Grazing alters environmental control mechanisms of evapotranspiration in an alpine meadow of the Tibetan Plateau

Aims Evapotranspiration (ET) is an important component of the terrestrial water cycle and is easily affected by external disturbances, such as climate change and grazing. Identifying ET responses to grazing is instructive for determining grazing activity and informative for understanding the water cycle. Methods This study utilized 2 years (2014 and 2017) of eddy covariance data to test how grazing regulated ET for an alpine meadow ecosystem on the Tibetan Plateau (TP) by path analysis. Important Findings Radiation dominated ET with a decision coefficient of 64-74%. The soil water content (SWC) worked as the limiting factor in the fenced site. However, in the grazing site, the limiting factor was the vapor pressure deficit (VPD). Grazing had large effects on ET because it greatly affected the water conditions. The SWC and VPD were enhanced by 14.63% and 4.36% in the grazing site, respectively. Therefore, sufficient water was supplied to ET, especially during drought, and strengthened the transpiration pull. As a result, a favorable micrometeorological environment was created for ET. Grazing shifted the limiting factor of ET from the SWC to VPD, which weakened the limiting effect of the water conditions on ET and advanced the ET peak time. In addition, grazing altered the compositions of ET by changing the community structure, which directly resulted in an increased ET. In summary, grazing enhanced ET through altering the community structure and micrometeorological environments. The findings of this study further improve our understanding of the driving mechanisms of grazing on ET and will improve our predictions for the global water cycle.