Lawrence Berkeley National Laboratory

Trees regulate canopy temperature (Tc) via transpiration to maintain an optimal temperature range. In diverse forests such as those of the eastern United States, the sensitivity of Tc 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 Tc data have historically been difficult to obtain. Recent advancement of thermal imaging cameras (TICs) enables Tc 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 Tc and vapor pressure deficit near the canopy surface (VPDc) in response to changing air temperature (Ta) and atmospheric VPD (VPDa). 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 Tc (up to 1.3°C) and VPDc (up to 0.3 kPa) in the afternoon and smaller sensitivity of both ∆T (=Tc − Ta) and ∆VPD (=VPDc − VPDa) to changing conditions. Furthermore, significant differences in Tc and VPDc between sunlit and shaded portions of a canopy implied a non-evaporative effect on Tc regulation. Specifically, Tc was more homogeneous within the pine canopy, reflecting differences in leaf morphology that allow higher canopy transmittance of solar radiation. The variability of Tc 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.