Drivers of variability in water use of two co-occurring species in a subalpine forest in Jiuzhaigou Valley, Southwest of China

Co-occur species with different sensitivity to soil water may be particularly useful in evaluating water use by different forest stands as well as the response of species distribution, forest structure and stand composition to soil water availability in water-limited area. To clarify the species-specific water use strategy and provide insights into the possible succession trend, variations in sap flow and environmental conditions were investigated for two co-occur species (Betula albo-sinensis and Pinus tabulaeformis) in a mixed forest in Jiuzhaigou Valley in 2014. Sap flow was measured by Granier-type thermal dissipation probes and soil water content was measured by time-domain reflectometry probes for a successive period. Pinus tabulaeformis and Betua albo-sinensis species showed different responses to meteorological factors under different soil water conditions. Despite that whole tree water use was much higher for Pinus tabulaeformis due to greater sapwood area, sap flux density of the other co-occurring species Betua albo-sinensis was higher throughout the growing season. Normalized sap flux density (Fd) could be mostly well fitted to solar radiation (Rs), vapor pressure deficit (VPD), or the variable of transpiration (VT) by the exponential saturation function. Much better fitted curves were found for Fd -VPD and Fd - VT datasets than Fd - Rs datasets. For most datasets, normalized Fd increased rapidly when the environmental factors were below their threshold values, but reached an asymptote thereafter. Based on the species' differences in fitting parameters and the average maximum sap flow level under different soil water conditions, it was concluded that Pinus tabulaeformis was sensitive to soil water conditions and tolerant of low soil water availability, while Betua albo-sinensis was insensitive to soil moisture and needed to access to similarly high amount of soil water in the growing season after leaf expansion. These results indicated possible successful succession for Pinus tabulaeformis rather than Betua albo-sinensis. Our results may be useful for proper reforestation practices and sustainable forest management in water-limited regions.