Leaf temperature is a key variable governing plant physiological processes, such as photosynthesis and respiration. Further, very high temperatures can lead to leaf necrosis. Increasing warming of the atmosphere and occurrence of heatwaves means that understanding physical and physiological processes of leaf temperature is of increasing importance. Despite this, there is little data on leaf temperatures, particularly addressing different thermoregulation strategies of tree species. Both microclimate (radiation, air temperature, vapour pressure deficit and wind speed) and species traits (leaf structure and stomatal conductance) are necessary to model leaf temperatures. Both leaf structure and the response of stomatal conductance to the microclimate varies between species. Hence, even given the same microclimate, leaf temperatures can differ between species.

The objective of this study was to explore the thermal behaviour of canopy dominant species under a range of climates in order to assess the extent of variation in leaf temperatures within and between sites, and to understand the microclimatic and species traits underlying this variation.

We monitored temperatures of transpiring and non-transpiring leaves, and microclimate conditions, and measured leaf traits, for four dominant tree species in a tropical seasonal forest, a sub-tropical montane forest, and a savannah in Yunnan Province, China. Leaf temperatures were measured using thermocouples on exposed canopy branches at one minute temporal resolution. Microclimate was monitored both at site and branch level using a weather station, and measuring air temperature and light environment of the branch. We measured diurnal cycles of leaf gas exchange and leaf structural traits. Exploration of continuous datasets such as these is challenging. Data were analysed using a range of approaches to determine differences in thermoregulation between species and sites, and how these are linked to species strategy.

Our data show differences between sites with the warm sites having leaves cooler than the ambient air, and the cooler site having leaves warmer than the ambient air. There were significant differences in leaf temperatures between species within all sites. In general, trees at the warmer site displayed both greater transpirational cooling and sensible heat losses. However, there was a variety of strategies shown within sites linked to differences in water use and the extent to which leaf temperature changes with radiation load.

This study allows greater understanding of tree thermoregulation strategies through extensive measurements and analysis. This can improve our understanding of the species and ecosystems that may be at greater risk from climate change.