Rainfall Manipulation Experiment Research Articles

Responses of Soil Water Potential and Plant Physiological Status to Pulsed Rainfall Events in Arid Northwestern China: Implications for Disclosing the Water‐Use Strategies of Desert Plants

ABSTRACTSoil water potential (SWP) strongly influences plant productivity and ecosystem functioning, particularly in arid regions characterized by sporadic and pulsed rainfall. This work aims to improve understanding of the response of SWP to varied rainfall pulses, and of the water‐use strategies of a widespread C4 shrub (Haloxylon ammodendron, HA) in arid northwestern China. Rainfall manipulation experiments and field measurements on HA were undertaken to explore the response features of SWP and plant physiological status to pulsed rainfall events of varied magnitudes and durations. The physiological state of HA was evaluated by quantifying critical metrics indicative of plant water‐use strategies, including leaf water potential, photosynthetic rate and transpiration rate. The response value of SWP increased with rainfall magnitude and was most affected by three vital factors (antecedent SWP, total rainfall and rainfall intensity). Low antecedent SWP amplifies SWP's sensitivity to subsequent events, accelerating its response to smaller rainfalls (<5 mm) compared with larger ones (>15 mm). Small rainfall can increase SWP by 0.5–2 MPa in the 20‐cm layer, sustaining plant physiological activities under high antecedent SWP conditions (>−3.5 MPa), with a maximum average rise in photosynthetic rate of 9.20 ± 0.45 μmol CO2 m−2 s−1, enhancing HA's water use efficiency to 1.79 ± 0.22 μmol CO2 mmol−1 H2O. Therefore, small events play a vital role in maintaining SWP and promoting water use of desert plants. Given the nature of plants' utilization of small rainfall events, re‐examining ecologically valid SWP thresholds of HA and other similar desert plants is critical.

Increased rainfall frequency enhances dry seasonal soil mineral-associated organic carbon in a subtropical forest

Climate models predict longer and more severe droughts, and alterations in the frequency and intensity of rainfall events. However, how changing precipitation patterns affect soil organic carbon (SOC), particulate organic carbon (POC), and mineral-associated organic carbon (MAOC) remains unclear. Here, we conducted a three-year rainfall manipulation experiment with ambient rainfall as the control, removal of half the total rainfall amount with unaltered frequency (DRA), and increased rainfall frequency with the total amounts unchanged (IRF) in a subtropical forest. The results showed that the rainfall treatments did not change SOC content or fractions during the wet season. In the dry season, DRA significantly increased topsoil POC but did not change SOC or MAOC. IRF significantly increased POC and MAOC levels. The increased MAOC was associated with the newly formed binary complexes of Fe3+, Al3+, and Ca2+, and the adsorption/precipitation of reduced short-range-ordered Fe oxides, likely derived from the enhanced reductive dissolution in the IRF treatment. Our results suggest that changes in rainfall frequency affect organo-mineral interactions more profoundly relative to the rainfall amount and that these effects are season-dependent. Therefore, moisture-sensitive geochemical processes play an essential role in SOC stabilization in subtropical forests.

Long-term drought promotes invasive species by reducing wildfire severity.

Anthropogenic climate change has increased the frequency of drought, wildfire, and invasions of non-native species. Although high-severity fires linked to drought can inhibit recovery of native vegetation in forested ecosystems, it remains unclear how drought impacts the recovery of other plant communities following wildfire. We leveraged an existing rainfall manipulation experiment to test the hypothesis that reduced precipitation, fuel load, and fire severity convert plant community composition from native shrubs to invasive grasses in a Southern California coastal sage scrub system. We measured community composition before and after the 2020 Silverado wildfire in plots with three rainfall treatments. Drought reduced fuel load and vegetation cover, which reduced fire severity. Native shrubs had greater prefire cover in added water plots compared to reduced water plots. Native cover was lower and invasive cover was higher in postfire reduced water plots compared to postfire added and ambient water plots. Our results demonstrate the importance of fuel load on fire severity and plant community composition on an ecosystem scale. Management should focus on reducing fire frequency and removing invasive species to maintain the resilience of coastal sage scrub communities facing drought. In these communities, controlled burns are not recommended as they promote invasive plants.

