Convergence in the temperature response of leaf respiration across biomes and plant functional types

Vertical canopy gradients of respiration drive plant carbon budgets and leaf area index

Despite its importance for determining global carbon fluxes, leaf respiration remains poorly constrained in land surface models (LSMs). We tested the sensitivity of the Energy Exascale Earth System Model Land Model - Functionally Assembled Terrestrial Ecosystem Simulator (ELM-FATES) to variation in the canopy gradients of leaf maintenance respiration (Rdark). We ran global and point simulations varying the canopy gradient of Rdark to explore the impacts on forest structure, composition, and carbon cycling. In global simulations, steeper canopy gradients of Rdark lead to increased understory survival and leaf biomass. Leaf area index (LAI) increased up to 77% in tropical regions compared with the default parameterization, improving alignment with remotely sensed benchmarks. Global vegetation carbon varied from 308 Pg C to 449 Pg C across the ensemble. In tropical forest simulations, steeper gradients of Rdark had a large impact on successional dynamics. Results show the importance of canopy gradients in leaf traits and fluxes for determining plant carbon budgets and emergent ecosystem properties such as competitive dynamics, LAI, and vegetation carbon. The high-model sensitivity to canopy gradients in Rdark highlights the need for more observations of how leaf traits and fluxes vary along light micro-environments to inform critical dynamics in LSMs.

Accounting for the impact of genotype and environment on variation in leaf respiration of wheat in Mexico and Australia

An approach to improving radiation use efficiency (RUE) in wheat is to screen for variability in rates of leaf respiration in darkness (Rdark). We used a high-throughput system to quantify variation in Rdark among a diverse range of spring wheat genotypes (301 lines) grown in two countries (Mexico and Australia) and two seasons (2017 and 2018), and in doing so quantify the relative importance of genotype (G) and environment (E) in influencing variations in leaf Rdark. Through careful design, residual (unexplained) variation represented <10% of the total observed. Up to a third of the variation in Rdark (and related traits) was under genetic control. This suggests opportunities for breeders to use Rdark as a novel selection tool. In addition, E accounted for more than half of the total variation in area-based rates of Rdark. Here, the day of measurement was crucial, suggesting that day-to-day variations in the environment influence rates of Rdark measured at a common temperature. Overall, this study provides new insights into the role G and E play in determining variation in rates of leaf Rdark of one of the most important cereal crops, with implications for future improvements in carbon use efficiency and yield.

Leaf Photosynthetic and Respiratory Thermal Acclimation in Terrestrial Plants in Response to Warming: A Global Synthesis

Leaf photosynthesis and respiration are two of the largest carbon fluxes between the atmosphere and biosphere. Although experiments examining the warming effects on photosynthetic and respiratory thermal acclimation have been widely conducted, the sensitivity of various ecosystem and vegetation types to warming remains uncertain. Here we conducted a meta-analysis on experimental observations of thermal acclimation worldwide. We found that the optimum temperature for photosynthetic rate (Topt) and the maximum rate of carboxylation of Rubisco (ToptV) in tropical forest plants increased by 0.51°C and 2.12°C per 1°C of warming, respectively. Similarly, Topt and the optimum temperature for maximum electron transport rate for RuBP regeneration (ToptJ) in temperate forest plants increased by 0.91°C and 0.15°C per 1°C of warming, respectively. However, reduced photosynthetic rates at optimum temperature (Aopt) were observed in tropical forest (17.2%) and grassland (16.5%) plants, indicating that they exhibited limited photosynthetic thermal acclimation to warming. Warming reduced respiration rate (R25) in boreal forest plants by 6.2%, suggesting that respiration can acclimate to warming. Photosynthesis and respiration of broadleaved deciduous trees may adapt to warming, as indicated by higher Aopt (7.5%) and Topt (1.08°C per 1°C of warming), but lower R25 (7.7%). We found limited photosynthetic thermal acclimation in needleleaved evergreen trees (-14.1%) and herbs (-16.3%), both associated with reduced Aopt. Respiration of needleleaved deciduous trees acclimated to warming (reduced R25 and temperature sensitivity of respiration (Q10)); however, broadleaved evergreen trees did not acclimate (increased R25). Plants in grasslands and herbaceous species displayed the weakest photosynthetic acclimation to warming, primarily due to the significant reductions in Aopt. Our global synthesis provides a comprehensive analysis of the divergent effects of warming on thermal acclimation across ecosystem and vegetation types, and provides a framework for modeling responses of vegetation carbon cycling to warming.

The leaf-scale mass-based photosynthetic optimization model better predicts photosynthetic acclimation than the area-based

Leaf-scale photosynthetic optimization models can quantitatively predict photosynthetic acclimation and have become an important means of improving vegetation and land surface models. Previous models have generally been based on the optimality assumption of maximizing the net photosynthetic assimilation per unit leaf area (i.e. the area-based optimality) while overlooking other optimality assumptions such as maximizing the net photosynthetic assimilation per unit leaf dry mass (i.e. the mass-based optimality). This paper compares the predicted results of photosynthetic acclimation to different environmental conditions between the area-based optimality and the mass-based optimality models. The predictions are then verified using the observational data from the literatures. The mass-based optimality model better predicted photosynthetic acclimation to growth light intensity, air temperature and CO2 concentration, and captured more variability in photosynthetic traits than the area-based optimality models. The findings suggest that the mass-based optimality approach may be a promising strategy for improving the predictive power and accuracy of optimization models, which have been widely used in various studies related to plant carbon issues.

Exponential or Unimodal Relationships Between Nighttime Ecosystem Respiration and Temperature at the Eddy Covariance Flux Tower Sites

Ecosystem respiration is a key flux in the terrestrial carbon cycle and is affected substantially by temperature. This work analysed the time series data of nighttime net ecosystem exchange of carbon dioxide (NEEnight) from 196 FLUXNET2015 sites to re-evaluate the relationships between NEEnight and temperature. A total of 93 sites (48%) were identified to have a unimodal relationship between NEEnight and temperature. Site-specific apparent optimum temperature parameters were then estimated at these sites. We further assessed the impacts of using exponential or unimodal equations on NEEnight predictions. The predicted NEEnight values at high temperatures were substantially higher from the exponential-type equations (mean: ~200%) than from the unimodal equation (mean: ~30%), compared to the observed NEEnight. This study calls for using a unimodal equation to predict NEEnight (often considered as nighttime ecosystem respiration, ERnight), which could substantially improve the accuracy and reduce uncertainty in ER estimates, in particular under the scenario of global warming.

Instantaneous Q10 of night-time leaf respiratory CO2 efflux - measurement and analytical protocol considerations

The temperature sensitivity (e.g. Q10) of night-time leaf respiratory CO2 efflux (RCO2) is a fundamental aspect of leaf physiology. The Q10 typically exhibits a dependence on measurement temperature, and it is speculated that this is due to temperature-dependent shifts in the relative control of leaf RCO2. Two decades ago, a review hypothesized that this mechanistically caused change in values of Q10 is predictable across plant taxa and biomes. Here, we discuss the most appropriate measuring protocol among existing data and for future data collection, to form the foundation of a future mechanistic understanding of Q10 of leaf RCO2 at different temperature ranges. We do this primarily via a review of existing literature on Q10 of night-time RCO2 and only supplement this to a lesser degree with our own original data. Based on mechanistic considerations, we encourage that instantaneous Q10 of leaf RCO2 to represent night-time should be measured: only at night-time; only in response to short-term narrow temperature variation (e.g. max. 10°C) to represent a given midpoint temperature at a time; in response to as many temperatures as possible within the chosen temperature range; and on still attached leaves.

Restricted internal diffusion weakens transpiration-photosynthesis coupling during heatwaves: Evidence from leaf carbonyl sulphide exchange

Increasingly frequent and intense heatwaves threaten ecosystem health in a warming climate. However, plant responses to heatwaves are poorly understood. A key uncertainty concerns the intensification of transpiration when heatwaves suppress photosynthesis, known as transpiration-photosynthesis decoupling. Field observations of such decoupling are scarce, and the underlying physiological mechanisms remain elusive. Here, we use carbonyl sulphide (COS) as a leaf gas exchange tracer to examine potential mechanisms leading to transpiration-photosynthesis decoupling on a coast live oak in a southern California woodland in spring 2013. We found that heatwaves suppressed both photosynthesis and leaf COS uptake but increased transpiration or sustained it at non-heatwave levels throughout the day. Despite statistically significant decoupling between transpiration and photosynthesis, stomatal sensitivity to environmental factors did not change during heatwaves. Instead, midday photosynthesis during heatwaves was restricted by internal diffusion, as indicated by the lower internal conductance to COS. Thus, increased evaporative demand and nonstomatal limitation to photosynthesis act jointly to decouple transpiration from photosynthesis without altering stomatal sensitivity. Decoupling offered limited potential cooling benefits, questioning its effectiveness for leaf thermoregulation in xeric ecosystems. We suggest that adding COS to leaf and ecosystem flux measurements helps elucidate diverse physiological mechanisms underlying transpiration-photosynthesis decoupling.

