Urgent need for a common metric to make precipitation manipulation experiments comparable

Can permanent grassland soils with elevated organic carbon buffer negative effects of more persistent precipitation regimes on forage grass performance?

Agricultural practices enhancing soil organic carbon (SOC) show potential to buffer negative effects of climate change on forage grass performance. We tested this by subjecting five forage grass varieties differing in fodder quality and drought/flooding resistance to increased persistence in summer precipitation regimes (PR) across sandy and sandy-loam soils from either permanent (high SOC) or temporary grasslands (low SOC) in adjacent parcels. Over the course of two consecutive summers, monoculture mesocosms were subjected to rainy/dry weather alternation either every 3 days or every 30 days, whilst keeping total precipitation equal. Increased PR persistence induced species-specific drought damage and productivity declines. Soils from permanent grasslands with elevated SOC buffered plant quality, but buffering effects of SOC on drought damage, nutrient availability and yield differed between texture classes. In the more persistent PR, Festuca arundinacea FERMINA was the most productive species but had the lowest quality under both ample water supply and mild soil drought, whilst under the most intense soil droughts, Festulolium FESTILO maintained the highest yields. The hybrid Lolium × boucheanum kunth MELCOMBI had intermediate productivity and both Lolium perenne varieties showed the lowest yields under soil drought, but the highest forage quality (especially the tetraploid variety MELFORCE). Performance varied with plant maturity stage and across seasons/years and was driven by altered water and nutrient availability and related nitrogen nutrition among species during drought and upon rewetting. Moreover, whilst permanent grassland soils showed the most consistent positive effects on plant performance, their available water capacity also declined under increased PR persistence. We conclude that permanent grassland soils with historically elevated SOC likely buffer negative effects of increasing summer weather persistence on forage grass performance, but may also be more sensitive to degradation under climate change.

Molinia caerulea alters forest Quercus petraea seedling growth through reduced mycorrhization

Oak regeneration is jeopardized by purple moor grass, a well-known competitive perennial grass in the temperate forests of Western Europe. Below-ground interactions regarding resource acquisition and interference have been demonstrated and have led to new questions about the negative impact of purple moor grass on ectomycorrhizal colonization. The objective was to examine the effects of moor grass on root system size and ectomycorrhization rate of oak seedlings as well as consequences on nitrogen (N) content in oak and soil. Oak seedlings and moor grass tufts were planted together or separately in pots under semi-controlled conditions (irrigated and natural light) and harvested 1 year after planting. Biomass, N content in shoot and root in oak and moor grass as well as number of lateral roots and ectomycorrhizal rate in oak were measured. Biomass in both oak shoot and root was reduced when planting with moor grass. Concurrently, oak lateral roots number and ectomycorrhization rate decreased, along with a reduction in N content in mixed-grown oak. An interference mechanism of moor grass is affecting oak seedlings performance through reduction in oak lateral roots number and its ectomycorrhization, observed in conjunction with a lower growth and N content in oak. By altering both oak roots and mycorrhizas, moor grass appears to be a species with a high allelopathic potential. More broadly, these results show the complexity of interspecific interactions that involve various ecological processes involving the soil microbial community and need to be explored in situ.

Does previous exposure to extreme precipitation regimes result in acclimated grassland communities?

Climate change will likely increase weather persistence in the mid-latitudes, resulting in precipitation regimes (PR) with longer dry and wet periods compared to historic averages. This could affect terrestrial ecosystems substantially through the increased occurrence of repeated, prolonged drought and water logging conditions. Climate history is an important determinant of ecosystem responses to consecutive environmental extremes, through direct damage, community restructuring as well as morphological and physiological acclimation in species or individuals. However, it is unclear how community restructuring and individual metabolic acclimation effects interact to determine ecosystem responses to subsequent climate extremes. Here, we investigated, if and how, differences in exposure to extreme or historically normal PR induced long-lasting (i.e. legacy) effects at the level of community (e.g., species composition), plant (e.g., biomass), and molecular composition (e.g., sugars, lipids, stress markers). Experimental grassland communities were exposed to long (extreme) or short (historically normal) dry/wet cycles in year 1 (Y1), followed by exposure to an identical PR or the opposite PR in year 2 (Y2). Results indicate that exposure to extreme PR in Y1, reduced diversity but induced apparent acclimation effects in all climate scenarios, stimulating biomass (higher productivity and structural sugar content) in Y2. In contrast, plants pre-exposed to normal PR, showed more activated stress responses (higher proline and antioxidants) under extreme PR in Y2. Overall, Y1 acclimation effects were strongest in the dominant grasses, indicating comparatively high phenotypical plasticity. However, Y2 drought intensity also correlated with grass productivity and structural sugar findings, suggesting that responses to short-term soil water deficits contributed to the observed patterns. Interactions between different legacy effects are discussed. We conclude that more extreme PR will likely alter diversity in the short-to midterm and select for acclimated grassland communities with increased productivity and attenuated molecular stress responses under future climate regimes.

