Dynamics Of Ecosystem Water Use Efficiency And Its Response To Biophysical Factors Of Typical Ecosystems In Dryland Of China

Climate change with continuous rise of global temperature,which leads to the change of rainfall pattern,the increase of drought frequency,and the intensification of drought,has a significant impact on the physical and energy cycle of the ecosystem.Water use efficiency(WUE)is an important index to evaluate the response of ecosystem to climate change and to reflect the interaction of carbon gain and water loss of the ecosystem.In this study,temporal and special dynamics of ecosystem WUE and its biophysical control were studied in drylands of China by means of single-site eddy-covariance(EC)flux measurements and multi-site meta-analysis of EC-related publications.Seasonal variations in ecosystem WUE and its biophysical control were studied in a desert steppe site in Yanchi,northwest China of Mu Us Desert.The magnitude,spatial patterns in WUE,and underlying drivers among ecosystems and climate zones were examined on the basis of published data from 34 EC sites.(1)Growing season mean ecosystem water use efficiency(EWUE)and inherent water use efficiency(IWUE)of desert steppe from 2012 to 2015 was 1.15 g C kg-1 H2 O and 1.47 g C k Pa kg-1 H2 O,respectively.EWUE and IWUE had obvious seasonal variation,which were regulated by different environmental and biological factors.EWUE was mostly controlled by air temperature(Ta),incident photosynthetically active radiation(PARin)and vapor pressure deficit(VPD).IWUE was mostly controlled by Ta,VPD,normalized difference vegetation index(NDVI)and canopy conductance(gc).IWUE was more sensitive to the regulation of biological factors on the carbon-water coupling.(2)Water use efficiency of desert steppe in Mu Us desert showed different sensitivity to biophysical factors among seasons.Variations in Ta had significantly positive effects on variations in EWUE in spring and autumn,but negative in summer.PARin dominantly regulated daily variation in EWUE with different sensitivity among seasons,the standardized correlation coefficient ranging from-0.49 to-0.27.Ta and VPD exerted affected daily variations in IWUE in spring and autumn,but not in summer.The gc affected the variations in IWUE mostly in summer,with a correlation coefficient of-0.30.(3)Precipitation affected WUE of desert steppe through its intensity,pattern and amount.The big rain increased WUE than the moderate rain.Frequently medium and low-intensity rainfall can stimulate the growth of gross ecosystem productivity(GEP)more than sporadic high-intensity rainfall,leading to a higher WUE.The decrease of rainfall amount on the monthly scale reduced the ecosystem WUE.The abnormal decrease of monthly rainfall in the middle of the growing season had a lagged effect of about one month on WUE.(4)Seasonal drought affected WUE of desert steppe by the duration and intensity of drought stress.Short drought stress had little effect on WUE,long time drought stress could reduce WUE by enhancing GEP and reducing evapotranspiration(ET).Mild drought stress(relative soil moisture content,REW=0.4)slightly enhanced WUE,and severe drought reduced WUE.WUE was mainly regulated by soil water content in 10 cm depth(VWC10)and gc during the drought stress.(5)Average growing-season EWUE at the 34 sites in drylands of China was 1.64 ± 0.93 g C kg-1 H2 O.EWUEs among vegetation types were significantly higher for cropland and forest,and lower for shrubland and grassland ecosystems.EWUEs among climate zones were significantly higher for arid and dry sub-humid zone,and lower for semi-arid zone.EWUE was shown to increase in direct association with increasing precipitation(PPT)and indirectly through PPT's effect on NDVI.Among ecosystem types,EWUE for cropland was most sensitive to PPT and NDVI,whereas shrubland EWUE was the least sensitive to these factors.EWUE in the semi-arid zone was more sensitive to the PPT and NDVI,unlike the other climatic zones.GEP and ET dominated spatial variation in EWUE in semi-arid and dry subhumid zones,respectively.In summary,precipitation was a key factor that controlled the ecosystem water use efficiency.The changes in distribution,frequency,intensity and extreme rainfall events of rainfall under future climate change,may result in changes in ecosystem composition and the carbon-water coupling,leading to differential outcomes on ecosystem carbon and water cycling along an aridity gradient,thus a crossgradient differences in ecosystem WUE's.