Increased temperature and prolonged soil moisture reduction have distinct impacts on tree photosynthetic properties. Yet, our knowledge of their combined effect is limited. Moreover, how species interactions alter photosynthetic responses to warming and moisture reduction remains unclear. 

Using mesocosms, we studied how photosynthetic properties of European beech and downy oak were impacted by multi-year warming and moisture reduction alone or combined, and how species interactions (intra- vs. interspecific interactions) modulated these effects.

Warming of +5°C enhanced photosynthetic properties in oak but not beech, while moisture reduction decreased them in both species. Combined warming and moisture reduction reduced photosynthetic properties for both species, but no exacerbated effects were observed. Oak was less impacted by combined warming and moisture reduction when interacting with beech than in intraspecific stands. For beech, species interactions had no impact on the photosynthetic responses to warming and moisture reduction, alone or combined.

Warming had either no or beneficial effects on the photosynthetic properties, while moisture reduction and their combined effects strongly reduced photosynthetic responses in both species. However, interspecific interactions can mitigate the negative impacts of the combined warming and moisture reduction in oak, thereby highlighting the need to deepen our understanding of interspecific interactions under climate change.

Five photosynthetic properties, including light‐saturated assimilation (A_max), maximal carboxylation rate (Vc_max), maximal photosynthetic electron transport rate (J_max), maximal photochemical efficiency (F_v/F_m), and chlorophyll concentration (Chl) were measured on 96 seedlings of European beech and downy oak trees grown in monoculture and in mixture. Measurement were done at the beginning, middle and late growing season during three years (from 2019 to 2021). During the first campaign of measurement, the trees did not exerce any climatic treatment, and afterward, a fully crossed combination of two air temperature and soil moisture regimes started. The measurements were carried out between 9 am to 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).

Excel files or R studio can open the datafiles included with the submission