Prolonged periods of high air temperatures combined with low soil moisture (i.e., "hot droughts") are increasing in frequency and intensity across Europe, with potentially severe consequences for forest net carbon (C) uptake and ecosystem services. Thermal acclimation of photosynthesis and respiration has been proposed as a key mechanism allowing trees to maintain optimal C uptake under warming, but this capacity may be hampered by concurrent soil drought. Additionally, mixtures of functionally contrasting tree species, such as beech (Fagus sylvatica L.) and oaks (Quercus petraea (Matt.) Liebl., Q. robur L., Q. pubescens Willd.), are promoted as forest management strategies for climate change adaptation, including the potential mitigation of heat and drought stress. However, how these mixtures influence trees' acclimation responses to warmer and drier conditions, how to manage them effectively, and their implications for forest ecosystem services remain poorly understood.

Using a long-term manipulative experiment on beech and downy oak saplings growing in mixtures and monocultures, I disentangled the individual and combined effects of warming and soil drought on leaf- and canopy-level photosynthesis and respiration. Results showed that both species exhibited, to a certain extent, thermal acclimation of photosynthesis and respiration, but that this acclimation was strongly constrained by soil drought, reducing its potential benefits during hot droughts. Drought-induced reductions in canopy size strongly contributed to declines in net C uptake, often outweighing the effect of foliar thermal acclimation. Furthermore, mixing the two species at this early developmental stage did not affect their responses to warming and drought. With measurements in mature mixed and pure forest stands along a beech-oak ecotone, I then assessed how species interactions modify canopy microclimate and foliar thermal acclimation. Vertical canopy gradients in foliar traits played a greater role than microclimatic variation in shaping photosynthetic and respiratory responses, highlighting the importance of within-canopy functional adjustments. Species composition and local environmental conditions strongly modified canopy structure and microclimate, but did not consistently enhance foliar thermal acclimation in mixed stands. Finally, with a comprehensive literature review, I described the occurrence of beech-oak mixtures, physiological interactions, and ecosystem services across Europe. I revealed that while beech and oak differ markedly in shade, heat, and drought tolerance, their mixtures do not inherently confer greater resistance to climate change. Silvicultural practices may promote positive species interactions to enhance forest resilience and multifunctionality, but they often require intensive and costly interventions.

Overall, this thesis demonstrates that forest responses to climate change are determined by cross-scale interactions between physiological processes, structural adjustments, species interactions, and site conditions. A multi-scale, context-dependent understanding of these processes is essential to better predict forest carbon dynamics and develop effective forest management strategies to support the long-term resilience and multifunctionality of European temperate forests.