Abstract: In order to clarify the environmental factors affecting the water use of typical urban tree species Magnolia liliiflora, an investigation was conducted on the responses of M. liliiflora whole-tree sap flow to the air temperature, air relative humidity, radiation, wind speed,  soil temperature and water  content, and precipitation in Beijing from April to October, 2008. The eight environmental factors affecting M. liliiflora whole-tree sap flow could be divided into three categories, i.e., evaporative demand index, soil index, and precipitation index. The evaporative demand index (air temperature, air relative humidity, total radiation, wind speed, and vapor pressure deficit) could explain 60% of the variation in the sap flow of individual trees, which presented S-type change trend, i.e., the sap flow reached an asymptote where higher light and evaporative demands could not cause sap flow to increase further. Soil index (soil temperature and water content) and precipitation index (precipitation amount) had little influence on the sap flow.

Key words: urban tree, thermal dissipation method, sap flow, principal components analysis, Magnolia liliiflora