No abstract

Trees regulate canopy temperature (T c) via transpiration to maintain an optimal temperature range. In diverse forests such as those of the eastern United States, the sensitivity of T c to changing environmental conditions may differ across species, reflecting wide variability in hydraulic traits. However, these links are not well understood in mature forests, where T c data have historically been difficult to obtain. Recent advancement of thermal imaging cameras (TICs) enables T c measurement of previously inaccessible tall trees. By leveraging TIC and sap flux measurements, we investigated how co-occurring trees (Quercus alba, Q. falcata, and Pinus virginiana) change their T c and vapor pressure deficit near the canopy surface (VPD c) in response to changing air temperature (T a) and atmospheric VPD (VPD a). We found a weaker cooling effect for the species that most strongly regulates stomatal function during dry conditions (isohydric; P. virginiana). Specifically, the pine had higher T c (up to 1.3°C) and VPD c (up to 0.3 kPa) in the afternoon and smaller sensitivity of both ΔT (=T c − T a) and ΔVPD (=VPD c − VPD a) to changing conditions. Furthermore, significant differences in T c and VPD c between sunlit and shaded portions of a canopy implied a non-evaporative effect on T c regulation. Specifically, T c was more homogeneous within the pine canopy, reflecting differences in leaf morphology that allow higher canopy transmittance of solar radiation. The variability of T c among species (up to 1.3°C) was comparable to the previously reported differences in surface temperature across land cover types (1°C to 2°C), implying the potential for significant impact of species composition change on local/regional surface temperature.

Life on Earth depends on the conversion of solar energy to chemical energy by plants through photosynthesis. A fundamental challenge in optimizing photosynthesis is to adjust leaf angles to efficiently use the intercepted sunlight under the constraints of heat stress, water loss and competition. Despite the importance of leaf angle, until recently, we have lacked data and frameworks to describe and predict leaf angle dynamics and their impacts on leaves to the globe. We review the role of leaf angle in studies of ecophysiology, ecosystem ecology and earth system science, and highlight the essential yet understudied role of leaf angle as an ecological strategy to regulate plant carbon–water–energy nexus and to bridge leaf, canopy and earth system processes. Using two models, we show that leaf angle variations have significant impacts on not only canopy‐scale photosynthesis, energy balance and water use efficiency but also light competition within the forest canopy. New techniques to measure leaf angles are emerging, opening opportunities to understand the rarely‐measured intraspecific, interspecific, seasonal and interannual variations of leaf angles and their implications to plant biology and earth system science. We conclude by proposing three directions for future research.