A common thermal niche among geographically diverse populations of the widely distributed tree species Eucalyptus tereticornis: No evidence for adaptation to climate‐of‐origin

Impacts of climate warming depend on the degree to which plants are constrained by adaptation to their climate‐of‐origin or exhibit broad climatic suitability. We grew cool‐origin, central and warm‐origin provenances of Eucalyptus tereticornis in an array of common temperature environments from 18 to 35.5°C to determine if this widely distributed tree species consists of geographically contrasting provenances with differentiated and narrow thermal niches, or if provenances share a common thermal niche. The temperature responses of photosynthesis, respiration, and growth were equivalent across the three provenances, reflecting a common thermal niche despite a 2,200 km geographic distance and 13°C difference in mean annual temperature at seed origin. The temperature dependence of growth was primarily mediated by changes in leaf area per unit plant mass, photosynthesis, and whole‐plant respiration. Thermal acclimation of leaf, stem, and root respiration moderated the increase in respiration with temperature, but acclimation was constrained at high temperatures. We conclude that this species consists of provenances that are not differentiated in their thermal responses, thus rejecting our hypothesis of adaptation to climate‐of‐origin and suggesting a shared thermal niche. In addition, growth declines with warming above the temperature optima were driven by reductions in whole‐plant leaf area and increased respiratory carbon losses. The impacts of climate warming will nonetheless vary across the geographic range of this and other such species, depending primarily on each provenance's climate position on the temperature response curves for photosynthesis, respiration, and growth.     

Evolutionary temperature compensation of carbon fixation in marine phytoplankton

The efficiency of carbon sequestration by the biological pump could decline in the coming decades because respiration tends to increase more with temperature than photosynthesis. Despite these differences in the short‐term temperature sensitivities of photosynthesis and respiration, it remains unknown whether the long‐term impacts of global warming on metabolic rates of phytoplankton can be modulated by evolutionary adaptation. We found that respiration was consistently more temperature dependent than photosynthesis across 18 diverse marine phytoplankton, resulting in universal declines in the rate of carbon fixation with short‐term increases in temperature. Long‐term experimental evolution under high temperature reversed the short‐term stimulation of metabolic rates, resulting in increased rates of carbon fixation. Our findings suggest that thermal adaptation may therefore have an ameliorating impact on the efficiency of phytoplankton as primary mediators of the biological carbon pump.     

Autumn photosynthetic decline and growth cessation in seedlings of white spruce are decoupled under warming and photoperiod manipulations

Climate warming is expected to increase the seasonal duration of photosynthetic carbon fixation and tree growth in high‐latitude forests. However, photoperiod, a crucial cue for seasonality, will remain constant, which may constrain tree responses to warming. We investigated the effects of temperature and photoperiod on weekly changes in photosynthetic capacity, leaf biochemistry and growth in seedlings of a boreal evergreen conifer, white spruce [Picea glauca (Moench) Voss]. Warming delayed autumn declines in photosynthetic capacity, extending the period when seedlings had high carbon uptake. While photoperiod was correlated with photosynthetic capacity, short photoperiods did not constrain the maintenance of high photosynthetic capacity under warming. Rubisco concentration dynamics were affected by temperature but not photoperiod, while leaf pigment concentrations were unaffected by treatments. Respiration rates at 25 °C were stimulated by photoperiod, although respiration at the growth temperatures was increased in warming treatments. Seedling growth was stimulated by increased photoperiod and suppressed by warming. We demonstrate that temperature is a stronger control on the seasonal timing of photosynthetic down‐regulation than is photoperiod. Thus, while warming can stimulate carbon uptake in boreal conifers, the extra carbon may be directed towards respiration rather than biomass, potentially limiting carbon sequestration under climate change.     

