Rising atmospheric CO₂ outweighs elevated vapor pressure deficit in explaining increased intrinsic water use...

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 CO₂ and intensified drought. However, the dominant effects of rising atmospheric CO₂2, vapor pressure deficit (VPD), and soil moisture on iWUE are not fully understood.

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 CO₂ and drought.

iWUE was significantly correlated with atmospheric CO₂ 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 CO₂ on iWUE outweighed that of VPD in Chinese pine.

Our findings highlight the dominant role of rising atmospheric CO₂ 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.