Revaluating forest drought experiments according to future precipitation patterns, ecosystem carbon and decomposition rate responses: A meta-analysis

Moisture availability is a strong determinant of decomposition rates in forests worldwide. Climate models suggest that many terrestrial ecosystems are at risk from future droughts, suggesting moisture limiting conditions will develop across a range of forests worldwide. The impacts of increasing drought conditions on forest carbon (C) fluxes due to shifts in organic matter decay rates may be poorly characterised due to limited experimental research. To appraise this question, we conducted a meta-analysis of forest drought experiment studies worldwide, examining spatial limits, knowledge gaps and potential biases. To identify limits to experimental knowledge, we projected the global distribution of forest drought experiments against spatially modelled estimates of (i) future precipitation change, (ii) ecosystem total above-ground C and (iii) soil C storage. Our assessment, involving 115 individual experimental study locations, found a mismatch between the distribution of forest drought experiments and regions with higher levels of future drought risk and C storage, such as Central America, Amazonia, the Atlantic Forest of Brazil, equatorial Africa and Indonesia. Decomposition rate responses in litter and soil were also relatively under-studied, with only 30 experiments specifically examining the potential experimental impacts of drought on C fluxes from soil or litter. We propose new approaches for engaging experimentally with forest drought research, utilising standardised protocols to appraise the impacts of drought on the C cycle, while targeting the most vulnerable and relevant forests.

Global positive gross primary productivity extremes and climate contributions during 1982-2016

Gross primary production (GPP) quantifies the photosynthetic uptake of carbon by the terrestrial ecosystem. Positive GPP extremes represent the potential capacity of the terrestrial ecosystem to uptake carbon dioxide. Studying the positive GPP extreme is vital for the global carbon cycle and mitigation of global warming. With increasing climate extreme events, many kinds of research focus on studying negative GPP and the negative impact of climatic extremes on GPP. There is still a lack of research on positive GPP extremes and whether climatic extremes could be beneficial to global carbon uptake. In this study, we used daily Boreal Ecosystem Productivity Simulator (BEPS) to simulate GPP of the global terrestrial ecosystem during 1982-2016 and combined TRENDY models to detect positive GPP extremes and investigate the effects of climate extremes on GPP. We found the results of the TRENDY models have large differences in some areas of the globe, and the BEPS model driven by remote sensing data could be more suitable for simulating the long-term time series of global terrestrial GPP. Compared to other plant functional types, grasslands contributed the most to positive GPP extremes, accounting for approximately 41.6% (TRENDY) and 34.8% (BEPS) of the global positive GPP extremes. The probabilities of positive GPP extremes caused by positive precipitation extremes were significantly higher than those caused by temperature and radiation in most areas of the globe, indicating that sufficient precipitation (not a flood) would boost the carbon uptake ability of the global terrestrial ecosystem to form positive GPP extremes. On the contrary, the partial correlation coefficients between temperature and GPP were negative in most areas of globe, suggesting that global warming will not be conducive to carbon uptake of the terrestrial ecosystem. This study may provide new knowledge on the global positive GPP extremes.

Fertilized graminoids intensify negative drought effects on grassland productivity

Droughts can strongly affect grassland productivity and biodiversity, but responses differ widely. Nutrient availability may be a critical factor explaining this variation, but is often ignored in analyses of drought responses. Here, we used a standardized nutrient addition experiment covering 10 European grasslands to test if full-factorial nitrogen, phosphorus, and potassium addition affected plant community responses to inter-annual variation in drought stress and to the extreme summer drought of 2018 in Europe. We found that nutrient addition amplified detrimental drought effects on community aboveground biomass production. Drought effects also differed between functional groups, with a negative effect on graminoid but not forb biomass production. Our results imply that eutrophication in grasslands, which promotes dominance of drought-sensitive graminoids over forbs, amplifies detrimental drought effects. In terms of climate change adaptation, agricultural management would benefit from taking into account differential drought impacts on fertilized versus unfertilized grasslands, which differ in ecosystem services they provide to society.

Why Coordinated Distributed Experiments Should Go Global

The performance of coordinated distributed experiments designed to compare ecosystem sensitivity to global-change drivers depends on whether they cover a significant proportion of the global range of environmental variables. In the present article, we described the global distribution of climatic and soil variables and quantified main differences among continents. Then, as a test case, we assessed the representativeness of the International Drought Experiment (IDE) in parameter space. Considering the global environmental variability at this scale, the different continents harbor unique combinations of parameters. As such, coordinated experiments set up across a single continent may fail to capture the full extent of global variation in climate and soil parameter space. IDE with representation on all continents has the potential to address global scale hypotheses about ecosystem sensitivity to environmental change. Our results provide a unique vision of climate and soil variability at the global scale and highlight the need to design globally distributed networks.

