Responses of sap flow in to environment under different weather conditions.

This study was conducted in Liupanshan Forest Ecological Positioning Station of the National Forestry and Grassland Administration. We monitored sap flow of plantation in the Xiangshui River sub-basin throughout the 2019 growing season (from May 17 to October 12), as well as the meteorological conditions and soil environment (soil temperature and soil water content), to analyze the comprehensive environmental responses of sap flow in under different weather conditions. The results showed that sap flow rate increased and then decreased on the daily scale, with the highest rate on sunny days, followed by overcast days and then rainy days. Sap flow rate had a single peak on sunny days and multiple peaks on overcast and rainy days. Sunny days had earlier and longer sap flow compared to overcast and rainy days. Dominant factors driving sap flow differed across different weather. Vapor pressure deficit was the dominant factor influencing sap flow in sunny and overcast days, while solar radiation was dominant one in rainy days. The contribution rates of main factors to sap flow on sunny, overcast and rainy days were 31.1%, 27.4% and 40.1%, respectively. Results of the principal component analysis showed the factors affecting sap flow on sunny days could be classified into hydrothermal complex factors (air temperature, soil temperature, and volumetric soil moisture), water vapor transpiration factors (relative humidity and vapor pressure deficit), and radiation factor (solar radiation). The factors affecting sap flow on overcast and rainy days were combined into transpiration (relative humidity, solar radiation, and vapor pressure deficit), heat (air temperature and soil temperature), and soil water factor volumetric (volumetric soil moisture). On sunny days, sap flow reached the peak value 110, 80, 70 min after the hydrothermal, water vapor transpiration, and radiation factors, respectively. On overcast and rainy days, sap flow reached its peak in 10, 20, 30 min and 140, 60, 150 min, respectively before the peaks of transpiration, heat, and soil water factors.