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.

Phenology (i.e., bud swelling, duration of bud development, onset and duration of senescence, growing season length), leaf-level carbon relations (i.e., starch and sugar concentrations, light-saturated assimilation, Asat) traits and growth traits (height, diameter, and aboveground biomass (AGB) increment) were measured on 96 seedlings of European beech and downy oak trees grown in monocultures and mixtures. Phenology, leaf-level carbon relations and growth traits were measured on the selected trees for three years (i.e., from 2019 to 2021). Phenology and growth traits were measured annually, and leaf-level carbon relations were measured three times a year during the growing season (i.e., early, middle, and late growing season). During the first campaign of measurement (i.e., early growing season 2019), trees did not exercise any climatic treatment, and afterward, a fully crossed combination of two air temperature (i.e., ambient air temperature and air warming at +5°C) and soil moisture regimes (i.e., soil moisture maintained at field capacity and soil moisture reduction of 50%) started. Light-saturated assimilation measurements were carried out between 9 am and 4 pm when the trees were the most active, excluding periods of midday stomatal closure if they occurred (mostly between 12am and 2pm in the middle of the growing season).

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