Thermal acclimation of photosynthesis, the physiological adjustment to temperature over weeks, may help plants mitigate adverse impacts of global warming, but is often under-represented in Earth System Models (ESMs). We evaluated a plant functional type (PFT)-agnostic, optimality-based model of C3ds photosynthesis with a global data set of leaf gas exchange measurements. We investigated how three key photosynthesis traits vary along a gradient of growing-season temperatures T_growth: optimal photosynthesis temperature T_opt , net photosynthesis rate at T_opt A_opt, and the width of the temperature response curve T_span . We analyzed how each trait is influenced by three acclimation processes: acclimation of photosynthetic capacities (carboxylation, electron transport, and respiration), their enzymatic responses, and stomatal sensitivity to vapor pressure deficit. The inclusion of all three acclimation processes was essential for reproducing observed patterns: a linear increase in T_opt with T_growth, and no correlations of A_opt and T_span with T_growth. Acclimation of enzymatic responses and stomatal sensitivity was crucial for accurately predicting T_opt and T_span. Acclimation of the photosynthetic capacities was necessary to avoid a bias in A_opt that can arise when relying on static, PFT-specific parameters. Comparing a model with all and a model without any acclimation processes showed that thermal acclimation buffers the response of photosynthesis to warming substantially, leading to smaller increases in photosynthesis in cold climates (+2% instead of +18%) and smaller declines in warm climates (−4% instead of −22%). Our observations-constrained photosynthesis predictions suggest an important role of thermal acclimation in ESM, partly mitigating adverse effects of a warming climate.

Summary Interactions between carbon (C) and nitrogen (N) cycles in terrestrial ecosystems are simulated in advanced vegetation models, yet methodologies vary widely, leading to divergent simulations of past land C balance trends. This underscores the need to reassess our understanding of ecosystem processes, given recent theoretical advancements and empirical data. We review current knowledge, emphasising evidence from experiments and trait data compilations for vegetation responses to CO₂ and N input, alongside theoretical and ecological principles for modelling. N fertilisation increases leaf N content but inconsistently enhances leaf-level photosynthetic capacity. Whole-plant responses include increased leaf area and biomass, with reduced root allocation and increased aboveground biomass. Elevated atmospheric CO₂ also boosts leaf area and biomass but intensifies belowground allocation, depleting soil N and likely reducing N losses. Global leaf traits data confirm these findings, indicating that soil N availability influences leaf N content more than photosynthetic capacity. A demonstration model based on the functional balance hypothesis accurately predicts responses to N and CO₂ fertilisation on tissue allocation, growth and biomass, offering a path to reduce uncertainty in global C cycle projections.

The rooting-zone water-storage capacity—the amount of water accessible to plants—controls the sensitivity of land–atmosphere exchange of water and carbon during dry periods. How the rooting-zone water-storage capacity varies spatially is largely unknown and not directly observable. Here we estimate rooting-zone water-storage capacity globally from the relationship between remotely sensed vegetation activity, measured by combining evapotranspiration, sun-induced fluorescence and radiation estimates, and the cumulative water deficit calculated from daily time series of precipitation and evapotranspiration. Our findings indicate plant-available water stores that exceed the storage capacity of 2-m-deep soils across 37% of Earth’s vegetated surface. We find that biome-level variations of rooting-zone water-storage capacities correlate with observed rooting-zone depth distributions and reflect the influence of hydroclimate, as measured by the magnitude of annual cumulative water-deficit extremes. Smaller-scale variations are linked to topography and land use. Our findings document large spatial variations in the effective root-zone water-storage capacity and illustrate a tight link among the climatology of water deficits, rooting depth of vegetation and its sensitivity to water stress.