Abstract: The eigenvalues of continuous sap flow pattern, i.e., skewness and kurtosis, were used to investigate the water usage of Schima superba with different diameter at breast height (DBH), and the method of normalization was firstly applied to eliminate the effects of strong affecting factor （photosynthetic active radiation，PAR) to explore the possible relationship between weak affecting factor (soil moisture) and sap flow. Generally, the trees with larger DBH had smaller skewness of sap flux density and later-appeared but larger peak values, suggesting that much more water was transpired, and the larger trees showed smaller skewness and later-appeared larger peak values in wet season than in dry season, suggesting that more water was transpired in wet season. On the other hand, smaller trees had lesser differences in the skewness between dry and wet seasons, suggesting that there was no significant difference in the transpiration between the two seasons. The relationship between individual tree’s transpiration and soil moisture was significant and positive after the two parameters being normalized with PAR peak values. When the soil moisture content was higher, the transpiration of the trees with larger DBH was steadily increasing with soil moisture, while that of the trees with moderate or smaller DBH had opposite trend, presumably due to their transpiration and water absorption were approached to the limit.

Key words: transpiration, soil moisture, diameter at breast height, Schima superba, sap flow, spatial pattern, vegetation NDVI, climate change, human activity, Xinjiang.