Abstract:In order to characterize the variance in sap flow within and between trees, and to quantify the water use of whole tree and of forest stand, Granier＇s thermal dissipation probes were applied to measure sap flux density （Js） in 14 sample trees in an Acacia mangium forest in Heshan Station, Guangdong. It showed that the differences of sap flux density recorded on a sunny day in July of 2004 were significant within tree （of different azimuthal sides） with coefficient of variability ranging from 15.51% to 37.26%, and among trees ranging from 37.46% to 50.73%. These variations were mainly attributed to the texture of sapwood and microclimate surrounding the point on trunk where the measurements were conducted. However, the sap flux density on different azimuthal sides showed significant linear correlation （p〈0.0001）, which provided a feature value for scaling up whole-tree transpiration from sap flux density measured on only one azimuthal side. The responses of tree sap flow to environmental factors were different and depended on the time scales. The daily sap flow was mainly controlled by climatic factors such as radiation and vapor pressure deficit, whereas the soil moisture had more influence of seasonal change on the sap flow. The morphological features of tree significantiy affected sap flow. Taller trees with larger sapwood area and canopy size that were load with more radiation demonsrated higher whole-tree transpiration. A proper integration of both azimuthal and radial variations in sap flux density within a tree can be scaled for whole-tree transpiration, which can relatively accurately estimate stand transpiration. The estimation of A. magium forest transpiration from sap flow showed obvious temporal and spatial differentiations of water use within the community.