Responses of Water Use Efficiency to climate change in evapotranspiration and transpiration ecosystems

Climate change intensifies the fluctuation of arid/semi-arid evaporation and rearranges vegetation distribution patterns. Therefore, popular studies were proceeded, such as the responses of ecosystem's water use efficiency (EWUE) and vegetation water use efficiency(VWUE) to precipitation and temperature fluctuations, which are essential to understand the relationship between carbon and water in ecosystems. Herein, this study defines two types of EWUE (GPP/ET; ET: evapotranspiration) and VWUE(GPP/T; T: transpiration to response to climate change, through which the spatial and temporal were revealed. We found that the decreasing rate of VWUE is greater than EWUE in winter (December). The interannual fluctuation(2000-2014 year) VWUE showed a decreasing trend (slope = -0.001). However, the EWUE showed an increasing trend (slope = 0.0014). There is a significant spatial pattern of differences between the stability of EWUE and VWUE. Precipitation negatively affects WUE (EWUE:79.4 %; VWUE:63.6 %), and temperature significantly affects WUE (EWUE:87.9 %; VWUE:87.7 %). The negative contribution of temperature and precipitation to VWUE is 81.73 % and 53.60 %, and the positive contribution is 12.27 % and 46.40 %, respectively. The negative contribution of temperature and precipitation to EWUE is 87.87 % and 66.55 %, and the positive contribution is 12.13 % and 33.45 %, respectively. The negative contribution rate indicates that climate factors have a promoting effect on WUE. The greater the negative contribution rate, the more pronounced the promoting effect. Through sensitivity and contribution rate methods analysis, the results of the two methods are consistent: precipitation attenuates WUE and temperature promotes WUE. This study enhanced our understanding of the climate response of ecosystems and vegetation canopy of carbon water fluxes.