Global response of terrestrial gross primary productivity to climate extremes

Extreme climate events undoubtedly have essential impacts on ecosystem gross primary productivity (GPP), but the global spatio-temporal patterns of GPP responses to climate extremes are unclear. In this study, we analyzed the responses of GPP to temperature and precipitation extremes during historical (1901-2016) and future (2006-2100) periods using climate extreme indices (CEIs) developed by the Expert Team on Climate Change Detection and Indices. Eight temperature-related CEIs and eight precipitation-related CEIs were used for this analysis, along with three future greenhouse gas concentration trajectory scenarios generated by the IPCC: RCP 2.6, RCP 4.5, and RCP 8.5. Our results show that under RCP 4.5 and RCP 8.5, most climate extremes are increasing from the historical period into the future, indicating a warming globe with more frequent and more intense extreme climate events. But the increasing rate is only persistently enhanced with time under scenario RCP 8.5. GPP shows a continuous negative relationship with cold CEIs and positive relationship with wet CEIs from the historical period into the future. In all zonal scales, the changed magnitude of GPP responds strongly to extreme value-related temperature extremes under different scenarios. However, the precipitation-related extremes with the strongest GPP response are various in different regions. In the future, GPP is most sensitive to temperature extremes in upper northern latitudes and in high-altitude regions (e.g., Qinghai-Tibet Plateau) and to precipitation extremes in the tropical zone. This study may provide a basis for predicting how GPP responds to climate extremes and explaining the underlying changes in the carbon cycle.

Stress gradients and biodiversity: monoculture vulnerability drives stronger biodiversity effects during drought years

Climate change will increase the likelihood and severity of droughts into the future. Although diversity may buffer plant communities against the negative effects of drought, the mechanisms underlying this pattern remain unclear. Higher-diversity plant communities may have a higher likelihood of including more drought-resistant species that can compensate for drought-sensitive species ("insurance effects"). Alternatively, higher-diversity communities may alter environmental conditions and improve performance of even drought-sensitive species. Here we planted nonleguminous forbs and grasses into monocultures and four- and eight-species mixtures, and measured species and plot productivity every year from 2000 to 2010. We found that six of our eight species were suppressed when growing in monoculture during dry years. These same species were unaffected by drought when growing in higher-diversity mixtures. Because of this poor performance in monoculture (not insurance effects), the biodiversity productivity relationship was strongest during the driest years. If biodiversity ameliorates hot/dry conditions and therefore improves performance of drought-sensitive species during periods of low rainfall, this may mean biodiversity can be used as a tool to protect individual species from drought conditions.

Consequences of drought severity for tropical live oak (Quercus oleoides) in Mesoamerica

In two Costa Rican and three Honduran sites that vary in rainfall and soil properties, we used natural isotopes, a soil water balance model, and broad-scale climate-based drought indices to study shifts in water use with ontogeny from seedlings to mature tropical live oak (Quercus oleoides) trees. Water use patterns help to explain persistence of this broadly distributed species in Mesoamerica and to evaluate likely threats of ongoing climate changes. At the end of the dry season, soil δ¹⁸ O profiles can be described by one-phase exponential decay curves. Minimum values reflect geographic origins of the last significant rain event, and curvature is inversely related to canopy closure, demonstrating its role in controlling topsoil evaporation. Partitioning of soil water sources for transpiration was analyzed with a mixing model. In the Costa Rican sites, in a relatively dry year, saplings and mature trees took up water from the upper soil. In a relatively wet year in the Honduran sites, we observed deeper water extraction. In all sites, soil storage dampens extreme variation in water availability. The size dependence of water uptake with larger stems exploiting deeper layers is translated into variation in bulk leaf δ¹³ C-based water use efficiency (WUE) with the exception of mature trees. From 1932 to 2015, drought severity was evaluated with the Standardized Precipitation Evapotranspiration Index (SPEI) concurrently with simulations of the soil water balance model. Drought occurrence increased, regardless of the time period, averaged across 6, 12, or 24 months. All ontogenetic stages in all populations experienced frequent water limitation. We found evidence for linear trends toward aridification with increases of return periods of drought for October SPEI-24 declining from 42 to 6 yr in Costa Rica and from 21 to 7 yr in Honduras and recent occurrence of multiyear droughts from 2013 to 2016. October SPEI-12 and SPEI-24 were significantly related to the Oceanic Niño Indices demonstrating that local inter-annual variations in drought severity in Mesoamerica are modulated by large-scale climate forces. Drought severity in the near-term future depends on the extent to which the Pacific will adopt a more La Niña-like vs. a more El Niño-like state under ongoing climatic changes.

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.

How ecologists define drought, and why we should do better

Drought, widely studied as an important driver of ecosystem dynamics, is predicted to increase in frequency and severity globally. To study drought, ecologists must define or at least operationalize what constitutes a drought. How this is accomplished in practice is unclear, particularly given that climatologists have long struggled to agree on definitions of drought, beyond general variants of "an abnormal deficiency of water." We conducted a literature review of ecological drought studies (564 papers) to assess how ecologists describe and study drought. We found that ecologists characterize drought in a wide variety of ways (reduced precipitation, low soil moisture, reduced streamflow, etc.), but relatively few publications (~32%) explicitly define what are, and are not, drought conditions. More troubling, a surprising number of papers (~30%) simply equated "dry conditions" with "drought" and provided little characterization of the drought conditions studied. For a subset of these, we calculated Standardized Precipitation Evapotranspiration Index values for the reported drought periods. We found that while almost 90% of the studies were conducted under conditions quantifiable as slightly to extremely drier than average, ~50% were within the range of normal climatic variability. We conclude that the current state of the ecological drought literature hinders synthesis and our ability to draw broad ecological inferences because drought is often declared but is not explicitly defined or well characterized. We suggest that future drought publications provide at least one of the following: (a) the climatic context of the drought period based on long-term records; (b) standardized climatic index values; (c) published metrics from drought-monitoring organizations; (d) a quantitative definition of what the authors consider to be drought conditions for their system. With more detailed and consistent quantification of drought conditions, comparisons among studies can be more rigorous, increasing our understanding of the ecological effects of drought.

