most responsive to precipitation variability followed by croplands and forests. Increased precipitation increased NPP, and NPP responded more strongly to higher precipitation than lower precipitation. We also found that increased precipitation concentration decreased NPP. GSP and SDP accounted for approximately 67% and 21%, respectively, of the variability in NPP. We concluded that growing season precipitation and its seasonal distribution were dominant factors controlling inter-annual variability in NPP in the arid region of northern China. Other factors, such as plant functional trait, antecedent soil moisture and human activities, might mediate NPP responses to precipitation variability through interaction with water availability. Our studies have implications for assessing and predicting vegetation responses to future climate change.

Climate change impacts in Latin America and the Caribbean and their implications for development

This paper synthesizes what is known about the physical and biophysical impacts of climate change and their consequences for societies and development under different levels of global warming in Latin America and the Caribbean (LAC). Projections show increasing mean temperatures by up to 4.5 °C compared to pre-industrial by the end of this century across LAC. Associated physical impacts include altered precipitation regimes, a strong increase in heat extremes, higher risks of droughts and increasing aridity. Moreover, the mean intensity of tropical cyclones, as well as the frequency of the most intense storms, is projected to increase while sea levels are expected to rise by ~0.2–1.1 mm depending on warming level and region. Tropical glacier volume is found to decrease substantially, with almost complete deglaciation under high warming levels. The much larger glaciers in the southern Andes are less sensitive to warming and shrink on slower timescales. Runoff is projected to be reduced in Central America, the southern Amazon basin and southernmost South America, while river discharge may increase in the western Amazon basin and in the Andes in the wet season. However, in many regions, there is uncertainty in the direction of these changes as a result of uncertain precipitation projections and differences in hydrological models. Climate change will also reduce agricultural yields, livestock and fisheries, although there may be opportunities such as increasing rice yield in several LAC countries or higher fish catch potential in the southernmost South American waters. Species range shifts threaten terrestrial biodiversity, and there is a substantial risk of Amazon rainforest degradation with continuing warming. Coral reefs are at increasing risk of annual bleaching events from 2040 to 2050 onwards irrespective of the climate scenario. These physical and biophysical climate change impacts challenge human livelihoods through, e.g., decreasing income from fisheries, agriculture or tourism. Furthermore, there is evidence that human health, coastal infrastructures and energy systems are also negatively affected. This paper concludes that LAC will be severely affected by climate change, even under lower levels of warming, due to the potential for impacts to occur simultaneously and compound one another.

The evolution of γ-Mg17Al12 intermetallic compound during accumulative back extrusion and subsequent ageing treatment

Accumulative back extrusion (ABE) processing, as a novel severe plastic deformation (SPD) method, has been recently justified to be capable of modifying the microstructural characteristics of alloys. In line to its ongoing researches, the present work has been planned to study the evolution of γ-Mg17Al12 intermetallic phase during ABE and subsequent ageing treatment in a high Al-bearing Mg–Al–Zn alloy. The behaviour of γ intermetallic has been systematically examined as following points of view: (i) strain–temperature-dependent morphology changes, (ii) strain-induced dissolution, and (iii) re-ageing behaviour as a function of time and temperature. Aiming to analyse the morphology of eutectic γ compound with respect to the strain and temperature, 2D projections of effective diameter, shape factor and globularity have been made in strain/temperature graphs. The processing conditions (strain and temperature) corresponding to the desired and undesired morphologies are introduced and microstructurally explained through underlying plasticity mechanisms, i.e., ‘necking-thinning-particle separation’ and ‘brittle fragmentation.’ The former mechanism is suggested to be in relation with partial strain-induced dissolution of eutectic γ phase, leading to generation of a supersaturated solid solution. This has resulted to the observation of ‘off-stoichiometry’ phenomena in Mg17Al12 phase and has been justified through dislocation-assisted deformation mechanism at elevated temperature. Surprisingly, a unique re-ageing behaviour has been found for the obtained solid solutions, where a modified kinetics and morphology of γ phase precipitation were characterized. The altered precipitation behaviour is attributed to the specific defect structure achieved by SPD acting as fast diffusion channel for Al solutes.

Resource colimitation governs plant community responses to altered precipitation.

Ecological theory and evidence suggest that plant community biomass and composition may often be jointly controlled by climatic water availability and soil nutrient supply. To the extent that such colimitation operates, alterations in water availability caused by climatic change may have relatively little effect on plant communities on nutrient-poor soils. We tested this prediction with a 5-y rainfall and nutrient manipulation in a semiarid annual grassland system with highly heterogeneous soil nutrient supplies. On nutrient-poor soils, rainfall addition alone had little impact, but rainfall and nutrient addition synergized to cause large increases in biomass, declines in diversity, and near-complete species turnover. Plant species with resource-conservative functional traits (low specific leaf area, short stature) were replaced by species with resource-acquisitive functional traits (high specific leaf area, tall stature). On nutrient-rich soils, in contrast, rainfall addition alone caused substantial increases in biomass, whereas fertilization had little effect. Our results highlight that multiple resource limitation is a critical aspect when predicting the relative vulnerability of natural communities to climatically induced compositional change and diversity loss.