The need to decipher plant drought stress along the soil–plant–atmosphere continuum

Lacking comparability among rainfall manipulation studies is still a major limiting factor for generalizations in ecological climate change impact research. A common framework for studying ecological drought effects is urgently needed to foster advances in ecological understanding the effects of drought. In this study, we argue, that the soil–plant–atmosphere‐continuum (SPAC), describing the flow of water from the soil through the plant to the atmosphere, can serve as a holistic concept of drought in rainfall manipulation experiments which allows for the reconciliation experimental drought ecology. Using experimental data, we show that investigations of leaf water potential in combination with edaphic and atmospheric drought – as the three main components of the SPAC – are key to understand the effect of drought on plants. Based on a systematic literature survey, we show that especially plant and atmospheric based drought quantifications are strongly underrepresented and integrative assessments of all three components are almost absent in current experimental literature. Based on our observations we argue, that studying dynamics of plant water status in the framework of the SPAC can foster comparability of different studies conducted in different ecosystems and with different plant species and can facilitate extrapolation to other systems, species or future climates.

Apparent fractionation of hydrogen isotope from precipitation to leaf wax n-alkanes from natural environments and manipulation experiments

Knowledge of hydrogen isotopic fractionation (ε) of plant leaf waxes is the foundation for applying hydrogen isotope values (δ2H) in environmental reconstructions. In this work, we systematically investigated plant ε values (εalk/precipitation, εalk/soil water, εalk/leaf water and εalk/lake water, representing the isotopic fractionation between plant n-alkane δ2H and precipitation δ2H, soil water δ2H, leaf water δ2H and lake water δ2H) from the natural environments and manipulation experiments. The results show that the εalk/precipitation values of terrestrial plants have large variations (from −190 ‰ to −20 ‰) and become more negative with increasing aridity index. This phenomenon is possibly caused by the δ2H changes in source water (from precipitation to soil water and then to leaf water) during plant leaf wax synthesis under various evapotranspiration conditions in different climatic zones. The rainfall manipulation experiments show that leaf water δ2H values are generally higher than soil water δ2H values, and the latter are higher than precipitation δ2H values. This finding further demonstrates that the evapotranspiration effect on source water δ2H affects the quantification of the leaf wax apparent ε values (εalk/leaf water < εalk/soil water < εalk/precipitation). The εalk/lake water values of submerged plants display a smaller range (−153 ± 5 ‰) than the εalk/precipitation values of terrestrial plants, which is close to the terrestrial εalk/precipitation values in humid areas. Therefore, the biosynthetic ε value of terrestrial plant leaf waxes is relatively constant (ca. –153 ± 5 ‰), and the observed variable apparent εalk/precipitation values are possibly caused by the varied degree of evapotranspiration effect on the water that plants used in different climatic conditions. This effect should be considered when applying δ2H values of leaf waxes to trace environmental changes.

Responses of a semiarid grassland to recurrent drought are linked to community functional composition.

Recurrent droughts are an inevitable consequence of climate change, yet how grasslands respond to such events is unclear. We conducted a 6-year rainfall manipulation experiment in a semiarid grassland that consisted of an initial 2-year drought (2015-2016), followed by a recovery period (2017-2018) and, finally, a second 2-year drought (2019-2020). In each year, we estimated aboveground net primary productivity (ANPP), species richness, community-weighted mean (CWM) plant traits, and several indices of functional diversity. The initial drought led to reduced ANPP, which was primarily driven by limited growth of forbs in the first year and grasses in the second year. Total ANPP completely recovered as the rapid recovery of grass productivity compensated for the slow recovery of forb productivity. The subsequent drought led to a greater reduction in total ANPP than the initial drought due to the greater decline of both grass and forb productivity. The structural equation models revealed that soil moisture influenced ANPP responses directly during the initial drought, and indirectly during the subsequent drought by lowering functional diversity, which resulted in reduced total ANPP. Additionally, ANPP was positively influenced by CWM plant height and leaf nitrogen during the recovery period and recurrent drought, respectively. Overall, the greater impact of the second drought on ecosystem function than the initial drought, as well as the underlying differential mechanism, underscores the need for an understanding of how increased drought frequency may alter semiarid grassland functioning.