Plastic and genetic responses of a common sedge to warming have contrasting effects on carbon cycle processes

Climate warming affects plant physiology through genetic adaptation and phenotypic plasticity, but little is known about how these mechanisms influence ecosystem processes. We used three elevation gradients and a reciprocal transplant experiment to show that temperature causes genetic change in the sedge Eriophorum vaginatum. We demonstrate that plants originating from warmer climate produce fewer secondary compounds, grow faster and accelerate carbon dioxide (CO₂) release to the atmosphere. However, warmer climate also caused plasticity in E. vaginatum, inhibiting nitrogen metabolism, photosynthesis and growth and slowing CO₂ release into the atmosphere. Genetic differentiation and plasticity in E. vaginatum thus had opposing effects on CO₂ fluxes, suggesting that warming over many generations may buffer, or reverse, the short‐term influence of this species over carbon cycle processes. Our findings demonstrate the capacity for plant evolution to impact ecosystem processes, and reveal a further mechanism through which plants will shape ecosystem responses to climate change.     

Contrasting thermal acclimation of leaf dark respiration and photosynthesis of Antarctic vascular plant species exposed to nocturnal warming

Leaf respiration and photosynthesis will respond differently to an increase in temperature during night, which can be more relevant in sensitive ecosystems such as Antarctica. We postulate that the plant species able to colonize the Antarctic Peninsula – Colobanthus quitensis (Kunth) Bartl. and Deschampsia antarctica Desv. – are able to acclimate their foliar respiration and to maintain photosynthesis under nocturnal warming to sustain a positive foliar carbon balance. We conducted a laboratory experiment to evaluate the effect of time of day (day and night) and nocturnal warming on dark respiration. Short (E₀ and Q₁₀) and long‐term acclimation of respiration, leaf carbohydrates, photosynthesis (A_sat) and foliar carbon balance (R/A) were evaluated. The results suggest that the two species have differential thermal acclimation respiration, where D. antarctica showed more thermosensitivity to short‐term changes in temperature than C. quitensis. Experimental nocturnal warming affected respiration at daytime differentially between the two species, with a significant increase of R₁₀ and A_sat in D. antarctica, while no changes on respiration were observed in C. quitensis. Long thermal treatments of the plants indicated that nocturnal but not diurnal respiration could acclimate in both species, and to a greater extent in C. quitensis. Non‐structural carbohydrates were related with respiration in C. quitensis but not in D. antarctica, suggesting that respiration in the former species is likely controlled by total soluble sugars and starch during day and night, respectively. Finally, foliar carbon balance was differentially improved under warming conditions in Antarctic plants by different mechanisms, with C. quitensis deploying respiratory acclimation, while D. antarctica increased its A_sat.     

Rapid thermal adaptation in a marine diatom reveals constraints and trade‐offs

Rapid evolution in response to environmental change will likely be a driving force determining the distribution of species across the biosphere in coming decades. This is especially true of microorganisms, many of which may evolve in step with warming, including phytoplankton, the diverse photosynthetic microbes forming the foundation of most aquatic food webs. Here we tested the capacity of a globally important, model marine diatom Thalassiosira pseudonana, for rapid evolution in response to temperature. Selection at 16 and 31°C for 350 generations led to significant divergence in several temperature response traits, demonstrating local adaptation and the existence of trade‐offs associated with adaptation to different temperatures. In contrast, competitive ability for nitrogen (commonly limiting in marine systems), measured after 450 generations of temperature selection, did not diverge in a systematic way between temperatures. This study shows how rapid thermal adaptation affects key temperature and nutrient traits and, thus, a population's long‐term physiological, ecological, and biogeographic response to climate change.     

Apparent thermal acclimation of soil heterotrophic respiration mainly mediated by substrate availability

Multiple lines of existing evidence suggest that increasing CO₂ emission from soils in response to rising temperature could accelerate global warming. However, in experimental studies, the initial positive response of soil heterotrophic respiration (R_H) to warming often weakens over time (referred to apparent thermal acclimation). If the decreased R_H is driven by thermal adaptation of soil microbial community, the potential for soil carbon (C) losses would be reduced substantially. In the meanwhile, the response could equally be caused by substrate depletion, and would then reflect the gradual loss of soil C. To address uncertainties regarding the causes of apparent thermal acclimation, we carried out sterilization and inoculation experiments using the soil samples from an alpine meadow with 6 years of warming and nitrogen (N) addition. We demonstrate that substrate depletion, rather than microbial adaptation, determined the response of R_H to long-term warming. Furthermore, N addition appeared to alleviate the apparent acclimation of R_H to warming. Our study provides strong empirical support for substrate availability being the cause of the apparent acclimation of soil microbial respiration to temperature. Thus, these mechanistic insights could facilitate efforts of biogeochemical modeling to accurately project soil C stocks in the future climate.     

