Simulation and Environment Factors of Platycladus orientalis Canopy-atmosphere Water Vapor Output Conductance

The water vapor resistance between forest canopy and atmosphere is an important factor in the study of forest transpiration, which is of great significance to forest management and ecosystem sustainability. However, there are still challenges in accurately quantifying this resistance. In this study, the Platycladus orientalis plantation in North China was taken as the research object. Based on the soil-plant-atmosphere continuum (SPAC) theory, the canopy water vapor output conductance model was constructed, and the response relationship between conductance and environmental factors was analyzed. The results showed that the diurnal variations of canopy stomatal conductance (g_c), aerodynamic conductance (g_a) and canopy water vapor output conductance (g_s) were basically similar, and g_s was significantly lower than g_c and g_a, indicating that if the water vapor exchange between canopy and atmosphere were only characterized by g_c, which would significantly overestimate g_s and could not accurately reflect the variation of g_s. There were significant positive correlations between g_c, g_a and g_s and PAR, temperature (T), vapor pressure deficit (VPD) and wind speed (u), but the responses of g_c, g_a and g_s to environmental factors were different. The effects of T, VPD and u on g_s did not show a significant threshold effect, while PAR had a strong effect on g_s within the threshold range. After exceeding this threshold, the sensitivity of g_s to PAR decreased significantly. These results provide a new perspective and technical reference for understanding the mechanism of forest canopy water output and its environmental driving factors.