The quandary of sources and sinks of CO2 efflux in tree stems-new insights and future directions

Stem respiration (RS) substantially contributes to the return of photo assimilated carbon to the atmosphere and, thus, to the tree and ecosystem carbon balance. Stem CO2 efflux (ECO2) is often used as a proxy for RS. However, this metric has often been challenged because of the uncertain origin of CO2 emitted from the stem due to post-respiratory processes. In this Insight, we (i) describe processes affecting the quantification of RS, (ii) review common methodological approaches to quantify and model RS and (iii) develop a research agenda to fill the most relevant knowledge gaps that we identified. Dissolution, transport and accumulation of respired CO2 away from its production site, reassimilation of respired CO2 via stem photosynthesis and the enzyme phosphoenolpyruvate carboxylase, axial CO2 diffusion in the gas phase, shifts in the respiratory substrate and non-respiratory oxygen (O2) consumption are the most relevant processes causing divergence between RS and measured stem gas exchange (ECO2 or O2 influx, IO2). Two common methodological approaches to estimate RS, namely the CO2 mass balance approach and the O2 consumption technique, circumvent some of these processes but have yielded inconsistent results regarding the fate of respired CO2. Stem respiration modelling has recently progressed at the organ and tree levels. However, its implementation in large-scale models, commonly operated from a source-driven perspective, is unlikely to reflect adequate mechanisms. Finally, we propose hypotheses and approaches to advance the knowledge of the stem carbon balance, the role of sap pH on RS, the reassimilation of respired CO2, RS upscaling procedures, large-scale RS modelling and shifts in respiratory metabolism during environmental stress.

Time-dependent regulation of respiration is widespread across plant evolution

Establishing the temperature dependence of respiration is critical for accurate predictions of the global carbon cycle under climate change. Diurnal temperature fluctuations, or changes in substrate availability, lead to variations in leaf respiration. Additionally, recent studies hint that the thermal sensitivity of respiration could be time-dependent. However, the role for endogenous processes, independent from substrate availability, as drivers of temporal changes in the sensitivity of respiration to temperature across phylogenies has not yet been addressed. Here, we examined the diurnal variation in the response of respiration to temperatures (R-T relationship) for different lycophyte, fern, gymnosperm and angiosperm species. We tested whether time-dependent changes in the R-T relationship would impact leaf level respiration modelling. We hypothesized that interactions between endogenous processes, like the circadian clock, and leaf respiration would be independent from changes in substrate availability. Overall, we observed a time-dependent sensitivity in the R-T relationship across phylogenies, independent of temperature, that affected modelling parameters. These results are compatible with circadian gating of respiration, but further studies should analyse the possible involvement of the clock. Our results indicate time-dependent regulation of respiration might be widespread across phylogenies, and that endogenous regulation of respiration is likely affecting leaf-level respiration fluxes.

Heatwaves do not limit recovery following defoliation but alter leaf drought tolerance traits

As heatwave frequency increases, they are more likely to coincide with other disturbances like insect defoliation. But it is unclear if high temperatures after defoliation impact canopy recovery or leaf traits which may affect response to further stressors like drought. To examine these stressor interactions, we subjected defoliated (DEF) and undefoliated (UNDEF) oak saplings to a simulated spring heatwave of +10°C for 25 days. We measured gas exchange, leaf area recovery, carbohydrate storage, turgor loss point (ΨTLP ), and minimum leaf conductance (gmin ). During the heatwave, stem respiration exhibited stronger thermal acclimation in DEF than UNDEF saplings, while stomatal conductance and net photosynthesis increased. The heatwave did not affect leaf area recovery or carbohydrate storage of DEF saplings, but reflush leaves had higher gmin than UNDEF leaves, and this was amplified by the heatwave. Across all treatments, higher gmin was associated with higher daytime stomatal conductance and a lower ΨTLP . The results suggest defoliation stress may not be exacerbated by higher temperatures. However, reflush leaves are less conservative in their water use, limiting their ability to minimise water loss. While lower ΨTLP could help DEF trees maintain gas exchange under mild drought, they may be more vulnerable to dehydration under severe drought.

Fast Assimilation-Temperature Response: a FAsTeR method for measuring the temperature dependence of leaf-level photosynthesis

We present the Fast Assimilation-Temperature Response (FAsTeR) method, a new method for measuring plant assimilation-temperature (AT) response that reduces measurement time and increases data density compared with conventional methods. The FAsTeR method subjects plant leaves to a linearly increasing temperature ramp while taking rapid, nonequilibrium measurements of gas exchange variables. Two postprocessing steps are employed to correct measured assimilation rates for nonequilibrium effects and sensor calibration drift. Results obtained with the new method are compared with those from two conventional stepwise methods. Our new method accurately reproduces results obtained from conventional methods, reduces measurement time by a factor of c. 3.3 (from c. 90 to 27 min), and increases data density by a factor of c. 55 (from c. 10 to c. 550 observations). Simulation results demonstrate that increased data density substantially improves confidence in parameter estimates and drastically reduces the influence of noise. By improving measurement speed and data density, the FAsTeR method enables users to ask fundamentally new kinds of ecological and physiological questions, expediting data collection in short-field campaigns, and improving the representativeness of data across species in the literature.

Temperature responses of leaf respiration in light and darkness are similar and modulated by leaf development

Our ability to predict temperature responses of leaf respiration in light and darkness (RL and RDk ) is essential to models of global carbon dynamics. While many models rely on constant thermal sensitivity (characterized by Q10 ), uncertainty remains as to whether Q10 of RL and RDk are actually similar. We measured short-term temperature responses of RL and RDk in immature and mature leaves of two evergreen tree species, Castanopsis carlesii and Ormosia henry in an open field. RL was estimated by the Kok method, the Yin method and a newly developed Kok-iterCc method. When estimated by the Yin and Kok-iterCc methods, RL and RDk had similar Q10 (c. 2.5). The Kok method overestimated both Q10 and the light inhibition of respiration. RL /RDk was not affected by leaf temperature. Acclimation of respiration in summer was associated with a decline in basal respiration but not in Q10 in both species, which was related to changes in leaf nitrogen content between seasons. Q10 of RL and RDk in mature leaves were 40% higher than in immature leaves. Our results suggest similar Q10 values can be used to model RL and RDk while leaf development-associated changes in Q10 require special consideration in future respiration models.

Reduced global plant respiration due to the acclimation of leaf dark respiration coupled with photosynthesis

Leaf dark respiration (Rd ) acclimates to environmental changes. However, the magnitude, controls and time scales of acclimation remain unclear and are inconsistently treated in ecosystem models. We hypothesized that Rd and Rubisco carboxylation capacity (Vcmax ) at 25°C (Rd,25 , Vcmax,25 ) are coordinated so that Rd,25 variations support Vcmax,25 at a level allowing full light use, with Vcmax,25 reflecting daytime conditions (for photosynthesis), and Rd,25 /Vcmax,25 reflecting night-time conditions (for starch degradation and sucrose export). We tested this hypothesis temporally using a 5-yr warming experiment, and spatially using an extensive field-measurement data set. We compared the results to three published alternatives: Rd,25 declines linearly with daily average prior temperature; Rd at average prior night temperatures tends towards a constant value; and Rd,25 /Vcmax,25 is constant. Our hypothesis accounted for more variation in observed Rd,25 over time (R2  = 0.74) and space (R2  = 0.68) than the alternatives. Night-time temperature dominated the seasonal time-course of Rd , with an apparent response time scale of c. 2 wk. Vcmax dominated the spatial patterns. Our acclimation hypothesis results in a smaller increase in global Rd in response to rising CO2 and warming than is projected by the two of three alternative hypotheses, and by current models.

