Water Use Efficiency Of Vegetation In The Mu Us Desert

Water use efficiency(WUE)is a key proxy to characterize the coupling process between carbon and water cycle in terrestrial ecosystems.Understanding how WUE varies with its impact factors helps to evaluate the adaptability on vegetation to local environmental conditions.Based on remote-sensing data and meteorological observations,the vegetation WUE in the Mu Us Desert from 2001 to 2016 was estimated by model calculation and data fitting.We implemented this study in the following aspects:(1)analyzing the spatial-temporal variations in WUE,(2)investigating the relationship between WUE and meteorological factors,and(3)probing the seasonal WUE responses to variability of land surface phenology parameters.The results are as follows:(1)Annual average WUE showed strong spatial heterogeneity in the Mu Us Desert,in which it was higher in the east but lower in the middle and west.The spatial distribution of WUE was determined by the hydrothermal conditions and the plant's own physiological characteristics.Owing to revegetation programme,significant increases were observed in WUE during the study period.On an annual scale,both net primary production(NPP)and WUE had increased in most areas since 2001,which contributed to the improvement on local ecological environment.However,in the central and western part of the Mu Us Desert with poor condition of water and heat,WUE showed a decrease trend due to faster increases in evapotranspiration(ET)as compared to NPP.This phenomenon was particularly evident in spring and summer,which determined the critical period of annual WUE level,and might be harmful to the sustainable development of local ecosystems over time.(2)Correlations between WUE and meteorological factors were totally different by contributing to non-codirectional changes or asymmetric co-directional changes of NPP and ET.Annual precipitation was generally positively correlated to WUE,while the decreasing spring precipitation in recent years was detrimental to the annual WUE.There were relatively insignificant relationship between WUE and air temperature.The positive correlation between WUE and solar radiation was subject to strong feedbacks between NPP and solar radiation,especially in spring and summer.Overall,solar radiation was the main control factor of WUE in the eastern regions.The local water and heat conditions were relatively good,so the ecological restoration worked well.Precipitation was the main control factor of WUE in the western regions,where local ecological environment was relatively fragile.Such areas were more prone to water conflicts,thus the risk of newly planted vegetation had a higher risk of degradation.In some areas where the annual precipitation were less than 300 mms,there had even been a decline in WUE,which should be the focus on attention of the future.(3)During the study period,both the start of growing season and the end of growing season showed an advance trend,which was conducive to the increase in WUE in spring and summer,while detrimental to autumn.The correlation between the length of growing season and annual WUE didn't reach the significant level.In general,under the joint influence of abiotic factors and biological factors,the phenological changes in recent years were conducive to the improvement of WUE on an annual scale.However,the land surface phenology parameters failed to explain the interannual changes of WUE in the western Mu Us Desert accurately.The results of response patterns indicated that the cooperative growth of NPP and WUE was of great significance for maintaining the stability of the ecosystem.The newly planted vegetation grew slowly in the western regions where water resources were relatively scarce.It also caused strong changes in evapotranspiration,leading to a decline in WUE in some regions.The mismatch between revegetation magnitude and water should be paid more attention.Our findings suggest that further studies should add WUE as one of the indicators to assess the quality of large-scale revegetation programme.An assessment framework based on NPP-WUE may achieve rapid assessments on ecosystem rehabilitation in wide geographic regions.