A meta-analysis of 1,119 manipulative experiments on terrestrial carbon-cycling responses to global change

Direct quantification of terrestrial biosphere responses to global change is crucial for projections of future climate change in Earth system models. Here, we synthesized ecosystem carbon-cycling data from 1,119 experiments performed over the past four decades concerning changes in temperature, precipitation, CO₂ and nitrogen across major terrestrial vegetation types of the world. Most experiments manipulated single rather than multiple global change drivers in temperate ecosystems of the USA, Europe and China. The magnitudes of warming and elevated CO₂ treatments were consistent with the ranges of future projections, whereas those of precipitation changes and nitrogen inputs often exceeded the projected ranges. Increases in global change drivers consistently accelerated, but decreased precipitation slowed down carbon-cycle processes. Nonlinear (including synergistic and antagonistic) effects among global change drivers were rare. Belowground carbon allocation responded negatively to increased precipitation and nitrogen addition and positively to decreased precipitation and elevated CO₂. The sensitivities of carbon variables to multiple global change drivers depended on the background climate and ecosystem condition, suggesting that Earth system models should be evaluated using site-specific conditions for best uses of this large dataset. Together, this synthesis underscores an urgent need to explore the interactions among multiple global change drivers in underrepresented regions such as semi-arid ecosystems, forests in the tropics and subtropics, and Arctic tundra when forecasting future terrestrial carbon-climate feedback.

Linking genes with ecological strategies in Arabidopsis thaliana

Arabidopsis thaliana is the most prominent model system in plant molecular biology and genetics. Although its ecology was initially neglected, collections of various genotypes revealed a complex population structure, with high levels of genetic diversity and substantial levels of phenotypic variation. This helped identify the genes and gene pathways mediating phenotypic change. Population genetics studies further demonstrated that this variation generally contributes to local adaptation. Here, we review evidence showing that traits affecting plant life history, growth rate, and stress reactions are not only locally adapted, they also often co-vary. Co-variation between these traits indicates that they evolve as trait syndromes, and reveals the ecological diversification that took place within A. thaliana. We argue that examining traits and the gene that control them within the context of global summary schemes that describe major ecological strategies will contribute to resolve important questions in both molecular biology and ecology.

A unified framework of plant adaptive strategies to drought: Crossing scales and disciplines

Plant adaptation to drought has been extensively studied at many scales from ecology to molecular biology across a large range of model species. However, the conceptual frameworks underpinning the definition of plant strategies, and the terminology used across the different disciplines and scales are not analogous. 'Drought resistance' for instance refers to plant responses as different as the maintenance of growth and productivity in crops, to the survival and recovery in perennial woody or grassland species. Therefore, this paper aims to propose a unified conceptual framework of plant adaptive strategies to drought based on a revised terminology in order to enhance comparative studies. Ecological strategies encapsulate plant adaptation to multidimensional variation in resource variability but cannot account for the dynamic and short-term responses to fluctuations in water availability. Conversely, several plant physiological strategies have been identified along the mono-dimensional gradient of water availability in a given environment. According to a revised terminology, dehydration escape, dehydration avoidance, dehydration tolerance, dormancy, and desiccation tolerance are clearly distinguishable. Their sequential expression is expressed as water deficit increases while cavitation tolerance is proposed here to be a major hydraulic strategy underpinning adaptive responses to drought of vascular plants. This continuum of physiological strategies can be interpreted in the context of the ecological trade-off between water-acquisition vs. water-conservation, since growth maintenance is associated with fast water use under moderate drought while plant survival after growth cessation is associated with slow water use under severe drought. Consequently, the distinction between 'drought resistance' and 'drought survival', is emphasized as crucial to ensure a correct interpretation of plant strategies since 'knowing when not to grow' does not confer 'drought resistance' but may well enhance 'drought survival'. This framework proposal should improve cross-fertilization between disciplines to help tackle the increasing worldwide challenges that drought poses to plant adaptation.

