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

Abstract

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 ¹³C _veg). The values of δ ¹³C _veg decreased by 1.34‰ during 1962–2010, similar to changes in δ ¹³C 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.