Ecosystem CO2 production during winter in a Swedish subarctic region: the relative importance of climate and vegetation type

General circulation models consistently predict that regional warming will be most rapid in the Arctic, that this warming will be predominantly in the winter season, and that it will often be accompanied by increasing snowfall. Paradoxically, despite the strong cold season emphasis in these predictions, we know relatively little about the plot and landscape‐level controls on tundra biogeochemical cycling in wintertime as compared to summertime. We investigated the relative influence of vegetation type and climate on CO₂ production rates and total wintertime CO₂ release in the Scandinavian subarctic. Ecosystem respiration rates and a wide range of associated environmental and substrate pool size variables were measured in the two most common vegetation types of the region (birch understorey and heath tundra) at four paired sites along a 50 km transect through a strong snow depth gradient in northern Sweden. Both climate and vegetation type were strong interactive controls on ecosystem CO₂ production rates during winter. Of all variables tested, soil temperature explained by far the largest amount of variation in respiration rates (41–75%). Our results indicate that vegetation type only exerted an influence on respiration when snow depth was below a certain threshold (～1 m). Thus, tall vegetation that enhanced snow accumulation within that threshold resulted in more effective thermal insulation from severe air temperatures, thereby significantly increasing respiratory activity. At the end of winter, within several days of snowmelt, gross ecosystem photosynthesis rates were of a similar magnitude to ecosystem respiration, resulting in significant net carbon gain in some instances. Finally, climate and vegetation type were also strong interactive controls on total wintertime respiration, suggesting that spatial variations in maximum snowdepth may be a primary determinant of regional patterns of wintertime CO₂ release. Together, our results have important implications for predictions of how the distribution of tundra vegetation types and the carbon balances of arctic ecosystems will respond to climate change during winter because they indicate a threshold (～1 m) above which there would be little effect of increased snow accumulation on wintertime biogeochemical cycling.     

Comparative experimental evolution reveals species-specific idiosyncrasies in marine phytoplankton adaptation to warming

A number of experimental studies have demonstrated that phytoplankton can display rapid thermal adaptation in response to warmed environments. While these studies provide insight into the evolutionary responses of single species, they tend to employ different experimental techniques. Consequently, our ability to compare the potential for thermal adaptation across different, ecologically relevant, species remains limited. Here, we address this limitation by conducting simultaneous long-term warming experiments with the same experimental design on clonal isolates of three phylogenetically diverse species of marine phytoplankton; the cyanobacterium Synechococcus sp., the prasinophyte Ostreococcus tauri and the diatom Phaeodoactylum tricornutum. Over the same experimental time period, we observed differing levels of thermal adaptation in response to stressful supra-optimal temperatures. Synechococcus sp. displayed the greatest improvement in fitness (i.e., growth rate) and thermal tolerance (i.e., temperature limits of growth). Ostreococcus tauri was able to improve fitness and thermal tolerance, but to a lesser extent. Finally, Phaeodoactylum tricornutum showed no signs of adaptation. These findings could help us understand how the structure of phytoplankton communities may change in response to warming, and possible biogeochemical implications, as some species show relatively more rapid adaptive shifts in their thermal tolerance.     

