Tree growth and intrinsic water use efficiency of Chinese pine plantations along a precipitation gradient in northern China

The combined effects of rising temperature, elevated atmospheric CO₂ levels and warming-induced droughts have led to an increase in tree intrinsic water use efficiency (iWUE). However, how and to what extent increasing iWUE compensates for tree growth in bioclimatic regions is not well understood. In this study, nine tree ring sampling sites of Chinese pine (Pinus tabulaeformis) plantations were selected along a precipitation gradient to explore the different contributions of iWUE to tree growth. The results showed that the coefficient of variation of tree ring chronology (TRI_cv) decreased significantly with the precipitation gradient. There was a significant increasing trend in iWUE over time at all sites, the rate of which decreased with the precipitation gradient. Tree growth in drier regions was positively correlated with the drought index, and TRI_cv showed an increasing trend as tree grew, which implies that tree growth in drier regions is unstable under the rapidly changing climate. At the three drier sites, drought was the dominant factor inducing increased iWUE, and iWUE explained between 17.51%, and 38.35% of tree growth. At the two wetter sites, there was a significant positive relationship between iWUE and temperature, and iWUE explained 18.61% and 12.10% of tree growth, respectively. Under dry conditions, the drier sites exhibited greater levels of reduced tree growth and maintained higher relative iWUE to adapt to the water deficit compared to other sites. Our results highlight that stomatal regulation has a critical role in maintaining tree growth and increasing iWUE could compensate for negative effects from drought in arid environments.