Long-term vegetation response to rainfall seasonality and fire in the heathlands and shrublands of the Cape Floristic Region (SW South Africa)

Mediterranean environments are biodiversity hotspots in which strongly seasonal winter rainfall regimes and fire play major roles in driving ecosystem dynamics. Global predictions forecast unreliability of winter rainfall and increases in summer rainfall that are expected to result in major changes in community structure. Mediterranean systems are difficult to model, and although ecophysiological responses can be studied at observational timescales, a long-term understanding is necessary to address uncertainties and refine predictive models at landscape scales. Here we provide a ∼ 1100 year-long palaeoecological reconstruction of vegetation (palynology), fire (sedimentary charcoal) and sedimentological change at a site adjacent to a multi-annual rainfall manipulation experiment designed to test plant population and community responses to altered seasonal regimes in the Greater Cape Floristic Region hotspot (southwestern Africa). We use these data to test whether long-term vegetation dynamics are controlled by changes in rainfall seasonality. We conclude that vegetation dynamics correlate with centennial-scale seasonality fluctuations, with transitions between two ecologically distinct fine-leaved shrub communities. These transitions are consistent with results of responses to experimental manipulations of summer rainfall. This study demonstrates the value of ecophysiological research in interpreting palaeoecological reconstructions and scaling up the results of observational research to answer long-term questions about environmental change.

Plant and herbivorous insect communities respond in complex ways to rainfall manipulation in an oak savanna grassland

Abstract Changes in precipitation due to climate change will have consequences for plant and herbivorous insect communities alike. Multiple hypotheses explain how changes in plant diversity and productivity can lead to changes in herbivore community composition. Yet as rainfall patterns change, the bottom‐up effects on the relationships between plant and herbivore communities are less well understood. Using a long‐term rainfall manipulation experiment in a remnant patch of Garry oak (Quercus garryana) savanna, we examined how plant diversity and productivity have responded to variation in soil moisture over 6 years. This highly endangered ecosystem is predicted to experience significantly wetter springs and drier summers by 2080. We also investigated plant‐mediated, indirect effects of manipulated rainfall on herbivore diversity and abundance, drawing on multiple hypotheses describing the relationships between plant and herbivore communities. For example, the more individuals hypothesis predicts that increased plant productivity results in increased herbivore abundance which, in turn, results in increased herbivore diversity. We found that plant productivity was influenced by soil moisture, but the direction and magnitude of the response varied across years, and found no support for plant diversity influencing productivity. We also found that the cover and productivity of grasses increased significantly with increasing precipitation. In addition to a direct negative effect on herbivore diversity, soil moisture had an indirect negative effect on herbivore abundance, via the negative effect of plant productivity on abundance, contradicting the more individuals hypothesis. Synthesis. Our results highlight that not only can drought result in significant reductions in plant productivity in this threatened ecosystem, but that these changes will also result in increases in herbivore abundance. In contrast, where soil moisture is higher, grasses will become more dominant resulting in decreased herbivore abundance. Ultimately, predicting how this system responds to changes in precipitation will depend on the ability to predict whether growing season soil moisture will be consistently drier or wetter in the future, a significant challenge. Going forward, investigating how variation in precipitation due to climate change affects the links between trophic levels, including how herbivores affect plant communities, remains critical for understanding ecosystem processes and stability.

