Impact of warmer and drier conditions on tree photosynthetic properties and the role of species interactions

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.