Thermal adaptation of soil microbial respiration to elevated temperature

In the short‐term heterotrophic soil respiration is strongly and positively related to temperature. In the long‐term, its response to temperature is uncertain. One reason for this is because in field experiments increases in respiration due to warming are relatively short‐lived. The explanations proposed for this ephemeral response include depletion of fast‐cycling, soil carbon pools and thermal adaptation of microbial respiration. Using a > 15 year soil warming experiment in a mid‐latitude forest, we show that the apparent ‘acclimation’ of soil respiration at the ecosystem scale results from combined effects of reductions in soil carbon pools and microbial biomass, and thermal adaptation of microbial respiration. Mass‐specific respiration rates were lower when seasonal temperatures were higher, suggesting that rate reductions under experimental warming likely occurred through temperature‐induced changes in the microbial community. Our results imply that stimulatory effects of global temperature rise on soil respiration rates may be lower than currently predicted.     

Increasing microbial carbon use efficiency with warming predicts soil heterotrophic respiration globally

The degree to which climate warming will stimulate soil organic carbon (SOC) losses via heterotrophic respiration remains uncertain, in part because different or even opposite microbial physiology and temperature relationships have been proposed in SOC models. We incorporated competing microbial carbon use efficiency (CUE)–mean annual temperature (MAT) and enzyme kinetic–MAT relationships into SOC models, and compared the simulated mass‐specific soil heterotrophic respiration rates with multiple published datasets of measured respiration. The measured data included 110 dryland soils globally distributed and two continental to global‐scale cross‐biome datasets. Model–data comparisons suggested that a positive CUE–MAT relationship best predicts the measured mass‐specific soil heterotrophic respiration rates in soils distributed globally. These results are robust when considering models of increasing complexity and competing mechanisms driving soil heterotrophic respiration–MAT relationships (e.g., carbon substrate availability). Our findings suggest that a warmer climate selects for microbial communities with higher CUE, as opposed to the often hypothesized reductions in CUE by warming based on soil laboratory assays. Our results help to build the impetus for, and confidence in, including microbial mechanisms in soil biogeochemical models used to forecast changes in global soil carbon stocks in response to warming.     

Warming will affect phytoplankton differently: evidence through a mechanistic approach

Although the consequences of global warming in aquatic ecosystems are only beginning to be revealed, a key to forecasting the impact on aquatic communities is an understanding of individual species' vulnerability to increased temperature. Despite their microscopic size, phytoplankton support about half of the global primary production, drive essential biogeochemical cycles and represent the basis of the aquatic food web. At present, it is known that phytoplankton are important targets and, consequently, harbingers of climate change in aquatic systems. Therefore, investigating the capacity of phytoplankton to adapt to the predicted warming has become a relevant issue. However, considering the polyphyletic complexity of the phytoplankton community, different responses to increased temperature are expected. We experimentally tested the effects of warming on 12 species of phytoplankton isolated from a variety of environments by using a mechanistic approach able to assess evolutionary adaptation (the so-called ratchet technique). We found different degrees of tolerance to temperature rises and an interspecific capacity for genetic adaptation. The thermal resistance level reached by each species is discussed in relation to their respective original habitats. Our study additionally provides evidence on the most resistant phytoplankton groups in a future warming scenario.     

Modelled net carbon gain responses to climate change in boreal trees: Impacts of photosynthetic parameter selection and acclimation

Boreal forests are crucial in regulating global vegetation‐atmosphere feedbacks, but the impact of climate change on boreal tree carbon fluxes is still unclear. Given the sensitivity of global vegetation models to photosynthetic and respiration parameters, we determined how predictions of net carbon gain (C‐gain) respond to variation in these parameters using a stand‐level model (MAESTRA). We also modelled how thermal acclimation of photosynthetic and respiratory temperature sensitivity alters predicted net C‐gain responses to climate change. We modelled net C‐gain of seven common boreal tree species under eight climate scenarios across a latitudinal gradient to capture a range of seasonal temperature conditions. Physiological parameter values were taken from the literature together with different approaches for thermally acclimating photosynthesis and respiration. At high latitudes, net C‐gain was stimulated up to 400% by elevated temperatures and CO₂ in the autumn but suppressed at the lowest latitudes during midsummer under climate scenarios that included warming. Modelled net C‐gain was more sensitive to photosynthetic capacity parameters (V_cmax, J_max, Arrhenius temperature response parameters, and the ratio of J_max to V_cmax) than stomatal conductance or respiration parameters. The effect of photosynthetic thermal acclimation depended on the temperatures where it was applied: acclimation reduced net C‐gain by 10%–15% within the temperature range where the equations were derived but decreased net C‐gain by 175% at temperatures outside this range. Thermal acclimation of respiration had small, but positive, impacts on net C‐gain. We show that model simulations are highly sensitive to variation in photosynthetic parameters and highlight the need to better understand the mechanisms and drivers underlying this variability (e.g., whether variability is environmentally and/or biologically driven) for further model improvement.     