Effects of altered precipitation patterns on soil nitrogen transformation in different landscape types during the growing season in northern China

The soil nitrogen (N) cycle and transformation rate are susceptible to precipitation regime changes in semiarid sandy ecosystems, and they play important roles in both the soil fertility and in plant interactions. However, the response mechanisms of soil N transformation to changing precipitation regimes in semiarid ecosystems are still unclear. Thus, we conducted a three-year rainfall manipulation experiment along a landscape evolution gradient from fixed dunes to dune grasslands in a semiarid sandy region of northern China during the growing season (from May to September) between 2017 and 2019. The study included a contrast (P0: natural precipitation) and four treatments in which precipitation was decreased and increased by 60 % [P(−S) and P(+S)] between May and September and decreased and increased by 100 % [(P(−J) and P(+J)] between May and June. The soil nitrate nitrogen (NO3−-N) and inorganic N concentrations significantly increased under P(−S) and P(−J) (P < 0.05) in both fixed dunes and dune grasslands during the growing season. P(−S) and P(−J) significantly increased the soil net nitrification rate (Rn) and net N mineralization rate (Rm) only in July, with Rm significantly increasing with sampling months and years (P < 0.001). The structural equation model showed that Rn and Rm did not change with water availability variation, and an increase in Rm caused by the direct effects of increased precipitation was offset by its indirect effects mediated by reducing electrical conductivity and soil NO3−-N concentration while increasing soil water content and vegetation cover. Soil properties and vegetation characteristics were the main factors affecting soil N transformation during the growing season. Our findings are vital for understanding the seasonal patterns, cumulative N transformation, and possible climate change feedback of semiarid sandy land.

Spatial turnover of soil viral populations and genotypes overlain by cohesive responses to moisture in grasslands.

Viruses shape microbial communities, food web dynamics, and carbon and nutrient cycling in diverse ecosystems. However, little is known about the patterns and drivers of viral community composition, particularly in soil, precluding a predictive understanding of viral impacts on terrestrial habitats. To investigate soil viral community assembly processes, here we analyzed 43 soil viromes from a rainfall manipulation experiment in a Mediterranean grassland in California. We identified 5,315 viral populations (viral operational taxonomic units [vOTUs] with a representative sequence ≥10 kbp) and found that viral community composition exhibited a highly significant distance-decay relationship within the 200-m2 field site. This pattern was recapitulated by the intrapopulation microheterogeneity trends of prevalent vOTUs (detected in ≥90% of the viromes), which tended to exhibit negative correlations between spatial distance and the genomic similarity of their predominant allelic variants. Although significant spatial structuring was also observed in the bacterial and archaeal communities, the signal was dampened relative to the viromes, suggesting differences in local assembly drivers for viruses and prokaryotes and/or differences in the temporal scales captured by viromes and total DNA. Despite the overwhelming spatial signal, evidence for environmental filtering was revealed in a protein-sharing network analysis, wherein a group of related vOTUs predicted to infect actinobacteria was shown to be significantly enriched in low-moisture samples distributed throughout the field. Overall, our results indicate a highly diverse, dynamic, active, and spatially structured soil virosphere capable of rapid responses to changing environmental conditions.

Response of soil carbon dioxide efflux to temporal repackaging of rainfall into fewer, larger events in a semiarid grassland

