Abstract: Largescale plantations of Eucalyptus urophylla × Eucalyptus grandis have been recently developed in southern China, especially in Guangxi Province. For sustainable forest and water resource management of these plantations, attention is increasingly paid to their water use efficiency. Sap flow measurement provides a powerful tool for quantifying plant water use, and it is widely employed to estimate the ecological and hydrological outcomes of plant growth. However, a potential error might occur if one applies the sap flow measurements of the outermost layer of these trees to the entire trunk or an entire plantation without considering the radial distribution of xylem sap flow density. Hence, we determined the radial distribution of xylem sap flow density in trunks of 4-year-old E. urophylla × E. grandis using the Granier thermal dissipation probe (TDP) method at the Qipo Forest Farm, Guangxi Province. We found that despite similar diurnal variation of sap flow at different depths of the trunk, sap flow at the 0-20 mm depth constituted bulk of the flow, which changed with seasons, while that of the 20-40 mm depth remain relatively stable. The curve regression analysis showed a significant exponential correlation in daily mean sap flow density (R²>0.90, P=0.00) between the 0-20 and 20-40 mm depths. Radial distribution pattern of sap flow of, E. urophylla × E. grandis showed a declining pattern with steep slope. The mean monthly sap flow density showed significant changes in the daytime, but it was relatively stable at night. Our findings will be helpful in accurately assessing the water use efficiency of this widely planted Eucalyptus hybrid by more precise calculation of its seasonal sap flow density, having significant implications for land management.

Key words: gap, bryophyte, alpine forest., coarse woody debris, nutrient