Temperature dependence of carbon metabolism in the leaves in sun and shade in a subtropical forest

Rising temperatures pose a threat to the stability of climate regulation by carbon metabolism in subtropical forests. Although the effects of temperature on leaf carbon metabolism traits in sun-exposed leaves are well understood, there is limited knowledge about its impacts on shade leaves and the implications for ecosystem-climate feedbacks. In this study, we measured temperature response curves of photosynthesis and respiration for 62 woody species in summer (including both evergreen and deciduous species) and 20 evergreen species in winter. The aim was to uncover the temperature dependence of carbon metabolism in both sun and shade leaves in subtropical forests. Our findings reveal that shade had no significant effects on the mean optimum photosynthetic temperatures (TOpt) or temperature range (T90). However, there were decreases observed in mean stomatal conductance, mean area-based photosynthetic rates at TOpt and 25 °C, as well as mean area-based dark respiration rates at 25 °C in both evergreen and deciduous species. Moreover, the respiration-temperature sensitivity (Q10) of sun leaves was higher than that of shade leaves in winter, with the reverse being true in summer. Leaf economics spectrum traits, such as leaf mass per area, and leaf concentration of nitrogen and phosphorus across species, proved to be good predictors of TOpt, T90, mass-based photosynthetic rate at TOpt, and mass-based photosynthetic and respiration rate at 25 °C. However, Q10 was poorly predicted by these leaf economics spectrum traits except for shade leaves in winter. Our results suggest that model estimates of carbon metabolism in multilayered subtropical forest canopies do not necessitate independent parameterization of T90 and TOpt temperature responses in sun and shade leaves. Nevertheless, a deeper understanding and quantification of canopy variations in Q10 responses to temperature are necessary to confirm the generality of temperature-carbon metabolism trait responses and enhance ecosystem model estimates of carbon dynamics under future climate warming.

Persistent net release of carbon dioxide and methane from an Alaskan lowland boreal peatland complex

Permafrost degradation in peatlands is altering vegetation and soil properties and impacting net carbon storage. We studied four adjacent sites in Alaska with varied permafrost regimes, including a black spruce forest on a peat plateau with permafrost, two collapse scar bogs of different ages formed following thermokarst, and a rich fen without permafrost. Measurements included year-round eddy covariance estimates of net carbon dioxide (CO2 ), mid-April to October methane (CH4 ) emissions, and environmental variables. From 2011 to 2022, annual rainfall was above the historical average, snow water equivalent increased, and snow-season duration shortened due to later snow return. Seasonally thawed active layer depths also increased. During this period, all ecosystems acted as slight annual sources of CO2 (13-59 g C m-2  year-1 ) and stronger sources of CH4 (11-14 g CH4  m-2 from ~April to October). The interannual variability of net ecosystem exchange was high, approximately ±100 g C m-2  year-1 , or twice what has been previously reported across other boreal sites. Net CO2 release was positively related to increased summer rainfall and winter snow water equivalent and later snow return. Controls over CH4 emissions were related to increased soil moisture and inundation status. The dominant emitter of carbon was the rich fen, which, in addition to being a source of CO2 , was also the largest CH4 emitter. These results suggest that the future carbon-source strength of boreal lowlands in Interior Alaska may be determined by the area occupied by minerotrophic fens, which are expected to become more abundant as permafrost thaw increases hydrologic connectivity. Since our measurements occur within close proximity of each other (≤1 km2 ), this study also has implications for the spatial scale and data used in benchmarking carbon cycle models and emphasizes the necessity of long-term measurements to identify carbon cycle process changes in a warming climate.

Warming inhibits increases in vegetation net primary productivity despite greening in India

India is the second-highest contributor to the post-2000 global greening. However, with satellite data, here we show that this 18.51% increase in Leaf Area Index (LAI) during 2001-2019 fails to translate into increased carbon uptake due to warming constraints. Our analysis further shows 6.19% decrease in Net Primary Productivity (NPP) during 2001-2019 over the temporally consistent forests in India despite 6.75% increase in LAI. We identify hotspots of statistically significant decreasing trends in NPP over the key forested regions of Northeast India, Peninsular India, and the Western Ghats. Together, these areas contribute to more than 31% of the NPP of India (1274.8 TgC.year-1). These three regions are also the warming hotspots in India. Granger Causality analysis confirms that temperature causes the changes in net-photosynthesis of vegetation. Decreasing photosynthesis and stable respiration, above a threshold temperature, over these regions, as seen in observations, are the key reasons behind the declining NPP. Our analysis shows that warming has already started affecting carbon uptake in Indian forests and calls for improved climate resilient forest management practices in a warming world.

Evidence for widespread thermal optimality of ecosystem respiration

Ecosystem respiration (ER) is among the largest carbon fluxes between the biosphere and the atmosphere. Understanding the temperature response of ER is crucial for predicting the climate change-carbon cycle feedback. However, whether there is an apparent optimum temperature of ER ([Formula: see text]) and how it changes with temperature remain poorly understood. Here we analyse the temperature response curves of ER at 212 sites from global FLUXNET. We find that ER at 183 sites shows parabolic temperature response curves and [Formula: see text] at which ER reaches the maximum exists widely across biomes around the globe. Among the 15 biotic and abiotic variables examined, [Formula: see text] is mostly related to the optimum temperature of gross primary production (GPP, [Formula: see text]) and annual maximum daily temperature (Tmax). In addition, [Formula: see text] linearly increases with Tmax across sites and over vegetation types, suggesting its thermal adaptation. The adaptation magnitude of [Formula: see text], which is measured by the change in [Formula: see text] per unit change in Tmax, is positively correlated with the adaptation magnitude of [Formula: see text]. This study provides evidence of the widespread existence of [Formula: see text] and its thermal adaptation with Tmax across different biomes around the globe. Our findings suggest that carbon cycle models that consider the existence of [Formula: see text] and its adaptation have the potential to more realistically predict terrestrial carbon sequestration in a world with changing climate.

Similar patterns of leaf temperatures and thermal acclimation to warming in temperate and tropical tree canopies

As the global climate warms, a key question is how increased leaf temperatures will affect tree physiology and the coupling between leaf and air temperatures in forests. To explore the impact of increasing temperatures on plant performance in open air, we warmed leaves in the canopy of two mature evergreen forests, a temperate Eucalyptus woodland and a tropical rainforest. The leaf heaters consistently maintained leaves at a target of 4 °C above ambient leaf temperatures. Ambient leaf temperatures (Tleaf) were mostly coupled to air temperatures (Tair), but at times, leaves could be 8-10 °C warmer than ambient air temperatures, especially in full sun. At both sites, Tleaf was warmer at higher air temperatures (Tair > 25 °C), but was cooler at lower Tair, contrary to the 'leaf homeothermy hypothesis'. Warmed leaves showed significantly lower stomatal conductance (-0.05 mol m-2 s-1 or -43% across species) and net photosynthesis (-3.91 μmol m-2 s-1 or -39%), with similar rates in leaf respiration rates at a common temperature (no acclimation). Increased canopy leaf temperatures due to future warming could reduce carbon assimilation via reduced photosynthesis in these forests, potentially weakening the land carbon sink in tropical and temperate forests.

Acclimation of photosynthetic capacity and foliar respiration in Andean tree species to temperature change

Climate warming is causing compositional changes in Andean tropical montane forests (TMFs). These shifts are hypothesised to result from differential responses to warming of cold- and warm-affiliated species, with the former experiencing mortality and the latter migrating upslope. The thermal acclimation potential of Andean TMFs remains unknown. Along a 2000 m Andean altitudinal gradient, we planted individuals of cold- and warm-affiliated species (under common soil and irrigation), exposing them to the hot and cold extremes of their thermal niches, respectively. We measured the response of net photosynthesis (A_net ), photosynthetic capacity and leaf dark respiration (R_dark ) to warming/cooling, 5 months after planting. In all species, A_net and photosynthetic capacity at 25°C were highest when growing at growth temperatures (T_g ) closest to their thermal means, declining with warming and cooling in cold-affiliated and warm-affiliated species, respectively. When expressed at T_g , photosynthetic capacity and R_dark remained unchanged in cold-affiliated species, but the latter decreased in warm-affiliated counterparts. R_dark at 25°C increased with temperature in all species, but remained unchanged when expressed at T_g . Both species groups acclimated to temperature, but only warm-affiliated species decreased R_dark to photosynthetic capacity ratio at T_g as temperature increased. This could confer them a competitive advantage under future warming.

It's only natural: Plant respiration in unmanaged systems

Respiration plays a key role in the terrestrial carbon cycle and is a fundamental metabolic process in all plant tissues and cells. We review respiration from the perspective of plants that grow in their natural habitat and how it is influenced by wide-ranging elements at different scales, from metabolic substrate availability to shifts in climate. Decades of field-based measurements have honed our understanding of the biological and environmental controls on leaf, root, stem, and whole-organism respiration. Despite this effort, there remain gaps in our knowledge within and across species and ecosystems, especially in more challenging-to-measure tissues like roots. Recent databases of respiration rates and associated leaf traits from species representing diverse biomes, plant functional types, and regional climates have allowed for a wider-lens view at modeling this important CO2 flux. We also re-analyze published data sets to show that maximum leaf respiration rates (Rmax) in species from around the globe are related both to leaf economic traits and environmental variables (precipitation and air temperature), but that root respiration does not follow the same latitudinal trends previously published for leaf data. We encourage the ecophysiological community to continue to expand their study of plant respiration in tissues that are difficult to measure and at the whole plant and ecosystem levels to address outstanding questions in the field.