High land-use intensity exacerbates shifts in grassland vegetation composition after severe experimental drought

Climate change projections anticipate increased frequency and intensity of drought stress, but grassland responses to severe droughts and their potential to recover are poorly understood. In many grasslands, high land-use intensity has enhanced productivity and promoted resource-acquisitive species at the expense of resource-conservative ones. Such changes in plant functional composition could affect the resistance to drought and the recovery after drought of grassland ecosystems with consequences for feed productivity resilience and environmental stewardship. In a 12-site precipitation exclusion experiment in upland grassland ecosystems across Switzerland, we imposed severe edaphic drought in plots under rainout shelters and compared them with plots under ambient conditions. We used soil water potentials to scale drought stress across sites. Impacts of precipitation exclusion and drought legacy effects were examined along a gradient of land-use intensity to determine how grasslands resisted to, and recovered after drought. In the year of precipitation exclusion, aboveground net primary productivity (ANPP) in plots under rainout shelters was -15% to -56% lower than in control plots. Drought effects on ANPP increased with drought severity, specified as duration of topsoil water potential ψ < -100 kPa, irrespective of land-use intensity. In the year after drought, ANPP had completely recovered, but total species diversity had declined by -10%. Perennial species showed elevated mortality, but species richness of annuals showed a small increase due to enhanced recruitment. In general, the more resource-acquisitive grasses increased at the expense of the deeper-rooted forbs after drought, suggesting that community reorganization was driven by competition rather than plant mortality. The negative effects of precipitation exclusion on forbs increased with land-use intensity. Our study suggests a synergistic impact of land-use intensification and climate change on grassland vegetation composition, and implies that biomass recovery after drought may occur at the expense of biodiversity maintenance.

Resilient Leaf Physiological Response of European Beech (Fagus sylvatica L.) to Summer Drought and Drought Release

Drought is a major environmental constraint to trees, causing severe stress and thus adversely affecting their functional integrity. European beech (Fagus sylvatica L.) is a key species in mesic forests that is commonly expected to suffer in a future climate with more intense and frequent droughts. Here, we assessed the seasonal response of leaf physiological characteristics of beech saplings to drought and drought release to investigate their potential to recover from the imposed stress and overcome previous limitations. Saplings were transplanted to model ecosystems and exposed to a simulated summer drought. Pre-dawn water potentials (ψpd), stomatal conductance (gS), intercellular CO2 concentration (ci), net-photosynthesis (AN), PSII chlorophyll fluorescence (PItot), non-structural carbohydrate concentrations (NSC; soluble sugars, starch) and carbon isotope signatures were measured in leaves throughout the growing season. Pre-dawn water potentials (ψpd), gS, ci, AN, and PItot decreased as drought progressed, and the concentration of soluble sugars increased at the expense of starch. Carbon isotopes in soluble sugars (δ13CS) showed a distinct increase under drought, suggesting, together with decreased ci, stomatal limitation of AN. Drought effects on ψpd, ci, and NSC disappeared shortly after re-watering, while full recovery of gS, AN, and PItot was delayed by 1 week. The fast recovery of NSC was reflected by a rapid decay of the drought signal in δ13C values, indicating a rapid turnover of assimilates and a reactivation of carbon metabolism. After recovery, the previously drought-exposed saplings showed a stimulation of AN and a trend toward elevated starch concentrations, which counteracted the previous drought limitations. Overall, our results suggest that the internal water relations of beech saplings and the physiological activity of leaves are restored rapidly after drought release. In the case of AN, stimulation after drought may partially compensate for limitations on photosynthetic activity during drought. Our observations suggest high resilience of beech to drought, contradicting the general belief that beech is particularly sensitive to environmental stressors.

Warming increases the sensitivity of seedling growth capacity to rainfall in six temperate deciduous tree species

Predicting the effects of climate change on tree species and communities is critical for understanding the future state of our forested ecosystems. We used a fully factorial precipitation (three levels; ambient, -50 % ambient, +50 % ambient) by warming (four levels; up to +4 °C) experiment in an old-field ecosystem in the northeastern USA to study the climatic sensitivity of seedlings of six native tree species. We measured whole plant-level responses: survival, total leaf area (TLA), seedling insect herbivory damage, as well as leaf-level responses: specific leaf area (SLA), leaf-level water content (LWC), foliar nitrogen (N) concentration, foliar carbon (C) concentration and C:N ratio of each of these deciduous species in each treatment across a single growing season. We found that canopy warming dramatically increased the sensitivity of plant growth (measured as TLA) to rainfall across all species. Warm, dry conditions consistently reduced TLA and also reduced leaf C:N in four species (Acer rubrum, Betula lenta, Prunus serotina, Ulmus americana), primarily as a result of reduced foliar C, not increased foliar N. Interestingly, these conditions also harmed the other two species in different ways, increasing either mortality (Populus grandidentata) or herbivory (Quercus rubra). Specific leaf area and LWC varied across species, but did not show strong treatment responses. Our results indicate that, in the northeastern USA, dry years in a future warmer environment could have damaging effects on the growth capacity of these early secondary successional forests, through species-specific effects on leaf production (total leaves and leaf C), herbivory and mortality.

