Whether increased water-use efficiency of Picea crassifolia promotes radial growth of trees in the eastern Qilian Mountains

Global change, characterized by elevated atmospheric CO2 concentrations and warming, is profoundly affecting terrestrial forests. However, whether the increasing atmospheric CO2 concentration has a fertilizer effect on tree growth and whether intrinsic water use efficiency (iWUE) promotes tree growth in semi-arid regions is not entirely clear. In this study, we evaluated the relationships between tree-ring widths, delta C-13 values and iWUE of Qinghai Spruce (Picea crassifolia Kom.) in the eastern Qilian Mountains, northwestern China. We found that the iWUE is significantly consistent with the delta C-13 of rings in the high-frequency signal, and the relationship between iWUE and tree-ring width is unstable. The first-order difference correlation analysis with meteorological data showed that both the iWUE and tree-ring width were drought-limited. The radial growth was mainly regulated by water availability in current April and July, while annual variation of iWUE was primarily controlled by that in current July. In the early stage of trees growth, there was a positive relationship between iWUE and radial growth due to the CO2 fertilization. During the 1970s-2000s, the effect of water availability in July on radial growth increases because of the decreased precipitation in July. This has decreased stomatal conductance, intercellular CO2 concentration, carbon uptake and growth, contributing to the negatively significant correlation between tree-ring widths and iWUE (p < .05). Since the year of 2010, the relationship between the radial growth and iWUE shifted because of the increased precipitation in July. This study provided new insights for whether increased water-use efficiency enhances tree growth under semixeric conditions.