Trait-based, spatial, and temporal variation in leaf temperature of tropical trees.

Current estimates of temperature effects on plants are usually based on air temperature (T_air), although it is well known that leaf temperature (T_leaf) can deviate considerably from T_air. In some studies, to overcome the problem of T_air often being a poor proxy of T_leaf, measurements of canopy temperature (T_can) have been used instead. However, T_can data do not capture the spatial variation in T_leaf among leaves with different thermoregulatory traits. This may be particularly problematic for highly diverse and heterogeneous tropical forest canopies. In this study, we used infrared thermometers to study T_leaf and T_can in multispecies tropical tree plantations established at three sites along an elevation gradient from 2,400 m a.s.l. (17.1°C mean daytime temperature) to 1,300 m a.s.l. (24.0°C) in Rwanda.  Measurements of chlorophyll fluorescence were also conducted to study the photosynthetic heat tolerance of these species. Our results showed high T_leaf (up to ~50°C) and leaf-to-air temperature differences (ΔT_leaf; on average 8-10°C and up to 24°C) of sun-exposed leaves. Both leaf size and stomatal conductance were important traits in controlling T_leaf. The T_leaf (and thus ΔT_leaf) of sun-exposed leaves greatly exceeded the simultaneously measured values of T_can (and ΔT_can). Photosynthetic heat tolerance partially acclimated to increased growth temperature; on average 0.31°C increase in heat tolerance per 1°C increase in growth temperature. Consequently, thermal safety margins were narrower for species at the warmer, lower-elevation sites. Our findings highlight the importance of leaf traits for leaf thermoregulation and show that monitoring of canopy temperature is not enough to capture the peak temperatures and heat stress experienced by individual leaves in diverse tropical forest canopies. They also suggest that tropical trees have limited abilities to thermally acclimate to increasing temperatures.

Keywords: Canopy temperature, elevation gradient, fluorescence, heat tolerance, leaf area, leaf temperature, stomatal conductance, thermoregulation, tropical forest.