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 separate impacts of warming and drier soils differ
from their cumulative ones 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.