Thermal acclimation of leaf respiration is consistent in tropical and subtropical populations of two mangrove species

Populations from different climates often show unique growth responses to temperature, reflecting temperature adaptation. Yet, whether populations from different climates differ in physiological temperature acclimation remains unclear. Here, we test whether populations from differing thermal environments exhibit different growth responses to temperature and differences in temperature acclimation of leaf respiration. We grew tropical and subtropical populations of two mangrove species (Avicennia germinans and Rhizophora mangle) under ambient and experimentally warmed conditions in a common garden at the species' northern range limit. We quantified growth and temperature responses of leaf respiration (R) at seven time points over ~10 months. Warming increased productivity of tropical populations more than subtropical populations, reflecting a higher temperature optimum for growth. In both species, R measured at 25 °C declined as seasonal temperatures increased, demonstrating thermal acclimation. Contrary to our expectations, acclimation of R was consistent across populations and temperature treatments. However, populations differed in adjusting the temperature sensitivity of R (Q10) to seasonal temperatures. Following a freeze event, tropical Avicennia showed greater freeze damage than subtropical Avicennia, while both Rhizophora populations appeared equally susceptible. We found evidence of temperature adaptation at the whole-plant scale but little evidence for population differences in thermal acclimation of leaf physiology. Studies that examine potential costs and benefits of thermal acclimation in an evolutionary context may provide new insights into limits of thermal acclimation.

Rubisco deactivation and chloroplast electron transport rates co-limit photosynthesis above optimal leaf temperature in terrestrial plants

Net photosynthetic CO₂ assimilation rate (A_n) decreases at leaf temperatures above a relatively mild optimum (T_opt) in most higher plants. This decline is often attributed to reduced CO₂ conductance, increased CO₂ loss from photorespiration and respiration, reduced chloroplast electron transport rate (J), or deactivation of Ribulose-1,5-bisphosphate Carboxylase Oxygenase (Rubisco). However, it is unclear which of these factors can best predict species independent declines in A_n at high temperature. We show that independent of species, and on a global scale, the observed decline in A_n with rising temperatures can be effectively accounted for by Rubisco deactivation and declines in J. Our finding that A_n declines with Rubisco deactivation and J supports a coordinated down-regulation of Rubisco and chloroplast electron transport rates to heat stress. We provide a model that, in the absence of CO₂ supply limitations, can predict the response of photosynthesis to short-term increases in leaf temperature.

Improving the spatial and temporal estimation of ecosystem respiration using multi-source data and machine learning methods in a rainfed winter wheat cropland

The investigation of ecosystem respiration (RE) and its vital influential factors along with the timely and accurate detection of spatiotemporal variations in RE are essential for guiding agricultural production planning. RE observation in the plot region is primarily based on the laborious chamber method. However, upscaling the spatial-temporal estimates of RE at the canopy scale is still challenging. The present study conducted a field experiment to determine RE using the chamber method. A multi-rotor unmanned aerial vehicle (UAV) equipped with a multispectral camera was employed to acquire the canopy spectral data of wheat during each RE test experiment. Moreover, the agronomic indicators of aboveground plant biomass, leaf area index, leaf dry mass as well as agrometeorological and soil data were measured simultaneously. The study analyzed the potential of multi-information for estimating RE at the field scale and proposed two strategies for RE estimation. In addition, a semiempirical, yet Lloyd and Taylor-based, remote sensing model (LT1-NIR_V) was developed for estimating RE observed across different growth stages with a small margin of error (coefficient of determination [R²] = 0.60-0.64, root-mean-square error [RMSE] = 285.98-316.19 mg m^-2 h^-1). Further, five machine learning (ML) algorithms were utilized to independently estimate RE using two different datasets. The rigorous analyses, which included statistical comparison and cross-validation for estimating RE, confirmed that the XGBoost model, with the highest R² and lowest RMSE (R² = 0.88 and RMSE = 172.70 mg m^-2 h^-1), performed the best among the evaluated ML models. The LT1-NIR_V model was less effective in estimating RE compared with the other ML models. Based on this comprehensive comparison analysis, the ML model can successfully estimate variations in wheat field RE using high-resolution UAV multispectral images and environmental factors from the wheat cropland system, thereby providing a valuable reference for monitoring and upscaling RE observations.

Toward mechanistic modeling and rational engineering of plant respiration

Plant respiration not only provides energy to support all cellular processes, including biomass production, but also plays a major role in the global carbon cycle. Therefore, modulation of plant respiration can be used to both increase the plant yield and mitigate the effects of global climate change. Mechanistic modeling of plant respiration at sufficient biochemical detail can provide key insights for rational engineering of this process. Yet, despite its importance, plant respiration has attracted considerably less modeling effort in comparison to photosynthesis. In this update review, we highlight the advances made in modeling of plant respiration, emphasizing the gradual but important change from phenomenological to models based on first principles. We also provide a detailed account of the existing resources that can contribute to resolving the challenges in modeling plant respiration. These resources point at tangible improvements in the representation of cellular processes that contribute to CO2 evolution and consideration of kinetic properties of underlying enzymes to facilitate mechanistic modeling. The update review emphasizes the need to couple biochemical models of respiration with models of acclimation and adaptation of respiration for their effective usage in guiding breeding efforts and improving terrestrial biosphere models tailored to future climate scenarios.

A high-resolution monitoring approach of urban CO2 fluxes. Part 1 - bottom-up model development

Monitoring carbon dioxide (CO₂) emissions of urban areas is increasingly important to assess the progress towards the Paris Agreement goals for climate neutrality. Cities are currently voluntarily developing their local inventories, however, the approaches used across different cities are not systematically assessed, present consistency issues, neglect the biogenic fluxes and have restricted spatial and temporal resolution. In order to assess the accuracy of the urban emission inventories and provide information which is useful for planning local climate change mitigation actions, high resolution modelling approaches combined or evaluated with atmospheric observations are needed. This study presents a new high-resolution bottom-up (BU) model which provides hourly maps of all major components contributing to the local urban surface CO₂ flux (i.e. building emissions, traffic emissions, human respiration, soil respiration, plant respiration, plant photosynthetic uptake) and can therefore be used for direct comparison with in-situ atmospheric observations and development of local scale atmospheric inversion methodologies. The model design aims to be simple and flexible using inputs that are available in most cities, facilitating transferability to different locations. The inputs are primarily based on open geospatial datasets, census information, road traffic monitoring and basic meteorological parameters. The model is applied on the city centre of Basel, Switzerland, for the year 2018 and the results are compared to a local inventory. It is demonstrated that the model captures the highly dynamic spatiotemporal variability of the urban CO₂ fluxes according to main environmental drivers, population activity dynamics and geospatial information proxies. The annual modelled emissions from buildings and traffic are estimated 14.8 % and 9 % lower than the respective information derived by the local inventory. The differences are mainly attributed to the emissions from the industrial areas and the highways which are beyond the geographical coverage of the model.

Precipitation and temperature regulate species diversity, plant coverage and aboveground biomass through opposing mechanisms in large-scale grasslands

INTRODUCTION

Although the relationships between species diversity and aboveground biomass (AGB) are highly debated in grassland ecosystems, it is not well understood how climatic factors influence AGB directly and indirectly via plant coverage and species diversity in large-scale grasslands along a topographic gradient. In doing so, we hypothesized that climatic factors would regulate plant coverage, species diversity and AGB due to maintaining plant metabolic and ecological processes, but the relationship of plant coverage with AGB would be stronger than species diversity due to covering physical niche space.

METHODS

To test the proposed hypothesis, we collected data for calculations of species richness, evenness, plant coverage and AGB across 123 grassland sites (i.e., the mean of 3 plots in each site) dominated by Leymus chinensis in northern China. We used a structural equation model for linking the direct and indirect effects of topographic slope, mean annual precipitation and temperature on AGB via plant coverage, species richness, and evenness through multiple complex pathways.

RESULTS

We found that plant coverage increased AGB, but species evenness declined AGB better than species richness. Topographic slope influenced AGB directly but not indirectly via plant coverage and species diversity, whereas temperature and precipitation increased with increasing topographic slope. Regarding opposing mechanisms, on the one hand, precipitation increased AGB directly and indirectly via plant coverage as compared to species richness and evenness. On the other hand, temperature declined AGB indirectly via plant coverage but increased via species evenness as compared to species richness, whereas the direct effect was negligible.