Land cover and precipitation controls over long‐term trends in carbon gains in the grassland biome of South America

Carbon gains are a key aspect of ecosystem functioning since they represent the energy available for upper trophic levels. Carbon gains (or primary production) are strongly correlated with other ecosystem attributes such as secondary production and they are also the support for the provision of many ecosystem services. Given the documented dependency of primary production on precipitation, we expect that altered precipitation regimes, such as those projected by climate models, will have a significant impact on carbon gains. Land use and land cover changes are also expected to have a significant impact on the dynamics of carbon gains. We generated a spectral database of the fraction of photosynthetically active radiation intercepted by vegetation (fPAR), in order to study long‐term trends (i.e., decades) in carbon gains and its spatial and temporal relationships with precipitation and land cover patterns in Uruguay, which is part of the Rio de la Plata Grasslands, one of the largest temperate grasslands biome of the world. We found that carbon gains of native forests and grassland afforestation exhibited the strongest positive spatial response to precipitation, whereas crops and rangelands the weakest. In addition, we found that the temporal response of carbon gains to precipitation was strong and positive for all land uses. Although there were not clear trends in precipitation, we found strong negative trends in carbon gains through time, particularly in rangelands of the “Northern Campos” of Uruguay, where these trends represent a decrease between 10% and 25% of the annual aboveground net primary production. On the other hand, positive trends in carbon gains through time were associated to grassland afforestation and native forests. Therefore, during the period analyzed, land cover had a stronger influence on the observed trends in carbon gains than precipitation. These patterns emerged as a consequence of the interaction among precipitation, temperature, edaphic factors and management. Present trends in the controlling factors of C gains would exacerbate the observed patterns with serious consequences for the provision of ecosystems services.

Responses of gross primary productivity to different sizes of precipitation events in a temperate grassland ecosystem in Inner Mongolia, China

Changes in the sizes of precipitation events in the context of global climate change may have profound impacts on ecosystem productivity in arid and semiarid grasslands. However, we still have little knowledge about to what extent grassland productivity will respond to an individual precipitation event. In this study, we quantified the duration, the maximum, and the time-integrated amount of the response of daily gross primary productivity (GPP) to an individual precipitation event and their variations with different sizes of precipitation events in a typical temperate steppe in Inner Mongolia, China. Results showed that the duration of GPP-response (τ R) and the maximum absolute GPP-response (GPPmax) increased linearly with the sizes of precipitation events (P es), driving a corresponding increase in time-integrated amount of the GPP-response (GPPtotal) because variations of GPPtotal were largely explained by τ R and GPPmax. The relative contributions of these two parameters to GPPtotal were strongly P es-dependent. The GPPmax contributed more to the variations of GPPtotal when P es was relatively small (<20 mm), whereas τ R was the main driver to the variations of GPPtotal when P es was relatively large. In addition, a threshold size of at least 5 mm of precipitation was required to induce a GPP-response for the temperate steppe in this study. Our work has important implications for the modeling community to obtain an advanced understanding of productivity-response of grassland ecosystems to altered precipitation regimes.

Influence of water Availability on Photosynthesis, Water Potential, Leaf δ13C, and Phenology in Dominant C4Grasses In Kansas, USA

Determining effects of drought on prairie grasses could help us understand shifts in plant distributions and abundances with altered precipitation. The mixed grass prairie of North America is the zone where tallgrass prairies of the east intermingle with shortgrass prairies of the west. Species transitions in the mixed grass prairie reflect precipitation gradients, but little is known regarding how dominant grasses respond physiologically to water availability. We measured the performance of Andropogon gerardii, Bouteloua curtipendula, Schizachyrium scoparium, and Sorghastrum nutans in three precipitation treatments (ambient rainfall, drought, irrigated) in a southern mixed grass prairie near Hays, Kansas, USA. Photosynthesis and water potential were measured monthly in June and July of 2010 and 2011. Phenology and height measurements were collected semimonthly in June, July, and August of 2011. Growth increased under irrigated treatments in all four species. Conversely, leaf δ13C became lower in drought ...

