Mechanisms and trait controls of canopy thermal regulation across a tropical rainfall gradient

Tropical forests play a vital role in the global carbon cycle. However, their ability to regulate canopy temperature is still poorly understood. In this study, we integrate observational data and modeling across three tropical forests along a rainfall gradient on the Panama Isthmus to examine how environmental drivers, plant traits, and biophysical mechanisms jointly regulate canopy-top temperature under distinct moisture and thermal regimes. A central finding is the divergent thermal responses among forests across the rainfall gradient. Solar radiation is the dominant warming driver at wetter and intermediate sites, while drier forests show stronger sensitivity to relative humidity and reduced evaporative cooling. During the wet season, observed evaporative cooling accounts for >30% of total cooling during periods of extreme canopy temperature; in the dry season, this pathway is constrained, increasing reliance on convective heat and radiative processes. Pre-dawn air temperature—driven by large-scale weather patterns—elevates daytime thermal baselines consistently across sites, while other drivers show site-specific effects. Among plant traits, leaf orientation is the most influential for regulating canopy-top temperature by distributing radiation more evenly through the canopy profile. Hydraulic traits—such as hydraulic conductance and turgor loss point—also play key roles by enabling evaporative cooling. Model simulations show that only a coordinated 10% increase across three key traits can reduce canopy-top temperature by ∼1 °C. In contrast, modifying any single trait results in negligible cooling, emphasizing the importance of multivariate trait interactions. These results highlight that tropical forests do not respond uniformly to warming. Thermal regulation strategies vary substantially along moisture gradients, with drier forests more vulnerable to the combined effects of water and heat stress, as evidenced by up to 5 °C higher top-canopy temperatures, larger seasonal warming, and stronger sensitivity to atmospheric humidity. Preserving functional trait diversity and understanding site-specific processes are critical for predicting and managing tropical forest resilience under climate change.