DISCUSSION

Our results show that niche complementarity and selection effects are jointly regulating AGB, but these processes are dependent on climatic factors. Plant coverage promoted the coexistence of species but depended greatly on precipitation and temperature. Our results highlight that precipitation and temperature are two key climatic drivers of species richness, evenness, plant coverage and AGB through complex direct and indirect pathways. Our study suggests that grasslands are sensitive to climate change, i.e., a decline in water availability and an increase in atmospheric heat. We argue that temperature and precipitation should be considered in grassland management for higher productivity in the context of both plant coverage and species diversity which underpin animals and human well-being.

Limited acclimation of leaf traits and leaf temperatures in a subtropical urban heat island

The consequences of rising temperatures for trees will vary between species based on their abilities to acclimate their leaf thermoregulatory traits and photosynthetic thermal tolerances. We tested the hypotheses that adult trees in warmer growing conditions (i) acclimate their thermoregulatory traits to regulate leaf temperatures, (ii) acclimate their thermal tolerances such that tolerances are positively correlated with leaf temperature and (iii) that species with broader thermal niche breadths have greater acclimatory abilities. To test these hypotheses, we measured leaf traits and thermal tolerances of seven focal tree species across steep thermal gradients in Miami's urban heat island. We found that some functional traits varied significantly across air temperatures within species. For example, leaf thickness increased with maximum air temperature in three species, and leaf mass per area and leaf reflectance both increased with air temperature in one species. Only one species was marginally more homeothermic than expected by chance due to acclimation of its thermoregulatory traits, but this acclimation was insufficient to offset elevated air temperatures. Thermal tolerances acclimated to higher maximum air temperatures in two species. As a result of limited acclimation, leaf thermal safety margins (TSMs) were narrower for trees in hotter areas. We found some support for our hypothesis that species with broader thermal niches are better at acclimating to maintain more stable TSMs across the temperature gradients. These findings suggest that trees have limited abilities to acclimate to high temperatures and that thermal niche specialists may be at a heightened risk of thermal stress as global temperatures continue to rise.

Nocturnal plant respiration is under strong non-temperature control

Most biological rates depend on the rate of respiration. Temperature variation is typically considered the main driver of daily plant respiration rates, assuming a constant daily respiration rate at a set temperature. Here, we show empirical data from 31 species from temperate and tropical biomes to demonstrate that the rate of plant respiration at a constant temperature decreases monotonically with time through the night, on average by 25% after 8 h of darkness. Temperature controls less than half of the total nocturnal variation in respiration. A new universal formulation is developed to model and understand nocturnal plant respiration, combining the nocturnal decrease in the rate of plant respiration at constant temperature with the decrease in plant respiration according to the temperature sensitivity. Application of the new formulation shows a global reduction of 4.5 -6 % in plant respiration and an increase of 7-10% in net primary production for the present-day.

No evidence of canopy-scale leaf thermoregulation to cool leaves below air temperature across a range of forest ecosystems

Understanding and predicting the relationship between leaf temperature (T_leaf) and air temperature (T_air) is essential for projecting responses to a warming climate, as studies suggest that many forests are near thermal thresholds for carbon uptake. Based on leaf measurements, the limited leaf homeothermy hypothesis argues that daytime T_leaf is maintained near photosynthetic temperature optima and below damaging temperature thresholds. Specifically, leaves should cool below T_air at higher temperatures (i.e., > ∼25-30°C) leading to slopes <1 in T_leaf/T_air relationships and substantial carbon uptake when leaves are cooler than air. This hypothesis implies that climate warming will be mitigated by a compensatory leaf cooling response. A key uncertainty is understanding whether such thermoregulatory behavior occurs in natural forest canopies. We present an unprecedented set of growing season canopy-level leaf temperature (T_can) data measured with thermal imaging at multiple well-instrumented forest sites in North and Central America. Our data do not support the limited homeothermy hypothesis: canopy leaves are warmer than air during most of the day and only cool below air in mid to late afternoon, leading to T_can/T_air slopes >1 and hysteretic behavior. We find that the majority of ecosystem photosynthesis occurs when canopy leaves are warmer than air. Using energy balance and physiological modeling, we show that key leaf traits influence leaf-air coupling and ultimately the T_can/T_air relationship. Canopy structure also plays an important role in T_can dynamics. Future climate warming is likely to lead to even greater T_can, with attendant impacts on forest carbon cycling and mortality risk.

Dynamic Energy Budget models: fertile ground for understanding resource allocation in plants in a changing world

Climate change is having dramatic effects on the diversity and distribution of species. Many of these effects are mediated by how an organism's physiological patterns of resource allocation translate into fitness through effects on growth, survival and reproduction. Empirically, resource allocation is challenging to measure directly and so has often been approached using mathematical models, such as Dynamic Energy Budget (DEB) models. The fact that all plants require a very similar set of exogenous resources, namely light, water and nutrients, integrates well with the DEB framework in which a small number of variables and processes linked through pathways represent an organism's state as it changes through time. Most DEB theory has been developed in reference to animals and microorganisms. However, terrestrial vascular plants differ from these organisms in fundamental ways that make resource allocation, and the trade-offs and feedbacks arising from it, particularly fundamental to their life histories, but also challenging to represent using existing DEB theory. Here, we describe key features of the anatomy, morphology, physiology, biochemistry, and ecology of terrestrial vascular plants that should be considered in the development of a generic DEB model for plants. We then describe possible approaches to doing so using existing DEB theory and point out features that may require significant development for DEB theory to accommodate them. We end by presenting a generic DEB model for plants that accounts for many of these key features and describing gaps that would need to be addressed for DEB theory to predict the responses of plants to climate change. DEB models offer a powerful and generalizable framework for modelling resource allocation in terrestrial vascular plants, and our review contributes a framework for expansion and development of DEB theory to address how plants respond to anthropogenic change.

Stomatal conductance, not biochemistry, drives low temperature acclimation of photosynthesis in Populus balsamifera, regardless of nitrogen availability

Low-temperature thermal acclimation may require adjustments to N and water use to sustain photosynthesis because of slow enzyme functioning and high water viscosity. However, understanding of photosynthetic acclimation to temperatures below 11 °C is limited. We acclimated Populus balsamifera to 6 °C and 10 °C (6A and 10A, respectively) and provided the trees with either high or low N fertilizer. We measured net CO₂ assimilation (A_net ), stomatal conductance (g_s ), maximum rates of Rubisco carboxylation (V_cmax ), electron transport (J_max ) and dark respiration (R_d ) at leaf temperatures of 2, 6, 10, 14 and 18 °C, along with leaf N concentrations. The 10A trees had higher A_net than the 6A trees at warmer leaf temperatures, which was correlated with higher g_s in the 10A trees. The instantaneous temperature responses of V_cmax , J_max and R_d were similar for trees from both acclimation temperatures. While soil N availability increased leaf N concentrations, this had no effect on acclimation of photosynthesis or respiration. Our results indicate that acclimation below 11 °C occurred primarily through changes in stomatal conductance, not photosynthetic biochemistry, and was unaffected by short-term N supply. Thermal acclimation of stomatal conductance should therefore be a priority for future carbon cycle model development.

Plant photosynthetic overcompensation under nocturnal warming: lack of evidence in subtropical evergreen trees

BACKGROUND AND AIMS

Increased plant photosynthesis under nocturnal warming is a negative feedback mechanism to overcompensate for night-time carbon loss to mitigate climate warming. This photosynthetic overcompensation effect has been observed in dry deciduous ecosystems but whether it exists in subtropical wet forest trees is unclear.

METHODS

Two subtropical evergreen tree species (Schima superba and Castanopsis sclerophylla) were grown in a greenhouse and exposed to ambient and elevated night-time temperature. The occurrence of the photosynthetic overcompensation effect was determined by measuring daytime and night-time leaf gas exchange and non-structural carbohydrate (NSC) concentration.

KEY RESULTS

A reduction in leaf photosynthesis for both species and an absence of persistent photosynthetic overcompensation were observed. The photosynthetic overcompensation effect was transient in S. superba due to respiratory acclimation and stomatal limitation. For S. superba, nocturnal warming resulted in insufficient changes in night-time respiration and NSC concentration to stimulate overcompensation and inhibited leaf stomatal conductance by increasing the leaf-to-air vapour pressure deficit.

CONCLUSIONS

The results indicate that leaf stomatal conductance is important for the photosynthetic overcompensation effect in different tree species. The photosynthetic overcompensation effect under nocturnal warming may be a transient occurrence rather than a persistent mechanism in subtropical forest ecosystems.

