Changing Trends And Association With Meteorological Drivers Of Water Use Efficiency Across Forest,Grassland And Cropland Ecosystems

Forests,grasslands,and croplands are the main types of terrestrial ecosystems that respond to global climate change,and are closely related to people's production and life,social economy,and ecological economy.Quantifying the response of water use efficiency under different vegetation types to the current ecological environment is the key to predicting and adapting to climate change.However,in forest,grassland and cropland ecosystems,the change trends of ecosystem water use efficiency(EWUE),inherent water use efficiency(IWUE),underlying water use efficiency(uWUE)and the essential process of climate control are still unclear.The study uses the data of 42 sites from four vegetation types,including deciduous broadleaf forests(DBFs),evergreen needleleaf forests(ENFs),grasslands(GRAs),croplands(CROs),in the global flux network FLUXNET2015 over 5 years to investigate the change trends of water use efficiency in forest,grassland,cropland ecosystems and the commonalities and differences among different water use efficiency indicators,and explore the common factors of different climatic factors on WUE trend in forest,grassland and cropland ecosystems.The main results of the paper are as follows:(1)The mean trend of all WUEs of forest,grassland and cropland ecosystems are overall increasing.The average growth rate of IWUE is the highest,followed by uWUE and EWUE among the two vegetation types,DBF and GRA,but nothing similar happened occurs in CROs and ENFs.Among them,the change of the rate of CROs rate as follows uWUE,IWUE and EWUE.In ENFs,the order is EWUE>IWUE>uWUE.(2)The mean slope of water use efficiency in ENFs is always higher than that of the other three vegetation types.In EWUE and uWUE,the order of ENFs>CROs>DBFs>GRAs is shown,while in IWUE,the mean trend is that grassland is no longer the smallest and ranks before cropland and forest(DBFs).Meanwhile,changes in climate factors also show certain commonalities and differences under different vegetation types.For instance,atmospheric carbon dioxide concentration(Ca)and shortwave radiation(SR)both show an upward trend,and other independent variables such as vapor pressure deficit(VPD),soil water content(SWC),air temperature(TA),and wind speed(WS)have different trends in different ecosystems.(3)The growth of EWUE is mainly due to the growth rate of gross primary productivity(GPP),and there is also a significant positive correlation between GPP and evapotranspiration(ET),which indicates that changes in ET may partially promote or offset the stimulation or inhibition of other factors on GPP.In addition,the three WUE indicators also show a significant positive correlation in the comparison of different vegetation types,reflecting the possibility of substitution between different WUE indicators.(4)There are certain commonalities in the response of WUE to climate change.WS has a negative effect on WUE in DBFs,ENFs,and GRA,,while Ca has a positive effect on WUE.Similarly,in DBFs,ENFs,and GRAs,VPD has a positive effect on IWUE and uWUE.Only VPD,TA affected the increase EWUE and IWUE in the CROs.From the perspective of interpretation,VPD is the most important meteorological driving factor for WUE,while WS only occupies the most important position under DBFs,ENFs,and GRAs.Based on the characteristics of the carbon-water cycle among the three terrestrial ecosystems of global forest grassland and cropland,this study explores the water use efficiency of different ecosystems by analyzing and comparing the performance of various water use efficiency indicators reflecting the carbon-water coupling characteristics of ecosystems.The commonalities and differences among forest,grassland and cropland ecosystems clarify the carbon and water laws and WUE in response to different impact factors(such as air temperature,wind speed,shortwave radiation,vapor pressure deficit,atmospheric CO2 concentration,soil water content,etc.).The feedback mechanism can provide a reference for the application of different WUE indicators in forest,grassland and cropland ecosystems.The results help to understand the carbon-water cycle mechanism of terrestrial ecosystems under global climate change,are of great significance in coping with climate change,formulating sustainable development strategies and measures,improving and protecting human life.