Summary: 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.

Leaf morphological traits vary along climate gradients, but it is currently unclear to what extent this results from acclimation rather than adaptation. Knowing so is important for predicting the functioning of long-lived organisms, such as trees, in a rapidly changing climate. We investigated the leaf morphological warming responses of 18 tropical tree species with early (ES) abd late (LS) successional strategies, planted at three sites along an elevation gradient from 2400 m a.s.l. (15.2 °C mean temperature) to 1300 m a.s.l. (20.6 °C mean temperature) in Rwanda. Leaf size expressed as leaf area (LA) and leaf mass per area (LMA) decreased, while leaf width-to-length ratio (W/L) increased with warming, but only for one third to half of the species. While LA decreased in ES species, but mostly not in LS species, changes in LMA and leaf W/L were common in both successional groups. ES species had lower LMA and higher LA and leaf W/L compared to LS species. Values of LMA and LA of juvenile trees in this study were mostly similar to corresponding data on four mature tree species in another elevation-gradient study in Rwanda, indicating that our results are applicable also to mature forest trees. We conclude that leaf morphological responses to warming differ greatly between both successional groups and individual species, with potential consequences for species competitiveness and community composition in a warmer climate.

The productivity and climate feedbacks of tropical forests depend on tree physiological responses to warmer and, over large areas, seasonally drier conditions. However, knowledge regarding such responses is limited due to data scarcity. We studied the impact of growth temperature on net photosynthesis (A_n), maximum rates of Rubisco carboxylation at 25 °C (V_cmax25), stomatal conductance (g_s) and the slope parameter of the stomatal conductance-photosynthesis model (g₁), in 10 early successional (ES) and 8 late-successional (LS) tropical tree species grown at three sites along an elevation gradient in Rwanda, differing by 6.8 °C in daytime ambient air temperature. The effect of seasonal drought on A_n was also investigated. We found that warm climate decreased wet-season A_n in LS species, but not in ES species. Values of V_cmax25 were lower at the warmest site across both successional groups, and A_n and V_cmax25 were higher in ES compared with LS species. Stomatal conductance exhibited no significant site differences and g₁ was similar across both sites and successional groups. Drought strongly reduced A_n at warmer sites but not at the coolest montane site and this response was similar in both ES and LS species. Our results suggest that warming has negative effects on leaf-level photosynthesis in LS species, while both LS and ES species suffer photosynthesis declines in a warmer climate with more pronounced droughts. The contrasting responses of A_n between successional groups may lead to shifts in species’ competitive balance in a warmer world, to the disadvantage of LS trees.