Thermal Acclimation of Respiration and Photosynthesis in the Marine Macroalga Gracilaria lemaneiformis (Gracilariales,Rhodophyta)

The responses of respiration and photosynthesis to temperature fluctuations in marine macroalgae have the potential to significantly affect coastal carbon fluxes and sequestration. In this study, the marine red macroalga Gracilaria lemaneiformis was cultured at three different temperatures (12, 19, and 26°C) and at high‐ and low‐nitrogen (N) availability, to investigate the acclimation potential of respiration and photosynthesis to temperature change. Measurements of respiratory and photosynthetic rates were made at five temperatures (7°C–33°C). An instantaneous change in temperature resulted in a change in the rates of respiration and photosynthesis, and the temperature sensitivities (i.e., the Q₁₀ value) for both the metabolic processes were lower in 26°C‐grown algae than 12°C‐ or 19°C‐grown algae. Both respiration and photosynthesis acclimated to long‐term changes in temperature, irrespective of the N availability under which the algae were grown; respiration displayed strong acclimation, whereas photosynthesis only exhibited a partial acclimation response to changing growth temperatures. The ratio of respiration to gross photosynthesis was higher in 12°C‐grown algae, but displayed little difference between the algae grown at 19°C and 26°C. We propose that it is unlikely that respiration in G. lemaneiformis would increase significantly with global warming, although photosynthesis would increase at moderately elevated temperatures.     

Contrasting acclimation responses to elevated CO2 and warming between an evergreen and a deciduous boreal conifer

Rising atmospheric carbon dioxide (CO₂) concentrations may warm northern latitudes up to 8°C by the end of the century. Boreal forests play a large role in the global carbon cycle, and the responses of northern trees to climate change will thus impact the trajectory of future CO₂ increases. We grew two North American boreal tree species at a range of future climate conditions to assess how growth and carbon fluxes were altered by high CO₂ and warming. Black spruce (Picea mariana, an evergreen conifer) and tamarack (Larix laricina, a deciduous conifer) were grown under ambient (407 ppm) or elevated CO₂ (750 ppm) and either ambient temperatures, a 4°C warming, or an 8°C warming. In both species, the thermal optimum of net photosynthesis (T_optA) increased and maximum photosynthetic rates declined in warm‐grown seedlings, but the strength of these changes varied between species. Photosynthetic capacity (maximum rates of Rubisco carboxylation, V_cmax, and of electron transport, J_max) was reduced in warm‐grown seedlings, correlating with reductions in leaf N and chlorophyll concentrations. Warming increased the activation energy for V_cmax and J_max (E_aV and E_aJ, respectively) and the thermal optimum for J_max. In both species, the T_optA was positively correlated with both E_aV and E_aJ, but negatively correlated with the ratio of J_max/V_cmax. Respiration acclimated to elevated temperatures, but there were no treatment effects on the Q₁₀ of respiration (the increase in respiration for a 10°C increase in leaf temperature). A warming of 4°C increased biomass in tamarack, while warming reduced biomass in spruce. We show that climate change is likely to negatively affect photosynthesis and growth in black spruce more than in tamarack, and that parameters used to model photosynthesis in dynamic global vegetation models (E_aV and E_aJ) show no response to elevated CO₂.     

