Contrasting response of growing season water use efficiency to precipitation changes between alpine meadows and alpine steppes over the Tibetan Plateau

Alpine grasslands are the dominant vegetation of the Tibetan Plateau (TP) and are essential to understanding the mechanism of change in water use efficiency (WUE) in high-cold ecosystems in a changing environment. However, it is still unclear how the WUE of different alpine grasslands respond to changes in precipitation. Therefore, we investigated the variations in growing season WUE and its drivers among different grassland types, including alpine meadow of permafrost (PAM), alpine meadow of seasonal frost (SAM), alpine steppe of permafrost (PAS), and alpine steppe of seasonal frost (SAS), in different precipitation years. The mean growing season WUE was significantly higher for alpine meadow and seasonal frost regions and lower for alpine steppe and permafrost regions. WUE increased significantly in the alpine meadow (PAM and SAM) but decreased (PAS) or varied little (SAS) in the alpine steppe during the period of 1982-2014. Compared to normal years, WUE in the PAS region increased by 22% and 10% in severe and mild dry years, respectively, and decreased by approximately 10% in wet years. For the alpine meadow, WUE decreased more in severe dry years than in mild dry years and increased slightly in wet years. In different precipitation years, WUE for all grassland types was mainly controlled by the vegetation factor. Results suggest that the WUE in alpine grasslands, especially in the PAS region, may decline if the "dry gets wet, wet gets dry" happen over the TP in the future.