Stable Water Use Efficiency of Tibetan Alpine Meadows in Past Half Century: Evidence from Wool delta C-13 Values

Understanding the influences of climatic changes on water use efficiency (WUE) of Tibetan alpine meadows is important for predicting their long-term net primary productivity (NPP) because they are considered very sensitive to climate change. Here, we collected wool materials produced from 1962 to 2010 and investigated the long-term WUE of an alpine meadow in Tibet on basis of the carbon isotope values of vegetation (delta C-13(veg)). The values of delta C-13(veg) decreased by 1.34% during 1962-2010, similar to changes in delta C-13 values of atmospheric CO2. Carbon isotope discrimination was highly variable and no trend was apparent in the past half century. Intrinsic water use efficiency (W-i) increased by 18 mu mol.mol(-1) (approximately 23.5%) during 1962-2010 because the increase in the intercellular CO2 concentration (46 mu mol.mol(-1)) was less than that in the atmospheric CO2 concentration (C-a, 73 mu mol.mol(-1)). In addition, Wi increased significantly with increasing growing season temperature and C-a. However, effective water use efficiency (W-e) remained relatively stable, because of increasing vapor pressure deficit. C-a, precipitation, and growing season temperature collectively explained 45% of the variation of W-e. Our findings indicate that the W-e of alpine meadows in the Tibetan Plateau remained relatively stable by physiological adjustment to elevated C-a and growing season temperature. These findings improve our understanding and the capacity to predict NPP of these ecosystems under global change scenarios