How did climate and CO2 concentration affect intrinsic water-use efficiency and tree growth in a semi-arid region of China?

Key message Although an increased intrinsic water-use efficiency has been observed, drought stress played a leading role in controlling Chinese pine growth in semi-arid region of northern China. Climate change, which is acknowledged as the most important factor in affecting plant performance, could be modulated by increasing atmospheric CO2 concentration. In this study, a total of 141 Chinese pine ( Pinus tabuliformis ) trees were studied to estimate tree-ring width. Based on tree-ring delta C-13 records, we measured the fluctuations in leaf intercellular CO2 (C-i) and intrinsic water-use efficiency (iWUE) since industrialization period on Hasi mountain, Gansu province. We found that the long-term variation of iWUE has been increasing since 1850. On average, 73% of the total variance in iWUE was explained by temperature, Standardized Precipitation-Evapotranspiration Index (SPEI) and CO2 concentration. Over 19 and 23% of the variance was attributed solely to drought stress and CO2 concentration, respectively. For BAI (basal area increment), nearly 50% of the variance was explained by temperature, SPEI and CO2, with 40% of the variance in tree growth attributed to drought stress. In addition, our results show that the positive relationship between tree growth and drought stress did not change over time. When there was a wetter condition, a larger proportion of higher tree growth occurred, otherwise more trees had lower tree growth. We therefore, conclude that despite the clear increase of iWUE, its beneficial effect of this higher iWUE on tree growth was overcome by drought stress at our study area.