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). To address this, some studies have used canopy temperature (T). However, T fails to capture the fine-scale variation in T among leaves and species in diverse canopies. We used infrared radiometers to study T and T and how they deviate from air temperature (ΔT and ΔT) in multispecies tropical tree plantations at three sites along an elevation and temperature gradient in Rwanda. Our results showed high T (up to c. 50°C) and ΔT (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 and ΔT, respectively, leading to strongly overestimated leaf thermal safety margins if basing those on T data. Stomatal conductance and leaf size affected T and T 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 is not enough to capture the peak temperatures and heat stress experienced by individual leaves of different species in tropical forest canopies.