Growing Season Stem Water Status Assessment of Qinghai Spruce through the Sap Flow and Stem Radial Variations in the Qilian Mountains of China

Global climate change is likely to change precipitation patterns with consequences for tree water use and growth in semi-arid areas. However, little is known about the effects of variability in precipitation on growth- and water-related physiological processes of native trees in dry areas of northwestern China. In this study, sap flow and stem radial variability in four Qinghai spruce trees (Picea crassifolia) were monitored in the Qilian Mountains, China. Tree water deficit (W) and basal area increment (BAI) were calculated using stem radial variation; water-use efficiency (WUE) was then estimated as the ratio of BAI and sap flow (Jt). The results showed that sap flow density (Js) increased logarithmically with increasing W when W < 50 m, and then gradually stabilized. Multiple factor generalized additive models (GAM) showed that Js was closely related to all measured environmental variables except for daily mean temperature and relative air humidity. W was related to the minimum daily temperature and soil water content. WUE exhibited higher values in early July. Low WUE was observed under conditions of prolonged dry weather, but it quickly increased during rainy days. WUE decreased after precipitation events due to high transpiration. We concluded that, in these semi-arid areas, precipitation is the most important controlling factor in tree growth and transpiration.