Canopy temperatures strongly overestimate leaf thermal safety margins of tropical trees

Current estimates of temperature effects on plants mostly rely on air temperature, although it can significantly deviate from leaf temperature (T_leaf). To address this, some studies have used canopy temperature (T_can). However, T_can fails to capture the fine-scale variation in T_leaf among leaves and species in diverse canopies. We used infrared radiometers to study T_leaf and T_can and how they deviate from air temperature (ΔT_leaf and ΔT_can) in multispecies tropical tree plantations at three sites along an elevation and temperature gradient in Rwanda. Our results showed high T_leaf (up to c. 50°C) and ΔT_leaf (on average 8-10°C and up to c. 20°C) of sun-exposed leaves during 10:00 h-15:00 h, being close to or exceeding photosynthetic heat tolerance thresholds. These values greatly exceeded simultaneously measured values of T_can and ΔT_can, respectively, leading to strongly overestimated leaf thermal safety margins if basing those on T_can data. Stomatal conductance and leaf size affected T_leaf and T_can in line with their expected influences on leaf energy balance. Our findings highlight the importance of leaf traits for leaf thermoregulation and show that monitoring T_can is not enough to capture the peak temperatures and heat stress experienced by individual leaves of different species in tropical forest canopies.