Species composition shapes canopy microclimate and foliar traits without modifying photosynthetic thermal acclimation in a beech-oak ecotone

Rising temperatures and increasingly frequent soil droughts are reshaping forest ecosystems and driving shifts in species composition, especially in transitional zones such as beech-oak ecotones. These shifts may alter canopy structure and microclimate, with potential feedbacks on tree physiological responses and carbon uptake under warming. Yet, their effects remain poorly understood. We investigated how species composition influences canopy size, microclimate, key foliar functional traits, and photosynthetic and respiratory thermal acclimation at the canopy top and bottom in mature European beech ( Fagus sylvatica L.) and downy oak ( Quercus pubescens Willd.) occurring in pure beech, mixed, and pure oak forest stands along a beech-oak ecotone in Switzerland. Canopy structure and microclimate differed markedly between pure and mixed forest stands, with beech exhibiting larger canopies (+34.5% of total leaf area) and stronger air cooling (reaching -3.2°C compared to -2.1 to -2.6°C) in the pure stand than in the mixture, whereas downy oak showed the opposite trend, with smaller canopies (41.5% lower total leaf area) in the pure stand than in the mixture, and similar canopy air cooling (-2.1°C across sites). These differences had little impact on photosynthetic and respiratory thermal acclimation, but led to species-specific adjustments in leaf traits in both species. Variation in respiration rates across sites and canopy levels was strongly associated with specific leaf area, suggesting that morphological adjustments play a greater role than physiological acclimation in regulating carbon exchange at this ecotone under current climate. Understanding these processes is critical as projected increases in temperature and soil drought intensity may further reduce microclimatic buffering and amplify physiological stress in beech, while potentially favoring more drought‑tolerant oak species. Further research should explore how tree-level structural adjustments and local environmental drivers modulate beech and oak foliar functional and physiological responses in mature forests to better predict their vulnerability to future climate.