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

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 five-year open-top chamber experiment, we
determined acclimation of leaf-level photosynthesis (thermal optimum Topt
and rate Aopt) 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 (Atot). Prolonged warming increased
Topt 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. In contrast, drought reduced
Topt (-1.93°C in oak), Aopt (~50%), and slightly reduced R25 and Q10 (in
beech). Mainly because of reduced leaf area, Atot 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.