Characteristics of daily sap flow for typical species in Jinyun Mountain of Chongqing in relation to meteorological factors

Using Granier-type thermal dissipation probes,we monitored sap flux density (Fd) of three species (Cunninghamia lanceolata, Pinus massoniana, and Symplocas setchuensis) in August and September during 2012--2015 in Jinyun Mountain of Chongqing, southwestern China. Solar radiation (ES), atmospheric temperature (T), atmospheric relative humidity (RH), wind speed (W), vapour pressure deficiency (VPD) and soil water content (SWC) synchronously with Fd were monitored by mini weather station. Sunny day, cloudy day and rainy day were chosen as three typical weather conditions. We aimed to analyze the differences and characteristics of sap flow among three species and among the three typical weather conditions, and to clarify the relationship between Fd and meteorological factors. The results showed that water transpiration ability of xylem for the three species was ranked as S. setchuensis＞P. massoniana＞C. lanceolata. Transpiration velocity of broadleaf species was higher than that of coniferous species. The variations of Fd for the three species all displayed single-peaked curves. S. setchuensis was the first one to start transpiration and reach the peak value of Fd, while C. lanceolata was the last. Sap flow for the three species under different weather conditions followed the order of sunny day＞cloudy day＞rainy day. Compared with sunny day sap flow, the extent of sap flow decreased on cloudy and rainy days ranged from 41% to 86%. The order of contribution rate of diurnal sap flow to whole day transpiration was sunny day (94.74%-98.04%)＞cloudy day (93.63%-96.71%)＞rainy day (81.43%-85.43%), and that of nighttime sap flow contribution was rainy day (14.57%-18.27%)＞sunny day (3.29%-6.37%)＞cloudy day(1.96%-5.26%). SWC was the major factor affecting the nighttime sap flow contribution on rainy days. ES and VPD were major meteorological factors affecting Fd. W, T and RH had little effect on Fd , and there were some differences among these three species. The regression models to relate Fd with meteorological parameters can well explain the changes of sap flow, in which coefficients of determination were 0.873, 0.873 and 0.903.