Changing rainfall patterns will alter soil water availability to plants and microbes and likely impact soil CO2 efflux (Fs) in semiarid ecosystems. However, our understanding of the response of Fs to compound changes in rainfall event size and frequency remains relatively limited. To address this knowledge gap, we examined how compound changes in rainfall size and frequency impact Fs in a semiarid grassland by deploying automated soil chambers at a rainfall manipulation experiment. All plots within the experiment received equal total summer growing season precipitation that was temporally repackaged into regular events of inversely varied size and frequency, with event sizes ranging from 5 to 50 mm and dry intervals ranging from 3.5 to 21 days. We found that repackaging rainfall into few/large events with long dry intervals decreased seasonal cumulative Fs. Repackaging influenced key aspects of pulses including mean, maximum, and antecedent (day before irrigation) values of soil moisture and Fs and their rate of decline during drying intervals. Soil moisture explained substantial variation in Fs (R2 &amp;gt; 0.84) for all treatments; however, the sensitivity of Fs to soil moisture decreased in the few/large regime compared to the reference and many/small regimes. Dynamics in plant phenology (quantified by plot greenness) and soil temperature interacted with soil moisture to influence the seasonal evolution of Fs pulses and cumulative efflux. Our findings demonstrate that soil moisture and vegetation responses to changes in rainfall size and frequency impact soil CO2 efflux pulses and seasonal emissions in semiarid grasslands. These results, coupled with the knowledge that CO2 efflux pulses play an outsized role in dryland carbon exchange, indicate the possibility of future climate-mediated shifts in the carbon cycling of semiarid ecosystems.

The effect of prolonged drought legacies on plant–soil feedbacks

AbstractQuestionClimate change has been shown to cause shifts in plant–soil feedbacks (PSFs) that may affect plant community dynamics, but the effect of prolonged drought is uncertain. We asked whether prolonged drought legacies cause shifts in PSFs due to changes in plant–soil biotic interactions.LocationRichmond, New South Wales, Australia.MethodsWe collected soils from a five‐year field‐based rainfall manipulation experiment simulating ambient rainfall and drought (50% reduction) in a mesic temperate grassland. PSFs of twelve plant species representing four functional groups (C3 and C4 grasses, forbs, and legumes) were assessed when grown alone and in mixed cultures (one species from each of the four functional groups) under laboratory conditions following a standard PSF protocol in soils with ambient rainfall and drought legacies. All soils were sterilised and then re‐inoculated to create the respective treatments including a non‐inoculated control for biota‐mediated PSFs.ResultsPSFs varied considerably among species and functional types in both legacy treatments. Overall, C3 grasses displayed less negative and C4 grasses less positive PSFs in soils with a legacy of prolonged drought compared with soils with ambient rainfall legacies, while PSFs for forbs and legumes were not significantly different from zero in either rainfall treatment. However, PSFs differed between species within functional groups. For example, Plantago showed positive PSFs in soils with ambient rainfall legacies but negative PSFs in soils with drought legacies while the opposite was observed for Medicago. PSFs of the mixed communities showed a trend to shift from positive to neutral in soils with drought legacies, with significant differences in PSFs observed when comparing home vs sterile soils, suggesting that drought may destabilise plant communities.ConclusionsOur results provide evidence that prolonged drought legacies can modify plant community dynamics due to species‐specific changes in PSFs that persist after droughts are alleviated.

Post-fire summer rainfall differentially affects reseeder and resprouter population recovery in fire-prone shrublands of South Africa

Summer rainfall can have strong effects on post-fire mediterranean-type shrubland recovery patterns, with potentially long-lasting implications on communities. Our three-year field rainfall manipulation experiment tested post-fire survival and physiological responses of reseeders and resprouters to contrasting summer rainfall patterns in Fynbos and Renosterveld shrublands in South Africa. Climate projections are uncertain for this region but indicate that increased convective summer rainfall events could occur. We irrigated treatment plots during the hottest summer months (i.e. Jan, Feb, March) to contrast the naturally dry summer conditions. This allowed for assessments of the potential limiting effects of summer drought on post-fire vegetation recovery and the responsiveness of vegetation to moisture inputs during this time.Natural summer droughts led to leaf dehydration, reduced photosynthesis and reduced photosynthetic capacity. This had a particularly severe effect on reseeders during the first summer after fire leading to high mortality rates. Summer irrigations strongly reduced levels of reseeder stress and mortality. Resprouters in both vegetation types were physiologically less sensitive to rainfall patterns and showed little drought-related mortality. Comparisons of final population sizes with emergence and survival patterns showed that summer rainfall during the first summer after fire had the potential to strongly alter reseeder population sizes. The physiological sensitivity of plants to summer rainfall patterns was higher in shrubland communities occurring on fine-textured, moderately fertile soils (e.g. Renosterveld). Shrublands occurring on sandy, nutrient-poor soils (e.g. Fynbos) were remarkably insensitive to summer drought after the first summer with lower irrigation responses. Our study demonstrated the potential for variation in post-fire summer rainfall to strongly affect reseeder and resprouter population recovery patterns.

