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

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 ∆ T (= T c − T a ) and ∆ VPD (=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.