Pushing precipitation to the extremes in distributed experiments: recommendations for simulating wet and dry years

Intensification of the global hydrological cycle, ranging from larger individual precipitation events to more extreme multiyear droughts, has the potential to cause widespread alterations in ecosystem structure and function. With evidence that the incidence of extreme precipitation years (defined statistically from historical precipitation records) is increasing, there is a clear need to identify ecosystems that are most vulnerable to these changes and understand why some ecosystems are more sensitive to extremes than others. To date, opportunistic studies of naturally occurring extreme precipitation years, combined with results from a relatively small number of experiments, have provided limited mechanistic understanding of differences in ecosystem sensitivity, suggesting that new approaches are needed. Coordinated distributed experiments (CDEs) arrayed across multiple ecosystem types and focused on water can enhance our understanding of differential ecosystem sensitivity to precipitation extremes, but there are many design challenges to overcome (e.g., cost, comparability, standardization). Here, we evaluate contemporary experimental approaches for manipulating precipitation under field conditions to inform the design of 'Drought-Net', a relatively low-cost CDE that simulates extreme precipitation years. A common method for imposing both dry and wet years is to alter each ambient precipitation event. We endorse this approach for imposing extreme precipitation years because it simultaneously alters other precipitation characteristics (i.e., event size) consistent with natural precipitation patterns. However, we do not advocate applying identical treatment levels at all sites - a common approach to standardization in CDEs. This is because precipitation variability varies >fivefold globally resulting in a wide range of ecosystem-specific thresholds for defining extreme precipitation years. For CDEs focused on precipitation extremes, treatments should be based on each site's past climatic characteristics. This approach, though not often used by ecologists, allows ecological responses to be directly compared across disparate ecosystems and climates, facilitating process-level understanding of ecosystem sensitivity to precipitation extremes.

Drought Effects on Proanthocyanidins in Sainfoin (Onobrychis viciifolia Scop.) Are Dependent on the Plant's Ontogenetic Stage

Sainfoin (Onobrychis viciifolia Scop.) is a forage legume, which improves animal health and the environmental impact of livestock farming due to its proanthocyanidin content. To identify the impact of drought on acetone/water-extractable proanthocyanidin (PA) concentration and composition in the generative and vegetative stages, a rain exclosure experiment was established. Leaves of 120 plants from 5 different sainfoin accessions were sampled repeatedly and analyzed by UPLC-ESI-MS/MS. The results showed distinct differences in response to drought between vegetative and generative plants. Whereas vegetative plants showed a strong response to drought in growth (-56%) and leaf PA concentration (+46%), generative plants showed no response in growth (-2%) or PA concentration (-9%). The PA composition was stable across environments. The five accessions varied in PA concentrations and composition but showed the same pattern of response to the experimental treatments. These results show that the ontogenetic stage at which drought occurs significantly affects the plant's response.

Characterizing the drivers of seedling leaf gas exchange responses to warming and altered precipitation: indirect and direct effects

Anthropogenic forces are projected to lead to warmer temperatures and altered precipitation patterns globally. The impact of these climatic changes on the uptake of carbon by the land surface will, in part, determine the rate and magnitude of these changes. However, there is a great deal of uncertainty in how terrestrial ecosystems will respond to climate in the future. Here, we used a fully factorial warming (four levels) by precipitation (three levels) manipulation experiment in an old-field ecosystem in the northeastern USA to examine the impact of climatic changes on leaf carbon exchange in five species of deciduous tree seedlings. We found that photosynthesis generally increased in response to increasing precipitation and decreased in response to warming. Respiration was less sensitive to the treatments. The net result was greater leaf carbon uptake in wetter and cooler conditions across all species. Structural equation modelling revealed the primary pathway through which climate impacted leaf carbon exchange. Net photosynthesis increased with increasing stomatal conductance and photosynthetic enzyme capacity (Vcmax), and decreased with increasing respiration of leaves. Soil moisture and leaf temperature at the time of measurement most heavily influenced these primary drivers of net photosynthesis. Leaf respiration increased with increasing soil moisture, leaf temperature, and photosynthetic supply of substrates. Counter to the soil moisture response, respiration decreased with increasing precipitation amount, indicating that the response to short- (i.e. soil moisture) versus long-term (i.e. precipitation amount) water stress differed, possibly as a result of changes in the relative amounts of growth and maintenance demand for respiration over time. These data (>500 paired measurements of light and dark leaf gas exchange), now publicly available, detail the pathways by which climate can impact leaf gas exchange and could be useful for testing assumptions in land surface models.

Few multiyear precipitation-reduction experiments find a shift in the productivity-precipitation relationship

Well-defined productivity-precipitation relationships of ecosystems are needed as benchmarks for the validation of land models used for future projections. The productivity-precipitation relationship may be studied in two ways: the spatial approach relates differences in productivity to those in precipitation among sites along a precipitation gradient (the spatial fit, with a steeper slope); the temporal approach relates interannual productivity changes to variation in precipitation within sites (the temporal fits, with flatter slopes). Precipitation-reduction experiments in natural ecosystems represent a complement to the fits, because they can reduce precipitation below the natural range and are thus well suited to study potential effects of climate drying. Here, we analyse the effects of dry treatments in eleven multiyear precipitation-manipulation experiments, focusing on changes in the temporal fit. We expected that structural changes in the dry treatments would occur in some experiments, thereby reducing the intercept of the temporal fit and displacing the productivity-precipitation relationship downward the spatial fit. The majority of experiments (72%) showed that dry treatments did not alter the temporal fit. This implies that current temporal fits are to be preferred over the spatial fit to benchmark land-model projections of productivity under future climate within the precipitation ranges covered by the experiments. Moreover, in two experiments, the intercept of the temporal fit unexpectedly increased due to mechanisms that reduced either water loss or nutrient loss. The expected decrease of the intercept was observed in only one experiment, and only when distinguishing between the late and the early phases of the experiment. This implies that we currently do not know at which precipitation-reduction level or at which experimental duration structural changes will start to alter ecosystem productivity. Our study highlights the need for experiments with multiple, including more extreme, dry treatments, to identify the precipitation boundaries within which the current temporal fits remain valid.

