High Heterogeneity in Canopy Temperature Among Co-occurring Tree Species in a Temperate Forest

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 (VPDc) in response to changing air temperature (T-a) 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 T-c (up to 1.3 degrees C) and VPDc (up to 0.3 kPa) in the afternoon and smaller sensitivity of both increment T (=T-c - T-a) and increment VPD (=VPDc - VPDa) to changing conditions. Furthermore, significant differences in T-c and VPDc 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 degrees C) was comparable to the previously reported differences in surface temperature across land cover types (1 degrees C to 2 degrees C), implying the potential for significant impact of species composition change on local/regional surface temperature.