Anecic earthworms (Lumbricus terrestris) alleviate negative effects of extreme rainfall events on soil and plants in field mesocosms

Intense rains are becoming more frequent. By causing waterlogging, they may increase soil erosion and soil surface compaction, hamper seedling establishment, and reduce plant growth. Since anecic earthworms make vertical burrows that improve water infiltration, we hypothesised that they can counteract such disturbance. In a field experiment, intact soil mesocosms with ryegrass (Lolium multiflorum), with or without introduced adult Lumbricus terrestris, underwent either a precipitation regime with two intense rain events (36 mm, at beginning and end of spring), or a control regime with the same cumulative rainfall but no intense events. Short-term response of soil moisture and lagged response of plant growth were measured, and soil macroporosity was quantified. Intense rains reduced ryegrass shoot biomass (by 16–21 % on average) only in the absence of earthworms. Waterlogging duration aboveground was not affected, whereas soil moisture contents after intense rainfall tended to drop faster with earthworms present. Continuous vertical macropores were found only in the mesocosms to which earthworms had been added. The number of such macropores was 2.4 times higher under the intense precipitation regime, despite similar earthworm survival. We found that anecic earthworms can offset negative effects of intense rainfall on plant growth aboveground. Underlying mechanisms, such as macropore formation and enhanced nutrient cycling, are discussed. We also observed that altered precipitation patterns can modify earthworm burrowing behaviour, as earthworms had produced more burrows under the intense regime.

Transpiration dynamics support resource partitioning in African savanna trees and grasses

It is still far from clear whether and to what extent trees and grasses partition soil moisture in tropical savannas. A major reason for this is that we don't know how snapshot data on rooting differences translate into ecologically relevant patterns of water use at seasonal scales. We used stable isotopes in soil and stem water to quantify functional rooting profiles in grasses and two tree species in a South African savanna. Concurrently, we measured tree sap‐flow velocity, grass canopy temperature (a transpiration correlate), and soil moisture content at multiple depths over the course of a growing season. We used lasso regression to identify the dominant soil moisture layers driving daily variation in tree and grass water‐use metrics while controlling for weather variables. We found clear rooting depth differences between grasses (shallow) and trees (deep) from the isotopic data, and these patterns were strongly supported by the water‐use data, which showed that grasses and trees predominantly responded to soil moisture availability at 5 and 40 cm depth, respectively. Our results provide a rare example of mechanistic support for the resource partitioning hypothesis in savannas, with important implications for our understanding of tree–grass dynamics under altered precipitation regimes.

Effects of water and nitrogen addition on ecosystem carbon exchange in a meadow steppe.

A changing precipitation regime and increasing nitrogen deposition are likely to have profound impacts on arid and semiarid ecosystem C cycling, which is often constrained by the timing and availability of water and nitrogen. However, little is known about the effects of altered precipitation and nitrogen addition on grassland ecosystem C exchange. We conducted a 3-year field experiment to assess the responses of vegetation composition, ecosystem productivity, and ecosystem C exchange to manipulative water and nitrogen addition in a meadow steppe. Nitrogen addition significantly stimulated aboveground biomass and net ecosystem CO2 exchange (NEE), which suggests that nitrogen availability is a primary limiting factor for ecosystem C cycling in the meadow steppe. Water addition had no significant impacts on either ecosystem C exchange or plant biomass, but ecosystem C fluxes showed a strong correlation with early growing season precipitation, rather than whole growing season precipitation, across the 3 experimental years. After we incorporated water addition into the calculation of precipitation regimes, we found that monthly average ecosystem C fluxes correlated more strongly with precipitation frequency than with precipitation amount. These results highlight the importance of precipitation distribution in regulating ecosystem C cycling. Overall, ecosystem C fluxes in the studied ecosystem are highly sensitive to nitrogen deposition, but less sensitive to increased precipitation.

Responses of a desert nematode community to changes in water availability

The most recent climate models unequivocally predict a strong drying trend for the southwestern USA within the next century. Soil nematodes are a highly important component of desert ecosystem functioning, but rely on water films for movement. However, it is currently poorly understood how different trophic groups of nematodes respond to chronic presses as well as short‐lived pulses of altered water availability, especially in arid systems where such changes are expected to have the greatest impact. The aim of this study was to assess the effects of both instantaneous and long‐term variation in water availability on desert soil nematode trophic groups. We hypothesized that nematode abundance would respond positively to both short‐ and long‐term increases in moisture. Based on the ecology of the different trophic groups we further made predictions about their relative rates of response. We increased or decreased precipitation from ambient levels in the Chihuahuan Desert for four consecutive years and sampled soil nematodes after two, three and four years. We tested the effects of altered precipitation treatments through time as well as gravimetric soil moisture at the time of sampling on the abundance of the different nematode trophic groups. In contrast to our hypotheses, the abundances of most nematode trophic groups were unaffected by the amount of precipitation, even after four years of altered precipitation. Plant‐parasitic nematodes from low moisture soils were the only group that reacted positively to increased precipitation from the third year onwards. Trophic groups responded differently to soil moisture, with bacterivores decreasing with increasing moisture and omnivores showing a positive relationship that diminished over time. We show that in general, these desert nematodes were not limited by precipitation, and were highly resilient to decreases therein. However, when also considering the effects of soil moisture, some more complex patterns and differences among trophic groups emerged. We discuss potential mechanisms explaining these observations and contrast our findings with those from other ecosystems around the world. We conclude that deserts harbor nematode communities that seem more resilient to altered water availability than other ecosystems.