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Current estimates of temperature effects on plants are usually based on air temperature (Tair), although it is well known that leaf temperature (Tleaf) can deviate considerably from Tair. In some studies, to overcome the problem of Tair often being a poor proxy of Tleaf, measurements of canopy temperature (Tcan) have been used instead. However, Tcan data do not capture the spatial variation in Tleaf 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 Tleaf and Tcan 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 Tleaf (up to ~50°C) and leaf-to-air temperature differences (ΔTleaf; 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 Tleaf. The Tleaf (and thus ΔTleaf) of sun-exposed leaves greatly exceeded the simultaneously measured values of Tcan (and ΔTcan). 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.

Warming climate increases the risk for harmful leaf temperatures in terrestrial plants, causing heat stress and loss of productivity. The heat sensitivity may be particularly high in equatorial tropical tree species adapted to a thermally stable climate. Thermal thresholds of the photosynthetic system of sun-exposed leaves were investigated in three tropical montane tree species native to Rwanda with different growth and water use strategies (Harungana montana, Syzygium guineense and Entandrophragma exselsum). Measurements of chlorophyll fluorescence, leaf gas exchange, morphology, chemistry and temperature were made at three common gardens along an elevation/temperature gradient. Heat tolerance acclimated to maximum leaf temperature (Tleaf ) across the species. At the warmest sites, the thermal threshold for normal function of photosystem II was exceeded in the species with the highest Tleaf despite their higher heat tolerance. This was not the case in the species with the highest transpiration rates and lowest Tleaf . The results point to two differently effective strategies for managing thermal stress: tolerance through physiological adjustment of leaf osmolality and thylakoid membrane lipid composition, or avoidance through morphological adaptation and transpiratory cooling. More severe photosynthetic heat stress in low-transpiring montane climax species may result in a competitive disadvantage compared to high-transpiring pioneer species with more efficient leaf cooling.

Climate models predict an increase in the severity and the frequency of droughts. Tropical forests are among the ecosystems that could be highly impacted by these droughts. Here, we explore how hydraulic and photochemical processes respond to drought stress and re-watering. We conducted a pot experiment on saplings of five tree species. Before the onset of drought, we measured a set of hydraulic traits, including minimum leaf conductance, leaf embolism resistance and turgor loss point. During drought stress, we monitored traits linked to leaf hydraulic functioning (leaf water potential (ψmd) and stomatal conductance (gs)) and traits linked to leaf photochemical functioning (maximum quantum yield of photosystem II (Fv/Fm) and maximum electron transport rate (ETRmax)) at different wilting stages. After re-watering, the same traits were measured after 3, 7 and 14 days. Hydraulic trait values decreased faster than photochemical trait values. After re-watering, the values of the four traits recovered at different rates. Fv/Fm recovered very fast close to their initial values only 3 days after re-watering. This was followed by ETRmax, Ψmd and gs. Finally, we show that species with large stomatal and leaf safety margin and low πtlp are not strongly impacted by drought, whereas they have a low recovery on photochemical efficiency. These results demonstrate that πtlp, stomatal and leaf safety margin are a good indicators of plant responses to drought stress and also to recovery for photochemical efficiency.