Interactions between drought and shade on the productivity of winter pea grown in a 25‐year‐old walnut‐based alley cropping system

AbstractIn the face of climate change, more frequent drought events are expected in the Mediterranean regions. Alley cropping is an agroforestry practice that represents a promising adaptation strategy to sustain yield productivity under drier conditions. However, the presence of trees limits the productivity of the intercrop by reducing light availability and by competing for soil water resources, which could potentially exacerbate the yield losses due to drought conditions. Furthermore, the effects of co‐occurring drought and shade stresses on annual crops are still poorly understood. To tackle this issue, we performed a rainfall manipulation experiment on winter pea (Pisum sativum L.) grown in full‐sun conditions (agricultural control) and under different levels of shade in a 25‐year walnut‐based alley cropping system located in southern France. We evaluated first the effect of trees on light and water availability, and we then studied the effects of early drought (135 mm excluded from April to the end of May) and light conditions on crop performances and yield components. At 3.5 m from the tree line, light availability was reduced on average by 19% at south and 35% at north of trees over the entire crop cycle and mostly after tree budburst. The impact of trees on soil water content in the crop root zone was weak thanks to the good complementarity of the respective root systems. Under normal rainfall conditions, tree shade decreased pea yield from −25% to −77% compared to full‐sun conditions. In case of spring drought, pea yield was decreased by −22% in full‐sun conditions. The negative effect of tree shade was reduced and decreased pea yield only by −1% to −47%. Under the most intense shade conditions, pea yield was even higher under drought than in normal rainfall conditions. The analysis of crop dynamics and yield components revealed that the vegetative development of pea ceased under drought to the benefit of biomass allocation towards the reproductive organs. Pea yield was less impacted by tree shade under spring drought because yield elaboration relied less on the success of pod set, sensitive to shade and, more on grain filling, improved in case of early drought event. This study supports the hypothesis that agroforestry systems may be more resilient in the case of early drought.

Effects of manipulated precipitation on aboveground net primary productivity of grassland fields: Controlled rainfall experiments in Inner Mongolia, China

AbstractRecovery from grassland degradation caused by climate change and human disturbance is critical to the sustainable development of drylands. Grassland in China accounts for 13% of the global grassland area, and more than half of it is still recovering from degradation. How climate change will contribute to the recovery of degraded grassland has not been clarified through field experiments. Here, we report the effects of manipulated precipitation regimes on above ground net primary productivity (ANPP) in five grassland fields along a degradation gradient using a 3‐year controlled rainfall manipulation experiment in Inner Mongolia, China. We found that differences of approximately 20–30% in growing season precipitation have limited impacts on the ANPP of grasslands, and the responses can depend crucially on the degradation status indicated by vegetation and soil features. ANPP sensitivity to spring drought increased with increasing degradation level, which was mainly determined by vegetation composition. ANPP was generally insensitive to summer drought except for mature grassland, in which drought led to severe soil water deficit, mainly mediated by soil water holding capacity. Our results suggest that the recovery of degraded grassland is expected to be delayed by increasing drought early in the growing season, and mature grassland would be more vulnerable to extreme summer drought. These findings highlight the negative impacts of seasonal drought on degraded grassland and emphasize the importance of accounting for climate change and degradation status in future grassland ecosystem restoration and management policies.

