Rising atmospheric CO2 outweighs elevated vapor pressure deficit in explaining increased intrinsic water use efficiency in Chinese pine

Introduction Tree intrinsic water use efficiency (iWUE) is an important metric for carbon and water balance in forest ecosystems. Tree iWUE has widely increased due to rising atmospheric CO 2 and intensified drought. However, the dominant effects of rising atmospheric CO 2 2, vapor pressure deficit (VPD), and soil moisture on iWUE are not fully understood. Methods We developed tree-ring width, stable carbon and oxygen isotopic chronologies derived from Chinese pine ( Pinus tabulaeformis ) stands across northern China to quantify the ecophysiological responses of iWUE to atmospheric CO 2 and drought. Results iWUE was significantly correlated with atmospheric CO 2 and VPD at both dry and wet sites. Furthermore, increased assimilation rate led to increased iWUE at both dry and wet sites. Notably, the beneficial effect of atmospheric CO 2 on iWUE outweighed that of VPD in Chinese pine. Discussion Our findings highlight the dominant role of rising atmospheric CO 2 in enhancing assimilation rate and increasing iWUE of Chinese pine and will aid in defining the performance of this tree species under climate change in northern China.