Heat and drought responses of tropical trees in a warming world

Tropical trees face increasing heat and drought stress under climate change, yet their physiological responses remain poorly understood. This thesis explores the mechanisms underlying heat and drought tolerance in tropical trees, focusing on leaf temperature regulation, heat tolerance, and tree water use responses. We studied different tree species grown in common gardens at three sites along an elevation and climate gradient in Rwanda. Paper I demonstrates that leaf temperatures (Tleaf) can exceed air temperatures (Tair) by up to 20°C. Moreover, Tleaf of sun-exposed leaves greatly exceeds simultaneously measured canopy temperatures (Tcan). As a result, thermal safety margins are overestimated when based on Tcan or Tair instead of Tleaf, emphasizing the need to account for within-canopy variation in Tleaf to understand and predict leaf physiological heat stress. Results also emphasizes the role of traits such as leaf size and stomatal conductance (gs) in controlling interspecific variation in Tleaf. Papers II and III reveal that species with traits predisposing them to higher Tleaf (i.e., larger leaves, lower gs) tend to exhibit greater photosynthetic heat tolerance. While partial thermal acclimation of heat tolerance through membrane lipid adjustments was observed, it was insufficient to offset the increase in growth temperature, resulting in reduced thermal safety margins. More costly leaves, with higher leaf mass per area, had higher heat tolerance. Paper IV highlights significant acclimation in tree water use traits, with increased gs (at standard conditions) alleviating leaf overheating under moist conditions and reduced minimum conductance limiting water losses under periods of water shortage. Leaf osmotic potential, however, showed no plasticity. Paper V identifies variability in tree water use and status responses to declining soil water content. Larger trees, likely supported by deeper root systems, maintained higher water status, transport and growth during drought, suggesting tree size-related advantages in young forests or tree plantations under water-limited conditions. Together, these findings advance our understanding of how tropical trees respond to heat and drought stress and how these responses vary among species. The results underscore the critical role of species-specific traits in modulating resilience and highlight limitations in acclimation capacity under changing climatic conditions. These insights contribute to improving vegetation models, guiding forest restoration efforts, and informing climate-resilient forest management strategies.