Long-term increase in rainfall decreases soil organic phosphorus decomposition in tropical forests

Increased rainfall may affect soil phosphorus (P) cycling in tropical forest ecosystems, yet the key biotic and abiotic factors that govern soil organic P transformations remain unclear. In this study, we conducted a long-term (7 years) rainfall manipulation experiment in the field to examine the effects of increased rainfall (+25%) in the wet season on soil P dynamics in a tropical forest. We found that an increase in rainfall in the wet season enhanced the maximum P adsorption capacity of forest soil as a result of a greater availability of soil organic matter and organic iron (Fe)/aluminum (Al) oxides. Newly formed stable ternary complexes between soil organic matter, metals (Mn, Mg and Ca) and P increased the surface adsorption of soil organic P. Moreover, the alkaline phosphomonoesterase (ALP) activity was reduced under our experimental treatment owing to a decrease in the abundance of both ALP-producing bacteria (Bradyrhizobium and Methylobacterium being the dominant genera) and bacterivores (Rhabditis and Acrobeloides). Our findings demonstrated an overall negative impact of elevated rainfall in wet season on soil organic P decomposition in tropical forests through the enhancement of soil P adsorption capacity and the inhibition of soil ALP activities. Our results implied that a long-term rainfall increase in the wet season could increase soil organic P accumulation, which in turn further reduce soil P availability for plant uptake in the tropical forests.

Short- and long-term responses of photosynthetic capacity to temperature in four boreal tree species in a free-air warming and rainfall manipulation experiment.

High latitude forests cope with considerable variation in moisture and temperature at multiple temporal scales. To assess how their photosynthetic physiology responds to short- and long-term temperature variation, we measured photosynthetic capacity for four tree species growing in an open-air experiment in the boreal-temperate ecotone `Boreal Forest Warming at an Ecotone in Danger' (B4WarmED). The experiment factorially manipulated temperature above- and below-ground (ambient, +3.2°C) and summer rainfall (ambient, 40% removal). We measured A/Ci curves at 18, 25 and 32°C for individuals of two boreal (Pinus banksiana Lamb., Betula papyrifera Marsh.) and two temperate species (Pinus strobus L., Acer rubrum L.) experiencing the long-term warming and/or reduced-rainfall conditions induced by our experimental treatments. We calculated the apparent photosynthetic capacity descriptors VCmax,Ci and Jmax,Ci and their ratio for each measurement temperate. We hypothesized that (i) VCmax,Ci and Jmax,Ci would be down-regulated in plants experiencing longer term (e.g., weeks to months) warming and reduced rainfall (i.e., have lower values at a given measurement temperature), as is sometimes found in the literature, and that (ii) plants growing at warmer temperatures or from warmer ranges would show greater sensitivity (steeper slope) to short-term (minutes to hours) temperature variation. Neither hypothesis was supported as a general trend across the four species, as there was not a significant main effect (across species) of either warming or rainfall reduction on VCmax,Ci and Jmax,Ci. All species markedly increased VCmax,Ci and Jmax,Ci (and decreased their ratio) with short-term increases in temperature (i.e., contrasting values at 18, 25 and 32°C), and those responses were independent of long-term treatments and did not differ among species. The Jmax,Ci:VCmax,Ci ratio was, however, significantly lower across species in warmed and reduced rainfall treatments. Collectively, these results suggest that boreal trees possess considerable short-term plasticity that may allow homeostasis of VCmax,Ci and Jmax,Ci to a longer term temperature treatment. Our results also caution against extrapolating results obtained under controlled and markedly contrasting temperature treatments to responses of photosynthetic parameters to more modest temperature changes expected in the near-term with climate warming in field conditions.

Rainfall manipulation experiments as simulated by terrestrial biosphere models: Where do we stand?

