Prolonged warming and drought reduce canopy-level net carbon uptake in beech and oak saplings despite photosynthetic...

Tree net carbon (C) uptake may decrease under global warming, as higher temperatures constrain photosynthesis while simultaneously increasing respiration. Thermal acclimation might mitigate this negative effect, but its capacity to do so under concurrent soil drought remains uncertain. Using a 5-yr open-top chamber experiment, we determined acclimation of leaf-level photosynthesis (thermal optimum T_opt and rate A_opt) and respiration (rate at 25°C R₂₅ and thermal sensitivity Q₁₀) to chronic +5°C warming, soil drought, and their combination in beech (Fagus sylvatica L.) and oak (Quercus pubescens Willd.) saplings. Process-based modeling was used to evaluate acclimation impacts on canopy-level net C uptake (A_tot). Prolonged warming increased T_opt by 3.03-2.66°C, but only by 1.58-0.31°C when combined with soil drought, and slightly reduced R₂₅ and Q₁₀. By contrast, drought reduced T_opt (-1.93°C in oak), A_opt (c. 50%), and slightly reduced R₂₅ and Q₁₀ (in beech). Mainly because of reduced leaf area, A_tot decreased by 47-84% with warming (in beech) and drought, but without additive effects when combined. Our results suggest that, despite photosynthetic and respiratory acclimation to warming and soil drought, canopy-level net C uptake will decline in a persistently hotter and drier climate, primarily due to the prevalent impact of leaf area reduction.