Contrasting Trends in Water Use Efficiency of the Alpine Grassland in Tibetan Plateau

Alpine grassland is the dominant land cover type of the Tibetan Plateau (TP). Previous studies suggested that TP's climate and vegetation condition have changed substantially in past few decades. However, the impacts of such changes on ecosystem water use efficiency (WUE), a vital index for depicting the tradeoff between water loss and carbon gain, of alpine grassland remain poorly understood. This study used a water-carbon coupled model, Penman-Monteith-Leuning Version 2 (PML_V2), to investigate the variation in WUE of alpine grassland in the TP during 1982-2016 and its drivers. PML_V2 performs well in estimating WUE at nine eddy-covariance flux sites, noticeably better than other main-stream products. The long-term mean WUE decreases from the southeastern to the northwestern TP, generally following the spatial pattens of surface greenness and climatic condition. During 1982-2016, WUE increased in most parts (71%) of alpine grassland but declined in certain regions of western TP. At the ecosystem scale, contrasting trends in WUE emerged between alpine meadow and alpine steppe over the last 35 years. The former's significant increasing WUE was primarily driven by increased leaf area index (LAI) and CO2 concentration (eCO(2)). While greening also occurred in the latter, its WUE remained stable because (a) the large negative effect of precipitation offset the positive effect of LAI on WUE, and; (b) the enhancement of photosynthesis resulted from elevated eCO(2) was weaker because of low LAI. Our results are of importance for understanding the dynamics and mechanisms of water-carbon coupling in high-elevation ecosystems under a changing climate.