Abstract: Exploring the characteristics of sap flow among different tree species and environmental regulation mechanisms is of great significance to sustainable forest management and regional water resource management. We investigated the diurnal distribution characteristics of sap flow in these two tree species, Castanopsis lamontii and Schima superba, in the monsoon evergreen broad-leaved forest in the Xinfengjiang Reservoir Area, and synchronously monitored the environmental factors outside the forest. The results showed that the average daytime and nighttime sap flow rates of C. lamontii were 4.7±0.2 and 0.20±0.01 cm³·cm^-2·h^-1, respectively. The average daytime and nighttime sap flow rates of S. superba were 3.3±0.1 and 0.22±0.02 cm³·cm^-2·h^-1, respectively. The total sap flow in C. lamontii was 236.7 mm during the whole study period, which was higher than that in S. superba (147.5 mm). The contributions of daytime sap flow to total sap flow were 93.1% and 91.9% for C. lamontii and S. superba, respectively. The contributions of nighttime sap flow were 6.9% and 8.1%, respectively. The daytime sap flow of the two tree species was significantly affected by various environmental factors (P<0.01), while the nighttime sap flow had different responses to environmental factors. Daytime temperature and daytime photosynthetically active radiation were the main factors affecting the daytime sap flow of both species in wet and dry seasons. For nighttime sap flow, daytime sap flow, nighttime precipitation, and nighttime vapor pressure deficit were the main factors influencing both species during the wet season. In the dry season, the dominant factors for C. lamontii were daytime sap flow, nighttime temperature, nighttime precipitation, and nighttime vapor pressure deficit. For S. superba, the only relevant factors were nighttime vapor pressure deficit and nighttime precipitation. Therefore, it is necessary to fully consider the nighttime variations in sap flow in order to improve the accuracy of estimating the tree water consumption. Our results provide a reference for accurately estimating forest water consumption and forest water management.

Key words: sap flow, day and night dynamics, transpiration, water conservation, hydrologic process, Xinfengjiang Reservoir Area