Analysis Of Controlling Factors Of Water Use Efficiency In An Alpine Meadow On The Tibetan Plateau And Its Response To Droughts

Under global climate change,more frequent and widespread droughts have dramatic impacts on the water and carbon cycles.Water use efficiency(WUE)is a commonly used indicator reflecting the trade-off relationship between carbon sequestration and water consumption.The alpine meadow ecosystem on the Tibetan Plateau is susceptible to water stress during the growing season,which will have a special and significant impact on the WUE of this fragile ecosystem.Therefore,systematically clarifying the variations in WUE and the effects of drought on WUE on the Tibetan Plateau at different time scales is crucial to accurately predict the response of fragile alpine ecosystems to future climate change.Based on 6-year(2013–2018)continuous carbon and water flux data observed by the eddy covariance system,WUE was calculated by gross ecosystem productivity(GPP)/evapotranspiration(ET).Combined with microclimate system observation and remote sensing data,the control mechanism of climatic factors and biotic factors on WUE of alpine meadow ecosystem and its response to drought were explored.The main conclusions are as follows:(1)The occurrence of droughts in each year.Long-term droughts were observed in the mid-growing season in 2015,early-growing season in 2016,mid-growing season in 2017,and early-growing season in 2018;short-term droughts were observed in the early-growing season and mid-growing season in 2013 and the late-growing season in 2015.(2)The control factors of WUE in different periods.WUE was regulated by climatic factors and biotic factors,but WUE was dominated by different factors during different periods of the growing season.In the early-and late-growing seasons,WUE was dominated by temperature.In the mid-growing season,WUE was mainly controlled by water conditions.In addition,the LAI had a significant effect on WUE throughout the growing season.(3)The occurrence time and duration of droughts had divergent effects on WUE.WUE increased when droughts occurred in the early-growing and late-growing seasons,whereas it decreased during mid-growing season droughts.In addition,long-term drought probably decreased WUE,while short-term drought could increase WUE.However,the occurrence time of droughts played a determinate role compared with the effects of drought duration.(4)The variations in WUE were determined by the coupling relationship between carbon and water fluxes.In the early-growing seasons,the GPP was low due to the smaller LAI.Therefore,the drought-induced reduction in GPP was less than that in ET,thus,WUE increased.In the mid-growing seasons,WUE was dominated by water conditions and the LAI.When drought occurred,GPP decreased faster than ET,and hence,WUE decreased.In the late-growing season,WUE was mainly driven by temperature and the LAI,but short-term drought slightly enhanced WUE.