Chronic warming and dry soils limit carbon uptake and growth despite a longer growing season in beech and oak

Progressively warmer and drier conditions impact tree phenology and carbon cycling with large consequences for forest carbon balance. However, it remains unclear how individual impacts of warming and drier soils differ from their combined one and how species interactions modulate tree responses. Using mesocosms, we assessed the multi-year impact of continuous air warming and lower soil moisture acting alone or combined on phenology, leaf-level photosynthesis, non-structural carbohydrate concentrations, and aboveground growth of young European beech and Downy oak trees. We further tested how species interactions (monocultures vs. mixtures) modulated these effects. Warming prolonged the growing season of both species but reduced growth for oak. In contrast, lower moisture did not impact phenology but reduced trees assimilation and growth for both species. Combined impacts of warming and drier soils did not differ from single ones. Performances of both species in the mixtures were enhanced compared to the monocultures under extreme conditions. Our work revealed that higher temperature and lower soil moisture have contrasting impacts on phenology vs. leaf-level assimilation and growth, with the former being driven by temperature and the latter by moisture. Furthermore, we show a compensation of the negative impacts of extreme events by tree species interactions.