Variation in White spruce needle respiration at the species range limits: A potential impediment to Northern expansion

White spruce (Picea glauca) spans a massive range, yet the variability in respiratory physiology and related implications for tree carbon balance at the extremes of this distribution remain as enigmas. Working at both the most northern and southern extents of the distribution range more than 5000 km apart, we measured the short-term temperature response of dark respiration (R/T) at upper and lower canopy positions. R/T curves were fit to both polynomial and thermodynamic models so that model parameters could be compared among locations, canopy positions, and with previously published data. Respiration measured at 25°C (R₂₅ ) was 68% lower at the southern location than at the northern location, resulting in a significantly lower intercept in R/T response in temperate trees. Only at the southern location did upper canopy leaves have a steeper temperature response than lower canopy leaves, likely reflecting canopy gradients in light. At the northern range limit respiration is nearly twice that of the average R₂₅ reported in a global leaf respiration database. We predict that without significant thermal acclimation, respiration will increase with projected end-of-the-century warming and will likely constrain the future range limits of this important boreal species.

An improved representation of the relationship between photosynthesis and stomatal conductance leads to more stable estimation of conductance parameters and improves the goodness-of-fit across diverse data sets

Stomata play a central role in surface-atmosphere exchange by controlling the flux of water and CO₂ between the leaf and the atmosphere. Representation of stomatal conductance (g_sw ) is therefore an essential component of models that seek to simulate water and CO₂ exchange in plants and ecosystems. For given environmental conditions at the leaf surface (CO₂ concentration and vapor pressure deficit or relative humidity), models typically assume a linear relationship between g_sw and photosynthetic CO₂ assimilation (A). However, measurement of leaf-level g_sw response curves to changes in A are rare, particularly in the tropics, resulting in only limited data to evaluate this key assumption. Here, we measured the response of g_sw and A to irradiance in six tropical species at different leaf phenological stages. We showed that the relationship between g_sw and A was not linear, challenging the key assumption upon which optimality theory is based-that the marginal cost of water gain is constant. Our data showed that increasing A resulted in a small increase in g_sw at low irradiance, but a much larger increase at high irradiance. We reformulated the popular Unified Stomatal Optimization (USO) model to account for this phenomenon and to enable consistent estimation of the key conductance parameters g₀ and g₁ . Our modification of the USO model improved the goodness-of-fit and reduced bias, enabling robust estimation of conductance parameters at any irradiance. In addition, our modification revealed previously undetectable relationships between the stomatal slope parameter g₁ and other leaf traits. We also observed nonlinear behavior between A and g_sw in independent data sets that included data collected from attached and detached leaves, and from plants grown at elevated CO₂ concentration. We propose that this empirical modification of the USO model can improve the measurement of g_sw parameters and the estimation of plant and ecosystem-scale water and CO₂  fluxes.

Tropical rainforest species have larger increases in temperature optima with warming than warm-temperate rainforest trees

While trees can acclimate to warming, there is concern that tropical rainforest species may be less able to acclimate because they have adapted to a relatively stable thermal environment. Here we tested whether the physiological adjustments to warming differed among Australian tropical, subtropical and warm-temperate rainforest trees. Photosynthesis and respiration temperature responses were quantified in six Australian rainforest seedlings of tropical, subtropical and warm-temperate climates grown across four growth temperatures in a glasshouse. Temperature-response models were fitted to identify mechanisms underpinning the response to warming. Tropical and subtropical species had higher temperature optima for photosynthesis (T_optA ) than temperate species. There was acclimation of T_optA to warmer growth temperatures. The rate of acclimation (0.35-0.78°C °C^-1 ) was higher in tropical and subtropical than in warm-temperate trees and attributed to differences in underlying biochemical parameters, particularly increased temperature optima of V_cmax25 and J_max25 . The temperature sensitivity of respiration (Q₁₀ ) was 24% lower in tropical and subtropical compared with warm-temperate species. Overall, tropical and subtropical species had a similar capacity to acclimate to changes in growth temperature as warm-temperate species, despite being grown at higher temperatures. Quantifying the physiological acclimation in rainforests can improve accuracy of future climate predictions and assess their potential vulnerability to warming.

Increased tropical vegetation respiration is dually induced by El Niño and upper atmospheric warm anomalies

Tropical vegetation respiration (TVR) is affected by extreme climate change. As it is very difficult to directly observe TVR, our understanding of the land-ocean-atmosphere carbon cycle, and particularly the regulatory effect of El Niño-Southern Oscillation (ENSO) on TVR and the land-atmosphere carbon balance, is very limited. Therefore, usingModerate Resolution Imaging Spectroradiometer (MODIS) products and meteorological data, we investigated the response of TVR to changes in ENSO during 2000-2015. The influence of El Niño on TVR was approximately 10.8% higher than that of La Niña. During El Niño years, a significant and anomalous increase in thermal measures related to TVR and ENSO and a significant and anomalous decrease in related hydrological measures favor the formation of warmer and drier climate conditions. Furthermore, the zonal distributions of air temperature and vertical velocity at 200-1000 hPa during El Niño years show that a stronger atmospheric inversion over tropical regions causes an increase in the surface temperature. Moreover, anomalous atmospheric subsidence inhibits the upward transport of water vapor, leading to a decrease in the cloud formation probability and reduced precipitation. In summary, increased surface temperatures caused by increased solar radiation and enhanced atmospheric inversion and decreased precipitation cause warmer and drier climate conditions, which forces TVR to increase. As TVR constitutes the key node of the land-atmosphere‑carbon cycle process, we focus on TVR and its close linkage with ENSO events and further establish a knowledge framework for understanding the land-atmosphere-ocean carbon cycle. This study deepens our understanding of not only the mechanism of the land-atmosphere carbon balance but also the ocean-induced terrestrial ecosystem processes spurred by ENSO-involved climate change. PLAIN LANGUAGE SUMMARY: Vegetation respiration regulates the carbon balance of the land and atmosphere. As it is very difficult to directly observe vegetation respiration, our understanding of the land-ocean-atmosphere carbon cycle involved and the roles of vegetation respiration and El Niño-Southern Oscillation (ENSO) in regulating the land-atmosphere carbon balance is very limited. Therefore, using MODIS products and meteorological data, we investigated the response of tropical vegetation respiration to changes in ENSO during 2000-2015. We found that during El Niño years, warmer and drier climate conditions over tropical regions increased vegetation respiration. Exacerbating the warmer and drier climate conditions, upper atmospheric warm anomalies further caused a remarkable increase in tropical vegetation respiration. Based on the land-atmosphere‑carbon cycle process, we establish a knowledge framework for understanding the land-atmosphere-ocean carbon cycle. This knowledge deepens our understanding of not only the mechanism of the land-atmosphere carbon balance but also the ocean-induced terrestrial ecosystem processes spurred by ENSO-involved climate change.

Mechanistic drivers of stem respiration: A modelling exercise across species and seasons

Stem respiration (R_S ) plays a crucial role in plant carbon budgets. However, its poor understanding limits our ability to model woody tissue and whole-tree respiration. A biophysical model of stem water and carbon fluxes (TReSpire) was calibrated on cedar, maple and oak trees during spring and late summer. For this, stem sap flow, water potential, diameter variation, temperature, CO₂ efflux, allometry and biochemistry were monitored. Shoot photosynthesis (P_N ) and nonstructural carbohydrates (NSC) were additionally measured to evaluate source-sink relations. The highest R_S and stem growth was found in maple and oak during spring, both being seasonally decoupled from P_N and [NSC]. Temperature largely affected maintenance respiration (R_M ) in the short term, but temperature-normalized R_M was highly variable on a seasonal timescale. Overall, most of the respired CO₂ radially diffused to the atmosphere (>87%) while the remainder was transported upward with the transpiration stream. The modelling exercise highlights the sink-driven behaviour of R_S and the significance of overall metabolic activity on nitrogen (N) allocation patterns and N-normalized respiratory costs to capture R_S variability over the long term. These insights should be considered when modelling plant respiration, whose representation is currently biased towards a better understanding of leaf metabolism.

