Abstract: By using Granier’s sap-flow method, the variations of sap flux density and whole-tree transpiration of Cyclobalanopsis glauca (syn. Quercus glauca) on a hilly slope in the karst region of South China were studied, with their driving factors analyzed. The sap flux density (Js) of C. glauca varied randomly with the diameter of breast height of individual trees, and its maximum occurred at 13：30〖KG-*2〗-〖KG-*7〗14:30. In a seasonal course, the daily Js was the maximum (5600 g H2O·m -2·s-1) in summer, and the minimum (3586 g H2O·m-2·s-1) in spring. The daily whole-tree transpiration had a greater change with weather condition, representing a power functional relationship with vapor pressure deficit (VPD) and photosynthesically active radiation (PAR) (R=097，P<001). The average daily whole-tree transpiration tended to be high in summer and low in winter and spring, and decreased with the decrease of soil moisture content in autumn (drought season). However, comparing with that of the tree species in other regions, the daily whole-tree transpiration amount of C. glauca in study region was still higher, even though the weather was dry and the soil was thin. It was presumed that in the dry season in karst region, the water supply for C. glauca could be mainly depended on the water-rich epikarst.

Key words: root interaction, root separation, chlorophyll content, yield, microorganism.