Photosynthetic and respiratory acclimation cannot compensate reduced plant-level carbon uptake in beech and oak saplings under prolonged warming and drought

The combination of higher air temperatures and lower precipitation has become increasingly frequent under global warming, potentially exacerbating their individual effects. Higher air temperatures constrain photosynthesis while simultaneously accelerating respiration, and might decrease tree net C uptake. Thermal acclimation may 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 European beech (Fagus sylvatica L.) and downy oak (Quercus pubescens Willd.) saplings. Using a process-based model, we evaluated the impacts of acclimation on plant-level net C uptake (Atot). Our study showed that both species acclimated to warmer conditions by shifting their Topt to higher temperatures, but to a lower extent when combined with drought, and slightly reducing R25 and Q10. In contrast, drought reduced Topt (in oak), Aopt, and, to a lower extent, R25 and Q10 (in beech). However, despite these acclimation processes, Atot decreased drastically under warming and drought, mainly due to reduced plant leaf area. Our results suggest that, while photosynthetic and respiratory acclimation might moderate the adverse impacts of warming and soil drought on leaf-level C exchange, plant-level net C uptake may still decline in a persistently hotter and drier climate because of structural adjustments toward sparser canopies.