Temperature responses of photosynthesis and respiration in evergreen trees from boreal to tropical latitudes

Evergreen species are widespread across the globe, representing two major plant functional forms in terrestrial models. We reviewed and analysed the responses of photosynthesis and respiration to warming in 101 evergreen species from boreal to tropical biomes. Summertime temperatures affected both latitudinal gas exchange rates and the degree of responsiveness to experimental warming. The decrease in net photosynthesis at 25°C (A_net25 ) was larger with warming in tropical climates than cooler ones. Respiration at 25°C (R₂₅ ) was reduced by 14% in response to warming across species and biomes. Gymnosperms were more sensitive to greater amounts of warming than broadleaved evergreens, with A_net25 and R₂₅ reduced c. 30-40% with > 10°C warming. While standardised rates of carboxylation (V_cmax25 ) and electron transport (J_max25 ) adjusted to warming, the magnitude of this adjustment was not related to warming amount (range 0.6-16°C). The temperature optimum of photosynthesis (T_optA ) increased on average 0.34°C per °C warming. The combination of more constrained acclimation of photosynthesis and increasing respiration rates with warming could possibly result in a reduced carbon sink in future warmer climates. The predictable patterns of thermal acclimation across biomes provide a strong basis to improve modelling predictions of the future terrestrial carbon sink with warming.

Pastures and Climate Extremes: Impacts of Cool Season Warming and Drought on the Productivity of Key Pasture Species in a Field Experiment

Shifts in the timing, intensity and/or frequency of climate extremes, such as severe drought and heatwaves, can generate sustained shifts in ecosystem function with important ecological and economic impacts for rangelands and managed pastures. The Pastures and Climate Extremes experiment (PACE) in Southeast Australia was designed to investigate the impacts of a severe winter/spring drought (60% rainfall reduction) and, for a subset of species, a factorial combination of drought and elevated temperature (ambient +3°C) on pasture productivity. The experiment included nine common pasture and Australian rangeland species from three plant functional groups (C3 grasses, C4 grasses and legumes) planted in monoculture. Winter/spring drought resulted in productivity declines of 45% on average and up to 74% for the most affected species (Digitaria eriantha) during the 6-month treatment period, with eight of the nine species exhibiting significant yield reductions. Despite considerable variation in species' sensitivity to drought, C4 grasses were more strongly affected by this treatment than C3 grasses or legumes. Warming also had negative effects on cool-season productivity, associated at least partially with exceedance of optimum growth temperatures in spring and indirect effects on soil water content. The combination of winter/spring drought and year-round warming resulted in the greatest yield reductions. We identified responses that were either additive (Festuca), or less-than-additive (Medicago), where warming reduced the magnitude of drought effects. Results from this study highlight the sensitivity of diverse pasture species to increases in winter and spring drought severity similar to those predicted for this region, and that anticipated benefits of cool-season warming are unlikely to be realized. Overall, the substantial negative impacts on productivity suggest that future, warmer, drier climates will result in shortfalls in cool-season forage availability, with profound implications for the livestock industry and natural grazer communities.

Temperature sensitivity of woody nitrogen fixation across species and growing temperatures

The future of the land carbon sink depends on the availability of nitrogen (N)^1,2 and, specifically, on symbiotic N fixation^3-8, which can rapidly alleviate N limitation. The temperature response of symbiotic N fixation has been hypothesized to explain the global distribution of N-fixing trees^9,10 and is a key part of some terrestrial biosphere models (TBMs)^3,7,8, yet there are few data to constrain the temperature response of symbiotic N fixation. Here we show that optimal temperatures for N fixation in four tree symbioses are in the range 29.0-36.9 °C, well above the 25.2 °C optimum currently used by TBMs. The shape of the response to temperature is also markedly different to the function used by TBMs (asymmetric rather than symmetric). We also show that N fixation acclimates to growing temperature (hence its range of optimal temperatures), particularly in our two tropical symbioses. Surprisingly, optimal temperatures were 5.2 °C higher for N fixation than for photosynthesis, suggesting that plant carbon and N gain are decoupled with respect to temperature. These findings may help explain why N-fixing tree abundance is highest where annual maximum temperatures are >35 °C (ref. ¹⁰) and why N-fixing symbioses evolved during a warm period in the Earth's history^11,12. Everything else being equal, our findings indicate that climate warming will probably increase N fixation, even in tropical ecosystems, in direct contrast to past projections⁸.

Wheat respiratory O2 consumption falls with night warming alongside greater respiratory CO2 loss and reduced biomass

Warming nights are correlated with declining wheat growth and yield. As a key determinant of plant biomass, respiration consumes O2 as it produces ATP and releases CO2 and is typically reduced under warming to maintain metabolic efficiency. We compared the response of respiratory O2 and CO2 flux to multiple night and day warming treatments in wheat leaves and roots, using one commercial (Mace) and one breeding cultivar grown in controlled environments. We also examined the effect of night warming and a day heatwave on the capacity of the ATP-uncoupled alternative oxidase (AOX) pathway. Under warm nights, plant biomass fell, respiratory CO2 release measured at a common temperature was unchanged (indicating higher rates of CO2 release at prevailing growth temperature), respiratory O2 consumption at a common temperature declined, and AOX pathway capacity increased. The uncoupling of CO2 and O2 exchange and enhanced AOX pathway capacity suggest a reduction in plant energy demand under warm nights (lower O2 consumption), alongside higher rates of CO2 release under prevailing growth temperature (due to a lack of down-regulation of respiratory CO2 release). Less efficient ATP synthesis, teamed with sustained CO2 flux, could thus be driving observed biomass declines under warm nights.

Growth temperature affects O2 consumption rates and plasticity of respiratory flux to support shoot growth at various growth temperatures

The temperature dependence of respiration rates and their acclimation to growth temperature vary among species/ecotypes, but the details remain unclear. Here, we compared the temperature dependence of shoot O₂ consumption rates among Arabidopsis thaliana ecotypes to clarify how the temperature dependence and their acclimation to temperature differ among ecotypes, and how these differences relate to shoot growth. We examined growth analysis, temperature dependence of O₂ consumption rates, and protein amounts of the respiratory chain components in shoots of twelve ecotypes of A. thaliana grown at three different temperatures. The temperature dependence of the O₂ consumption rates were fitted to the modified Arrhenius model. The dynamic response of activation energy to measurement temperature was different among growth temperatures, suggesting that the plasticity of respiratory flux to temperatures differs among growth temperatures. The similar values of activation energy at growth temperature among ecotypes suggest that a similar process may determine the O₂ consumption rates at the growth temperature in any ecotype. These results suggest that the growth temperature affects not only the absolute rate of O₂ consumption but also the plasticity of respiratory flux in response to temperature, supporting the acclimation of shoot growth to various temperatures.

Model responses to CO2 and warming are underestimated without explicit representation of Arctic small-mammal grazing

We use a simple model of coupled carbon and nitrogen cycles in terrestrial ecosystems to examine how "explicitly representing grazers" vs. "having grazer effects implicitly aggregated in with other biogeochemical processes in the model" alters predicted responses to elevated carbon dioxide and warming. The aggregated approach can affect model predictions because grazer-mediated processes can respond differently to changes in climate compared with the processes with which they are typically aggregated. We use small-mammal grazers in a tundra as an example and find that the typical three-to-four-year cycling frequency is too fast for the effects of cycle peaks and troughs to be fully manifested in the ecosystem biogeochemistry. We conclude that implicitly aggregating the effects of small-mammal grazers with other processes results in an underestimation of ecosystem response to climate change, relative to estimations in which the grazer effects are explicitly represented. The magnitude of this underestimation increases with grazer density. We therefore recommend that grazing effects be incorporated explicitly when applying models of ecosystem response to global change.

Temperature acclimation of leaf respiration differs between marsh and mangrove vegetation in a coastal wetland ecotone

Temperature acclimation of leaf respiration (R) is an important determinant of ecosystem responses to temperature and the magnitude of temperature-CO₂ feedbacks as climate warms. Yet, the extent to which temperature acclimation of R exhibits a common pattern across different growth conditions, ecosystems, and plant functional types remains unclear. Here, we measured the short-term temperature response of R at six time points over a 10-month period in two coastal wetland species (Avicennia germinans [C₃ mangrove] and Spartina alterniflora [C₄ marsh grass]) growing under ambient and experimentally warmed temperatures at two sites in a marsh-mangrove ecotone. Leaf nitrogen (N) was determined on a subsample of leaves to explore potential coupling of R and N. We hypothesized that both species would reduce R at 25°C (R²⁵ ) and the short-term temperature sensitivity of R (Q₁₀ ) as air temperature (T_air ) increased across seasons, but the decline would be stronger in Avicennia than in Spartina. For each species, we hypothesized that seasonal temperature acclimation of R would be equivalent in plants grown under ambient and warmed temperatures, demonstrating convergent acclimation. Surprisingly, Avicennia generally increased R²⁵ with increasing growth temperature, although the Q₁₀ declined as seasonal temperatures increased and did so consistently across sites and treatments. Weak temperature acclimation resulted in reduced homeostasis of R in Avicennia. Spartina reduced R²⁵ and the Q₁₀ as seasonal temperatures increased. In Spartina, seasonal temperature acclimation was largely consistent across sites and treatments resulting in greater respiratory homeostasis. We conclude that co-occurring coastal wetland species may show contrasting patterns of respiratory temperature acclimation. Nonetheless, leaf N scaled positively with R²⁵ in both species, highlighting the importance of leaf N in predicting respiratory capacity across a range of growth temperatures. The patterns of respiratory temperature acclimation shown here may improve the predictions of temperature controls of CO₂ fluxes in coastal wetlands.

