About two-thirds of the Tibetan Plateau is covered by alpine grassland, which includes two primary ecosystem types, alpine meadow in the east and alpine steppe in the west. The unique environmental condition and plant communities make Tibetan alpine grassland often ecologically fragile, and therefore more vulnerable to climate change than other low-elevation ecosystems.

Ecosystem water use efficiency (WUE) is a key index depicting the relationship between carbon gain and water loss, serving as an indicator of plant capacity to adapt to the changing climate. However, it remains poorly understood that how WUE varies across Tibetan alpine grassland under a warming climate.

In a new study published in Journal of Geophysical Research: Atmospheres, researchers from the Institute of Geographic Sciences and Natural Resources Research of the Chinese Academy of Sciences found that the water use efficiency of the alpine grassland on Tibetan Plateau obviously increased during 1982-2016. Interestingly, the significantly increasing WUE mainly occurred in the alpine meadow, while in the alpine steppe WUE remained almost stable.

 "The increases in greenness and atmospheric CO₂ concentration are the two main reasons for the enhancement of WUE in alpine meadow," said Dr. MA Ning, lead author of this study.  

Although alpine steppe also became greener, the negative effect of wetting counteracted the positive effect of greening on WUE trend. Moreover, the CO₂ fertilization effect in alpine steppe is much smaller because of low biomass.

The results suggested divergent responses of WUE to ongoing environmental changes among different terrestrial ecosystems on Tibetan Plateau.

"Such differences are tightly related not only to external climate forcings but also to vegetation intrinsic properties," said Prof. ZHANG Yongqiang, corresponding author of the study. 

(a) The spatial pattern of trends in WUE of the alpine grassland during 1982-2016; (b) the anomaly of WUE averaged across alpine grassland and across its two ecosystems during 1982-2016; (c) the dominate driver of WUE trend across the alpine grassland of Tibetan Plateau. (Image by MA Ning)