Canopy Conductance Characteristics and Model Development for Three Representative Tree Species in the Hanjiang River Basin in China

ABSTRACT Canopy conductance (Gc) is critical for assessing forest ecosystem responses to climate change. The Hanjiang River Basin represents a typical subtropical humid region in China and serves as the water source for the middle route of the South‐to‐North Water Diversion Project. Studies on canopy conductance of trees in this region remains lacking, limiting the corresponding understanding of forest transpiration and hydrological cycle. In this study, we measured sap flux density and derived canopy conductance (Gc) of three representative tree species of oak, pine and poplar from 2021 to 2023. We then explored characteristics of Gc and the corresponding responses to key environmental factors. Results showed that incoming short‐wave radiation (Rsi) and vapour pressure deficit (VPD) are the two major controlling factors of Gc, and a crossed hysteresis pattern exists in the diurnal response of Gc to Rsi for all the three species, whereas the response of diurnal Gc to VPD (and air temperature) exhibits a closed clockwise hysteresis. The response of Gc to VPD has a clear VPD threshold: A linear relation was found when VPD is lower than the threshold, and a logarithmic relation was found when VPD is higher than the threshold. When taking Rsi and VPD as controlling factors of Gc, a statistical model for canopy conductance was developed; the model can explain over 64% of the variation in Gc for all the three tree species. This study contributes important knowledge to understanding the canopy conductance of representative tree species in the Hanjiang River Basin.