Effects Of Vegetation Restoration On Regional Water Use Efficiency Over The Agropastoral Ecotone Of Northern China

Vegetation restoration is not only the effective measure to improve regional environment and global change,but also an important way to construct the national ecological security strategic pattern.The northern agro-pastoral ecotone,the transition zone between farmland,grassland,and desert,is located in the arid and semi-arid region of China.In this zone,the ecological environment is fragile,and the relationship between human and environment is tense;hence,it is sensitive to climate change.In order to control the degradation of the ecological environment,a large number of ecological restoration projects have been implemented in this area since 2000.Therefore,accurate evaluating the spatiotemporal dynamics of vegetation and the characteristics of water use efficiency under ecological restoration program is of great guiding significance for the construction and sustainable development of regional ecological environment.Based on the long-term remote sensing data and meteorological data,the ensemble empirical mode decomposition(EEMD)and residual trend(RESTREND)methods were applied to analyze the regional vegetation dynamic tendency and its driving forces at the northern agro-pastoral ecotone during 1982-2015period.Then,the gross primary production(GPP)and evapotranspiration(ET)were simulated by the two-leaf light use efficiency(TL-LUE)and priestley-taylor jet propulsion laboratory(PT-JPL),and further the regional water use efficiency(WUE)was obtained.Furthermore,the spatial patterns as well as the influencing factors of GPP,ET and WUE under the vegetation restoration conditions were also analyzed.Finally,both of the contribution rate statistical method,partial correlation analysis method and multiple linear regression method were used to make comprehensive attribution of vegetation and climate factors on regional water use efficiency from qualitative and quantitative perspectives.The main conclusions were as follows,(1)In general,the vegetation showed a significant greening trend with an annual change rate of NDVI(Normalized difference in vegetation index)at 0.001 year^-1 in the northern agro-pastoral ecotone of China during the period of 1982-2015.However,vegetation browning also existed in some areas.By differentiating the contribution rates of human activities and climate factors to vegetation change,we found that human activities were the main driving factor of vegetation change,contributing to 63.85%to NDVI change,and the positive effect of human activities on vegetation change increased significantly after 2000.Among these climatic factors,precipitation was the main factor affecting the vegetation change in the study area.(2)the WUE significantly increased during the study period with an interannual change rate of 0.007g C kg H₂O year^-1(P<0.001),respectively.Specially,the interannual change rate increased significantly after 2000.The results of comprehensive attribution analysis showed that more than 79%of WUE variation in the study area was driven by GPP.Among the vegetation and climate factors,NDVI,precipitation and solar radiation were the three main factors affecting WUE variation,and their contribution rates to WUE were 41.36%,16.82%and 16.71%,respectively.(3)According to the influence intensity of WUE in unit step size,when NDVI changed by 0.1,the response intensities were 0.022 g C kg H₂O.Among the climatic factors,the WUE showed the strongest responses to solar radiation.When the solar radiation changed by 100 MJ m^-2,the effect intensities on WUE were-0.017 g C kg H₂O,respectively.(4)Generally,this study suggests that vegetation was the most important factor controlling the change of WUE in the northern agro-pastoral ecotone of China.According to the qualitative and quantitative results,changes of NDVI generated the greatest and strongest impact on WUE.Particularly,with the implementation of ecological restoration projects since 2000,the increase of NDVI have driven the increase of WUE in the study area.This study reveals that vegetation restoration plays a dominant role in influencing the regional WUE,and analyzes the influence degree of vegetation change and climate change on WUE,which may provide a scientific basis for evaluating the regional ecological effects of vegetation restoration.