Stable Water Use Efficiency of Tibetan Alpine Meadows in Past Half Century: Evidence from Wool δ13C 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.