Changes in rainfall amounts and patterns have been observed and are expected to continue in the near future with potentially significant ecological and societal consequences. Modelling vegetation responses to changes in rainfall is thus crucial to project water and carbon cycles in the future. In this study, we present the results of a new model-data intercomparison project, where we tested the ability of 10 terrestrial biosphere models to reproduce the observed sensitivity of ecosystem productivity to rainfall changes at 10 sites across the globe, in nine of which, rainfall exclusion and/or irrigation experiments had been performed. The key results are as follows: (a) Inter-model variation is generally large and model agreement varies with timescales. In severely water-limited sites, models only agree on the interannual variability of evapotranspiration and to a smaller extent on gross primary productivity. In more mesic sites, model agreement for both water and carbon fluxes is typically higher on fine (daily-monthly) timescales and reduces on longer (seasonal-annual) scales. (b) Models on average overestimate the relationship between ecosystem productivity and mean rainfall amounts across sites (in space) and have a low capacity in reproducing the temporal (interannual) sensitivity of vegetation productivity to annual rainfall at a given site, even though observation uncertainty is comparable to inter-model variability. (c) Most models reproduced the sign of the observed patterns in productivity changes in rainfall manipulation experiments but had a low capacity in reproducing the observed magnitude of productivity changes. Models better reproduced the observed productivity responses due to rainfall exclusion than addition. (d) All models attribute ecosystem productivity changes to the intensity of vegetation stress and peak leaf area, whereas the impact of the change in growing season length is negligible. The relative contribution of the peak leaf area and vegetation stress intensity was highly variable among models.

Critical transitions in rainfall manipulation experiments on grasslands.

As a result of climate and land‐use changes, grasslands have been subjected to intensifying drought regimes. Extreme droughts could interfere in the positive feedbacks between grasses and soil water content, pushing grasslands across critical thresholds of productivity and leading them to collapse. If this happens, systems may show hysteresis and costly management interventions might be necessary to restore predrought productivity. Thus, neglecting critical transitions may lead to mismanagement of grasslands and to irreversible loss of ecosystem services. Rainfall manipulation experiments constitute a powerful approach to investigate the risk of such critical transitions. However, experiments performed to date have rarely applied extreme droughts and have used resilience indices that disregard the existence of hysteresis. Here, we suggest how to incorporate critical transitions when designing rainfall manipulation experiments on grasslands and when measuring their resilience to drought. The ideas presented here have the potential to trigger a perspective shift among experimental researchers, into a new state where the existence of critical transitions will be discussed, experimentally tested, and largely considered when assessing and managing vegetation resilience to global changes.

Rainfall pulses increased short-term biocrust chlorophyll but not fungal abundance or N availability in a long-term dryland rainfall manipulation experiment

Microbial activity in drylands is mediated by the magnitude and frequency of growing season rain events that will shift as climate change progresses. Nitrogen is often co-limiting with water availability to dryland plants. This study investigated how microbes important to the nitrogen (N) cycle and soil N availability varied temporally and spatially in the context of a long-term rainfall variability experiment in the northern Chihuahuan Desert. Specifically, biological soil crust (biocrust) chlorophyll content, fungal abundance, and inorganic N were measured in soils adjacent to individuals of the grassland foundation species, Bouteloua eriopoda, and in the unvegetated interspace at multiple time points associated with experimental monsoon rainfall treatments. Treatments included 12 small weekly (5 mm) or 3 large monthly (20 mm) rain events, which had been applied during the summer monsoon for nine years prior to this study. Additionally, target plant C:N ratios were measured, and 15N-glutamate was added to biocrusts to determine potential for nutrient transport to B. eriopoda. Biocrust chlorophyll was up to 67% higher in the small weekly and large monthly rainfall treatments compared to ambient controls. Fungal biomass was 57% lower in soil interspaces than adjacent to plants but did not respond to rainfall treatments. Ammonium and nitrate concentrations near plants declined through the sampling period but varied little in soil interspaces. There was limited movement of 15N from interspace biocrusts to leaves but high 15N retention occurred in the soils even after additional ambient and experimental rain events. Plant C:N ratio was unaffected by rainfall treatments. The long-term alteration in rainfall regime in this experiment did not change how short-term microbial abundance or N availability responded to the magnitude or frequency of rain events at the end of the growing season, suggesting a limited response of N availability to future climate change.