Effects of Experimental Water Table and Temperature Manipulations on Ecosystem CO2 Fluxes in an Alaskan Rich Fen

Peatlands store 30% of the world’s terrestrial soil carbon (C) and those located at northern latitudes are expected to experience rapid climate warming. We monitored growing season carbon dioxide (CO₂) fluxes across a factorial design of in situ water table (control, drought, and flooded plots) and soil warming (control vs. warming via open top chambers) treatments for 2 years in a rich fen located just outside the Bonanza Creek Experimental Forest in interior Alaska. The drought (lowered water table position) treatment was a weak sink or small source of atmospheric CO₂ compared to the moderate atmospheric CO₂ sink at our control. This change in net ecosystem exchange was due to lower gross primary production and light-saturated photosynthesis rather than increased ecosystem respiration. The flooded (raised water table position) treatment was a greater CO₂ sink in 2006 due largely to increased early season gross primary production and higher light-saturated photosynthesis. Although flooding did not have substantial effects on rates of ecosystem respiration, this water table treatment had lower maximum respiration rates and a higher temperature sensitivity of ecosystem respiration than the control plot. Surface soil warming increased both ecosystem respiration and gross primary production by approximately 16% compared to control (ambient temperature) plots, with no net effect on net ecosystem exchange. Results from this rich fen manipulation suggest that fast responses to drought will include reduced ecosystem C storage driven by plant stress, whereas inundation will increase ecosystem C storage by stimulating plant growth.     

Thermal acclimation of leaf respiration of tropical trees and lianas: response to experimental canopy warming,and consequences for tropical forest carbon balance

Climate warming is expected to increase respiration rates of tropical forest trees and lianas, which may negatively affect the carbon balance of tropical forests. Thermal acclimation could mitigate the expected respiration increase, but the thermal acclimation potential of tropical forests remains largely unknown. In a tropical forest in Panama, we experimentally increased nighttime temperatures of upper canopy leaves of three tree and two liana species by on average 3  ° C for 1 week, and quantified temperature responses of leaf dark respiration. Respiration at 25  ° C (R₂₅) decreased with increasing leaf temperature, but acclimation did not result in perfect homeostasis of respiration across temperatures. In contrast, Q₁₀ of treatment and control leaves exhibited similarly high values (range 2.5–3.0) without evidence of acclimation. The decrease in R₂₅ was not caused by respiratory substrate depletion, as warming did not reduce leaf carbohydrate concentration. To evaluate the wider implications of our experimental results, we simulated the carbon cycle of tropical latitudes (24 ° S–24 ° N) from 2000 to 2100 using a dynamic global vegetation model (LM3VN) modified to account for acclimation. Acclimation reduced the degree to which respiration increases with climate warming in the model relative to a no‐acclimation scenario, leading to 21% greater increase in net primary productivity and 18% greater increase in biomass carbon storage over the 21st century. We conclude that leaf respiration of tropical forest plants can acclimate to nighttime warming, thereby reducing the magnitude of the positive feedback between climate change and the carbon cycle.     

Severe drought effects on ecosystem CO2 and H2O fluxes at three Mediterranean evergreen sites: revision of current hypotheses?