Remotely-sensed detection of effects of extreme droughts on gross primary production

Severe droughts strongly impact photosynthesis (GPP), and satellite imagery has yet to demonstrate its ability to detect drought effects. Especially changes in vegetation functioning when vegetation state remains unaltered (no browning or defoliation) pose a challenge to satellite-derived indicators. We evaluated the performance of different satellite indicators to detect strong drought effects on GPP in a beech forest in France (Hesse), where vegetation state remained largely unaffected while GPP decreased substantially. We compared the results with three additional sites: a Mediterranean holm oak forest (Puéchabon), a temperate beech forest (Hainich), and a semi-arid grassland (Bugacpuszta). In Hesse, a three-year reduction in GPP following drought was detected only by the Enhanced Vegetation Index (EVI). The Photochemical Reflectance Index (PRI) also detected this drought effect, but only after normalization for absorbed light. In Puéchabon normalized PRI outperformed the other indicators, while the short-term drought effect in Hainich was not detected by any tested indicator. In contrast, most indicators, but not PRI, captured the drought effects in Bugacpuszta. Hence, PRI improved detection of drought effects on GPP in forests and we propose that PRI normalized for absorbed light is considered in future algorithms to estimate GPP from space.

Effects of climate extremes on the terrestrial carbon cycle: concepts, processes and potential future impacts

Extreme droughts, heat waves, frosts, precipitation, wind storms and other climate extremes may impact the structure, composition and functioning of terrestrial ecosystems, and thus carbon cycling and its feedbacks to the climate system. Yet, the interconnected avenues through which climate extremes drive ecological and physiological processes and alter the carbon balance are poorly understood. Here, we review the literature on carbon cycle relevant responses of ecosystems to extreme climatic events. Given that impacts of climate extremes are considered disturbances, we assume the respective general disturbance-induced mechanisms and processes to also operate in an extreme context. The paucity of well-defined studies currently renders a quantitative meta-analysis impossible, but permits us to develop a deductive framework for identifying the main mechanisms (and coupling thereof) through which climate extremes may act on the carbon cycle. We find that ecosystem responses can exceed the duration of the climate impacts via lagged effects on the carbon cycle. The expected regional impacts of future climate extremes will depend on changes in the probability and severity of their occurrence, on the compound effects and timing of different climate extremes, and on the vulnerability of each land-cover type modulated by management. Although processes and sensitivities differ among biomes, based on expert opinion, we expect forests to exhibit the largest net effect of extremes due to their large carbon pools and fluxes, potentially large indirect and lagged impacts, and long recovery time to regain previous stocks. At the global scale, we presume that droughts have the strongest and most widespread effects on terrestrial carbon cycling. Comparing impacts of climate extremes identified via remote sensing vs. ground-based observational case studies reveals that many regions in the (sub-)tropics are understudied. Hence, regional investigations are needed to allow a global upscaling of the impacts of climate extremes on global carbon-climate feedbacks.

What have we learned from global change manipulative experiments in China? A meta-analysis

Although China has the largest population in the world, a faster rate of warming than the global average, and an active global change research program, results from many of the global change experiments in Chinese terrestrial ecosystems have not been included in global syntheses. Here, we specifically analyze the observed responses of carbon (C) and nitrogen (N) cycling in global change manipulative experiments in China, and compare these responses to those from other regions of the world. Most global change factors, vegetation types, and treatment methods that have been studied or used elsewhere in the world have also been studied and applied in China. The responses of terrestrial ecosystem C and N cycles to N addition and climate warming in China are similar in both direction and intensity to those reported in global syntheses. In Chinese ecosystems as elsewhere, N addition significantly increased aboveground (AGB) and belowground biomass (BGB), litter mass, dissolved organic C, net ecosystem productivity (NEP), and gross ecosystem productivity (GEP). Warming stimulated AGB, BGB and the root-shoot ratio. Increasing precipitation accelerated GEP, NEP, microbial respiration, soil respiration, and ecosystem respiration. Our findings complement and support previous global syntheses and provide insight into regional responses to global change.

