Stable Water Use Efficiency of Tibetan Alpine Meadows in Past Half Century: Evidence from Wool δ(13)C 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 (δ (13)C(veg)). The values of δ (13)C(veg) decreased by 1.34‰ during 1962–2010, similar to changes in δ (13)C values of atmospheric CO(2). 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 μmol·mol(–1) (approximately 23.5%) during 1962–2010 because the increase in the intercellular CO(2) concentration (46 μmol·mol(–1)) was less than that in the atmospheric CO(2) concentration (C (a), 73 μmol·mol(–1)). In addition, W (i) 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.