Eddy covariance and sapflow data from three Mediterranean ecosystems were analysed via top‐down approaches in conjunction with a mechanistic ecosystem gas‐exchange model to test current assumptions about drought effects on ecosystem respiration and canopy CO₂/H₂O exchange. The three sites include two nearly monospecific Quercus ilex L. forests – one on karstic limestone (Puéchabon), the other on fluvial sand with access to ground water (Castelporziano) – and a typical mixed macchia on limestone (Arca di Noè). Estimates of ecosystem respiration were derived from light response curves of net ecosystem CO₂ exchange. Subsequently, values of ecosystem gross carbon uptake were computed from eddy covariance CO₂ fluxes and estimates of ecosystem respiration as a function of soil temperature and moisture. Bulk canopy conductance was calculated by inversion of the Penman‐Monteith equation. In a top‐down analysis, it was shown that all three sites exhibit similar behaviour in terms of their overall response to drought. In contrast to common assumptions, at all sites ecosystem respiration revealed a decreasing temperature sensitivity ( Q ₁₀) in response to drought. Soil temperature and soil water content explained 70–80% of the seasonal variability of ecosystem respiration. During the drought, light‐saturated ecosystem gross carbon uptake and day‐time averaged canopy conductance declined by up to 90%. These changes were closely related to soil water content. Ecosystem water‐use efficiency of gross carbon uptake decreased during the drought, regardless whether evapotranspiration from eddy covariance or transpiration from sapflow had been used for the calculation. We evidence that this clearly contrasts current models of canopy function which predict increasing ecosystem water‐use efficiency (WUE) during the drought. Four potential explanations to those results were identified (patchy stomatal closure, changes in physiological capacities of photosynthesis, decreases in mesophyll conductance for CO₂, and photoinhibition), which will be tested in a forthcoming paper. It is suggested to incorporate the new findings into current biogeochemical models after further testing as this will improve estimates of climate change effects on (semi)arid ecosystems' carbon balances.     

Responses of terrestrial ecosystems to temperature and precipitation change: a meta‐analysis of experimental manipulation

Global mean temperature is predicted to increase by 2–7 °C and precipitation to change across the globe by the end of this century. To quantify climate effects on ecosystem processes, a number of climate change experiments have been established around the world in various ecosystems. Despite these efforts, general responses of terrestrial ecosystems to changes in temperature and precipitation, and especially to their combined effects, remain unclear. We used meta‐analysis to synthesize ecosystem‐level responses to warming, altered precipitation, and their combination. We focused on plant growth and ecosystem carbon (C) balance, including biomass, net primary production (NPP), respiration, net ecosystem exchange (NEE), and ecosystem photosynthesis, synthesizing results from 85 studies. We found that experimental warming and increased precipitation generally stimulated plant growth and ecosystem C fluxes, whereas decreased precipitation had the opposite effects. For example, warming significantly stimulated total NPP, increased ecosystem photosynthesis, and ecosystem respiration. Experimentally reduced precipitation suppressed aboveground NPP (ANPP) and NEE, whereas supplemental precipitation enhanced ANPP and NEE. Plant productivity and ecosystem C fluxes generally showed higher sensitivities to increased precipitation than to decreased precipitation. Interactive effects of warming and altered precipitation tended to be smaller than expected from additive, single‐factor effects, though low statistical power limits the strength of these conclusions. New experiments with combined temperature and precipitation manipulations are needed to conclusively determine the importance of temperature–precipitation interactions on the C balance of terrestrial ecosystems under future climate conditions.     

Plant community structure regulates responses of prairie soil respiration to decadal experimental warming

Soil respiration is recognized to be influenced by temperature, moisture, and ecosystem production. However, little is known about how plant community structure regulates responses of soil respiration to climate change. Here, we used a 13‐year field warming experiment to explore the mechanisms underlying plant community regulation on feedbacks of soil respiration to climate change in a tallgrass prairie in Oklahoma, USA. Infrared heaters were used to elevate temperature about 2 °C since November 1999. Annual clipping was used to mimic hay harvest. Our results showed that experimental warming significantly increased soil respiration approximately from 10% in the first 7 years (2000–2006) to 30% in the next 6 years (2007–2012). The two‐stage warming stimulation of soil respiration was closely related to warming‐induced increases in ecosystem production over the years. Moreover, we found that across the 13 years, warming‐induced increases in soil respiration were positively affected by the proportion of aboveground net primary production (ANPP) contributed by C₃ forbs. Functional composition of the plant community regulated warming‐induced increases in soil respiration through the quantity and quality of organic matter inputs to soil and the amount of photosynthetic carbon (C) allocated belowground. Clipping, the interaction of clipping with warming, and warming‐induced changes in soil temperature and moisture all had little effect on soil respiration over the years (all P > 0.05). Our results suggest that climate warming may drive an increase in soil respiration through altering composition of plant communities in grassland ecosystems.