Abstract: We examined the diurnal variation characteristics of stem sap flow of Castanopsis carlesii, a natural evergreen broad-leaved tree species occurring in the Mopanshan Forest Park in central Yunnan. Environmental factors were recorded with a small automatic weather station. The results showed that the diurnal variation of the sap flow rate of trees from large (35-45 cm), medium (25-35 cm) and small (15-25 cm) diameter classes showed obvious single-peak curves of ‘day high and night low’. The sap flow rate in the first half of the night was slightly higher than that in the second half. The change in sap flow rate on the daily scale showed a trend characterized by an initial increase followed by a decrease. The overall change range was proportional to the size of the diameter class. The sizes of sap flow rates across the diameter classes were in the order of middle diameter class > large diameter class > small diameter class. The daily sap flow rate across diameter classes ranged from 19.21 to 72.27 mL·cm^-2·d^-1, while the nighttime sap flow rate ranged from 0.97 to 6.04 mL·cm^-2·d^-1. The contribution rate of nighttime sap flow to the daily sap flow ranged between 2.6% and 21.5%. The contribution rate of nighttime sap flow in the diameter classes was in the order of small diameter class > middle diameter class > large diameter class. The sap flow rate in C. carlesii was significantly correlated with solar radiation, saturated water vapour pressure deficit, air temperature, relative humidity, and wind speed. The main factors influencing daytime sap flow rate were solar radiation, saturated water vapour pressure deficit, air temperature, relative humidity, and wind speed. The main factor influencing nighttime sap flow rate was air temperature. In the subalpine habitats of Southwest China characterized by low temperatures and high humidity, the sap flow of C. carlesii exhibited pronounced diurnal variation, with significant differences in the factors influencing sap flow between day and night. When estimating the total daily transpiration water consumption of C. carlesii, it is essential to consider the contribution of nighttime sap flow.

Key words: subalpine forest, Castanopsis carlesii, diameter class, daytime and nighttime stem sap flow, environmental impact