Allocation dynamics of recently fixed carbon in beech saplings in response to increased temperatures and drought

The response of carbon allocation to drought has often been studied in terms of short-term transport velocity of recently fixed carbon from leaves to roots and root respiration. However, its dynamic response to other environmental conditions, e.g., to changes in temperature, is less clear. Here, we investigated the effects of drought, increased temperatures and their combination on transport velocity as well as on distribution of recent photoassimilates for different compounds, such as sugars, starch, organic acids and amino acids. We used a (13)CO(2) pulse-labelling approach and studied the recovery of (13)C in different plant tissues and compounds of beech saplings (Fagus sylvatica L.) during a 9-day chase period. Neither total dry biomass nor dry weights of leaves or roots were affected by drought or increased temperatures. Generally, the fast transfer of recently fixed assimilates from leaves to roots took about 1 day, while (13)C enrichment in soil CO(2) efflux peaked only 2 days after labelling. Increased temperatures prolonged mean transfer times of recent photoassimilates from the leaves to the roots, probably caused by enhanced intermediate storage alongside basipetal transfer, clearly impacting short-term carbon allocation. This temperature effect was seen in the delayed peak in (13)C excess of root sugars, decoupling the roots from the leaves in the short term. On average, ∼40% of the (13)C label initially present in the plant was recovered in the roots (over all treatment combinations), providing strong evidence for preferred carbon allocation into the roots at the end of the growing season. Root starch was the principal compound for long-term storage of carbon, whereas leaf (transitory) starch was remobilized again after some days, exhibiting the longest mean residence times under dry and warm conditions. These observation clearly point to different functionalities of the same compound (i.e., starch) in different plant tissues and the crucial role of roots for long-term carbon storage.

Fate of recently fixed carbon in European beech (Fagus sylvatica) saplings during drought and subsequent recovery

Drought reduces the carbon (C) assimilation of trees and decouples aboveground from belowground carbon fluxes, but little is known about the response of drought-stressed trees to rewetting. This study aims to assess dynamics and patterns of C allocation in beech saplings under dry and rewetted soil conditions. In October 2010, 5-year-old beech saplings from a forest site were transplanted into 20 l pots. In 2011, the saplings were subjected to different levels of soil drought ranging from non-limiting water supply (control) to severe water limitation with soil water potentials of less than -1.5 MPa. As a physiologically relevant measure of drought, the cumulated soil water potential (i.e., drought stress dose (DSD)) was calculated for the growing season. In late August, the saplings were transferred into a climate chamber and pulse-labeled with (13)C-depleted CO2 (δ(13)C of -47‰). Isotopic signatures in leaf and soil respiration were repeatedly measured. Five days after soil rewetting, a second label was applied using 99 atom% (13)CO2. After another 12 days, the fate of assimilated C in each sapling was assessed by calculating the (13)C mass balance. Photosynthesis decreased by 60% in saplings under severe drought. The mean residence time (MRT) of recent assimilates in leaf respiration was more than three times longer than under non-limited conditions and was positively correlated to DSD. Also, the appearance of the label in soil respiration was delayed. Within 5 days after rewetting, photosynthesis, MRT of recent assimilates in leaf respiration and appearance of the label in soil respiration recovered fully. Despite the fast recovery, less label was recovered in the biomass of the previously drought-stressed plants, which also allocated less C to the root compartment (45 vs 64% in the control). We conclude that beech saplings quickly recover from extreme soil drought, although transitional after-effects prevail in C allocation, possibly due to repair-driven respiratory processes.

Climate extremes and the carbon cycle

The terrestrial biosphere is a key component of the global carbon cycle and its carbon balance is strongly influenced by climate. Continuing environmental changes are thought to increase global terrestrial carbon uptake. But evidence is mounting that climate extremes such as droughts or storms can lead to a decrease in regional ecosystem carbon stocks and therefore have the potential to negate an expected increase in terrestrial carbon uptake. Here we explore the mechanisms and impacts of climate extremes on the terrestrial carbon cycle, and propose a pathway to improve our understanding of present and future impacts of climate extremes on the terrestrial carbon budget.

Separating drought effects from roof artifacts on ecosystem processes in a grassland drought experiment

1: Given the predictions of increased drought probabilities under various climate change scenarios, there have been numerous experimental field studies simulating drought using transparent roofs in different ecosystems and regions. Such roofs may, however, have unknown side effects, called artifacts, on the measured variables potentially confounding the experimental results. A roofed control allows the quantification of potential artifacts, which is lacking in most experiments. 2: We conducted a drought experiment in experimental grasslands to study artifacts of transparent roofs and the resulting effects of artifacts on ecosystems relative to drought on three response variables (aboveground biomass, litter decomposition and plant metabolite profiles). We established three drought treatments, using (1) transparent roofs to exclude rainfall, (2) an unroofed control treatment receiving natural rainfall and (3) a roofed control, nested in the drought treatment but with rain water reapplied according to ambient conditions. 3: Roofs had a slight impact on air (+0.14°C during night) and soil temperatures (-0.45°C on warm days, +0.25°C on cold nights), while photosynthetically active radiation was decreased significantly (-16%). Aboveground plant community biomass was reduced in the drought treatment (-41%), but there was no significant difference between the roofed and unroofed control, i.e., there were no measurable roof artifact effects. 4: Compared to the unroofed control, litter decomposition was decreased significantly both in the drought treatment (-26%) and in the roofed control treatment (-18%), suggesting artifact effects of the transparent roofs. Moreover, aboveground metabolite profiles in the model plant species Medicago x varia were different from the unroofed control in both the drought and roofed control treatments, and roof artifact effects were of comparable magnitude as drought effects. 5: Our results stress the need for roofed control treatments when using transparent roofs for studying drought effects, because roofs can cause significant side effects.