Climate change impacts plant carbon balance, increasing mean future carbon use efficiency but decreasing total forest extent at dry range edges

Carbon use efficiency (CUE) represents how efficient a plant is at translating carbon gains through gross primary productivity (GPP) into net primary productivity (NPP) after respiratory costs (R_a ). CUE varies across space with climate and species composition, but how CUE will respond to climate change is largely unknown due to uncertainty in R_a at novel high temperatures. We use a plant physiological model validated against global CUE observations and LIDAR vegetation canopy height data and find that model-predicted decreases in CUE are diagnostic of transitions from forests to shrubland at dry range edges. Under future climate scenarios, we show mean growing season CUE increases in core forested areas, but forest extent decreases at dry range edges, with substantial uncertainty in absolute CUE due to uncertainty in R_a . Our results highlight that future forest resilience is nuanced and controlled by multiple competing mechanisms.

Foliar respiration is related to photosynthetic, growth and carbohydrate response to experimental drought and elevated temperature

Short-term plant respiration (R) increases exponentially with rising temperature, but drought could reduce respiration by reducing growth and metabolism. Acclimation may alter these responses. We examined if species with different drought responses would differ in foliar R response to +4.8°C temperature and -45% precipitation in a field experiment with mature piñon and juniper trees, and if any differences between species were related to differences in photosynthesis rates, shoot growth and nonstructural carbohydrates (NSCs). Short-term foliar R had a Q₁₀ of 1.6 for piñon and 2.6 for juniper. Piñon foliar R did not respond to the +4.8°C temperatures, but R increased 1.4× for juniper. Across treatments, piñon foliage had higher growth, lower NSC content, 29% lower photosynthesis rates, and 44% lower R than juniper. Removing 45% precipitation had little impact on R for either species. Species differences in the response of R under elevated temperature were related to substrate availability and stomatal response to leaf water potential. Despite not acclimating to the higher temperature and having higher R than piñon, greater substrate availability in juniper suggests it could supply respiratory demand for much longer than piñon. Species responses will be critical in ecosystem response to a warmer climate.

Genotypic variation of the interactive effects of elevated temperature and CO2 on leaf gas exchange and early growth of sugarcane

Increased atmospheric CO₂ and consequent increases in temperature are two prominent features of climate change, a major challenge to crops. Here, our objectives were to determine: (1) the responses of sugarcane during the first 90 days of elevated CO₂ (ECO₂ ) and elevated temperature (ETem), both individually and together, and (2) the genotypic variation of these responses. Eight varieties were grown both in open-top chambers in a factorial combination of ambient/ECO₂ concentrations (344-351/777-779 ppm) and ambient/ETem (34.9-35.6/36.6-38.4°C) and in open fields. Significant treatment × variety interaction effects were observed on leaf net photosynthetic rate (A_n ), stomatal conductance (g_s ), transpiration rate (E_l ), and instantaneous transpiration efficiency (T_E ). In most varieties, ECO₂ alone did not affect A_n, but the combination of ECO₂ and ETem decreased A_n . ECO₂ decreased g_s and E_l while increasing T_E in all varieties. These effects were amplified when ETem was combined with ECO₂ . ETem alone had variable effects on A_n and g_s depending on variety, while it increased E_l and did not affect T_E in a majority of varieties. Germination, tillering and stem diameter were not affected by treatments and did not show varietal variation. Leaf water potential, chlorophyll (spad), leaf area, and aboveground dry weight per plant showed varietal variations but were not affected by treatments. The variable responses to ETem and the significant genotypic variation to ECO₂ and elevated temperature (ETem) observed in this work, both individually and together, demonstrate a considerable scope to breed sugarcane varieties for a future high-CO₂ and warmer climate.

The physiological acclimation and growth response of Populus trichocarpa to warming

Plant metabolic acclimation to thermal stress remains underrepresented in current global climate models. Gaps exist in our understanding of how metabolic processes (i.e., photosynthesis, respiration) acclimate over time and how aboveground versus belowground acclimation differs. We measured the thermal acclimation of Populus trichocarpa, comparing aboveground versus belowground physiology over time. Ninety genetically identical ramets were propagated in mesocosms that separated root and microbial components. After establishment at 25°C for 6 weeks, 60 clones were warmed +4 or +8°C and monitored for 10 weeks, measuring photosynthesis (A), leaf respiration (R), soil respiration (R_s ), root plus soil respiration (R_s+r ), and root respiration (R_r ). We observed thermal acclimation in both A and R, with rates initially increasing, then declining as the thermal photosynthetic optimum (T_opt ) and the temperature-sensitivity (Q₁₀ ) of respiration adjusted to warmer conditions. Photosynthetic acclimation was constructive, based on an increase in both T_opt and peak A. Belowground, R_s+r decreased linearly with warming, while R_s rates declined abruptly, then remained constant with additional warming. Plant biomass was greatest at +4°C, with 30% allocated belowground. Rates of mass-based R_r were similar among treatments; however, root nitrogen declined at +8°C leading to less mass nitrogen-based R_r in that treatment. The Q₁₀ -temperature relationship of R_r was affected by warming, leading to differing values among treatments. Aboveground acclimation exceeded belowground acclimation, and plant nitrogen-use mediated the acclimatory response. Results suggest that moderate climate warming (+4°C) may lead to acclimation and increased plant biomass production but increases in production could be limited with severe warming (+8°C).

High Leaf Respiration Rates May Limit the Success of White Spruce Saplings Growing in the Kampfzone at the Arctic Treeline

Arctic Treeline is the transition from the boreal forest to the treeless tundra and may be determined by growing season temperatures. The physiological mechanisms involved in determining the relationship between the physical and biological environment and the location of treeline are not fully understood. In Northern Alaska, we studied the relationship between temperature and leaf respiration in 36 white spruce (Picea glauca) trees, sampling both the upper and lower canopy, to test two research hypotheses. The first hypothesis is that upper canopy leaves, which are more directly coupled to the atmosphere, will experience more challenging environmental conditions and thus have higher respiration rates to facilitate metabolic function. The second hypothesis is that saplings [stems that are 5-10cm DBH (diameter at breast height)] will have higher respiration rates than trees (stems ≥10cm DBH) since saplings represent the transition from seedlings growing in the more favorable aerodynamic boundary layer, to trees which are fully coupled to the atmosphere but of sufficient size to persist. Respiration did not change with canopy position, however respiration at 25°C was 42% higher in saplings compared to trees (3.43±0.19 vs. 2.41±0.14μmolm^-2 s^-1). Furthermore, there were significant differences in the temperature response of respiration, and seedlings reached their maximum respiration rates at 59°C, more than two degrees higher than trees. Our results demonstrate that the respiratory characteristics of white spruce saplings at treeline impose a significant carbon cost that may contribute to their lack of perseverance beyond treeline. In the absence of thermal acclimation, the rate of leaf respiration could increase by 57% by the end of the century, posing further challenges to the ecology of this massive ecotone.

Principles of resilient coding for plant ecophysiologists

Plant ecophysiology is founded on a rich body of physical and chemical theory, but it is challenging to connect theory with data in unambiguous, analytically rigorous and reproducible ways. Custom scripts written in computer programming languages (coding) enable plant ecophysiologists to model plant processes and fit models to data reproducibly using advanced statistical techniques. Since many ecophysiologists lack formal programming education, we have yet to adopt a unified set of coding principles and standards that could make coding easier to learn, use and modify. We identify eight principles to help in plant ecophysiologists without much programming experience to write resilient code: (i) standardized nomenclature, (ii) consistency in style, (iii) increased modularity/extensibility for easier editing and understanding, (iv) code scalability for application to large data sets, (v) documented contingencies for code maintenance, (vi) documentation to facilitate user understanding; (vii) extensive tutorials and (viii) unit testing and benchmarking. We illustrate these principles using a new R package, {photosynthesis}, which provides a set of analytical and simulation tools for plant ecophysiology. Our goal with these principles is to advance scientific discovery in plant ecophysiology by making it easier to use code for simulation and data analysis, reproduce results and rapidly incorporate new biological understanding and analytical tools.