| Water use efficiency(WUE)of vegetation,as one of the comprehensive indicators to assess the adaptation of vegetation growth in the background of environmental changes,reflects the water distribution in ecosystems and determines the vegetation growth in different environments.Because of the relative scarcity of water resources in arid and semi-arid ecosystems,vegetation growth is generally under relatively intense water stress.Under the background of drastic global climate change,vegetation productivity has increased in high latitudes,but the negative impact of climate change on water resources should not be neglected.As a key area for ecological management,the vegetation productivity and the effectiveness of water use in arid and semi-arid regions are also receiving increasing attention.The Mongolian Plateau,as a typical arid and semi-arid region,has extremely fragile ecosystems and vegetation is sensitive to climate change and human activities.Previous studies on the WUE in the Mongolian plateau have been relatively few and focused on some typical areas,and few studies have been reported for the entire Mongolian plateau.Therefore,it is significant to clarify the spatial and temporal distribution and evolution of vegetation WUE on the Mongolian plateau,and to further reveal the driving mechanism based on this,in order to clarify the response mechanism of the plateau ecosystem carbon and water cycle to natural environmental changes.This study takes the Mongolian plateau as the study area,and evaluates the spatial and temporal distribution and dynamic changes of vegetation WUE in different seasons on the Mongolian plateau quantitatively based on multisource remote sensing,field measurements and reanalysis data sets,and reveals the influence mechanisms of spatial and temporal variation and changes of vegetation WUE by combining with geographic detector and structural equation model.The main research findings are as follows:(1)The modified CASA model has better applicability in the Mongolian Plateau,and the determination coefficients of simulated NPP with measured NPP and MODIS NPP are 0.67 and 0.83,respectively.During the study period,the overall spatial pattern of WUE in the growing season in the Mongolian Plateau is lower in the west and southwest and higher in the east.From different seasonal scales,the WUE in spring shows an increasing pattern from west to east,and the WUE in summer is higher in the west and southwest of the plateau and lower in the north and northeast.In summer,WUE is higher in the western part of the plateau and south of the Yinshan,while it is lower in the north and northeast;in autumn,WUE increases from the vegetation-unvegetated junction zone in the west and southwest to the east and north,and is the lowest relative to the spring and summer seasons.Except for summer,the overall WUE of different vegetation showed that cropland > forest > grassland.(2)WUE distribution was mainly related to the distribution of vegetation and hydrothermal conditions.Except for spring,the spatial distribution of WUE in the growing season and different seasons was dominated by NDVI,while the spatial variation of WUE in spring was mainly influenced by hydrothermal and light conditions such as temperature(q = 0.36),solar radiation(q = 0.33),and elevation(q = 0.32).Slope and slope orientation had the least influence on WUE distribution(q < 0.05).Except for broadleaf forests,the WUE distributions of different vegetation ecosystems were controlled by NDVI as a single factor,and the degree of explanation gradually decreased with the change of vegetation from grassland-corpland-shrub-forest.In addition,the WUE distributions of coniferous and broad-leaved forests were more influenced by temperature and vapor pressure deficient.In contrast,the bi-factor interaction enhances the explanatory power of WUE spatial distribution,and the interactions of NDVI ∩ vapor pressure deficient and NDVI ∩ temperature are dominant in different seasons,with the strength of the interactions showing autumn >spring > growing season > summer.(3)During the study period,the WUE in the growing season and different seasons on the Mongolian plateau showed a significant decreasing trend,but the WUE showed more obvious fluctuations in spring,and the rate of change of WUE in summer was the largest and most significant.Coniferous forests,broad-leaved forests and alpine steppe showed a stable change in WUE during the growing season,while typical steppe,sandy land,shrub,desert steppe and cropland showed a higher rate of change in WUE;both spring and summer showed a high rate of change in low vegetation cover and a low rate of change in high vegetation cover;the change in WUE in autumn was generally low.Spatially,95.03% of the growing season WUE on the Mongolian Plateau showed a decreasing trend,of which 54.14%showed a significant decrease,and the percentage of the area in spring showed a significant decrease.The results of Hurst index analysis indicate that most of the regions will maintain the same WUE trend in the future,while the regions with anti-lasting WUE changes are concentrated in the areas with insignificant WUE changes in the historical period.(4)Vapor pressure deficient difference has a negative effect on WUE in the growing season(0.90),while precipitation enhances vegetation transpiration and soil evaporation while promoting vegetation growth,which has a positive effect on WUE(0.45).At the beginning of the growing season(spring)and the end of the growing season(autumn),the physiological activity of vegetation was weak,and NDVI changes were the main control factor of WUE changes,with total effects of 0.34 and 0.57,respectively,while WUE changes in the middle of the growing season(summer)were mainly influenced by changes in vapor pressure deficient(-0.62)and precipitation(0.46).The increase in vapor pressure deficient was the main driver of WUE reduction in each vegetation ecosystem,but the intensity of the effect varied with the hydrothermal conditions,with a stronger negative effect on WUE change when vapor pressure deficient was used as an intermediate variable of temperature.(5)WUE residual variation ranged from-0.13 to 0.06 and showed a decrease in most parts of the Mongolian Plateau.In which,55.60% of the area with a decreasing trend of WUE influenced by human activities is distributed near Hangay Mountain-Sajan-Kente Mountain,eastern Mongolia,near Inner Mongolia Yinshan Mountains-central-eastern Inner Mongolia,and the area with an increase of WUE influenced by human activities only accounts for 0.21% of the vegetation coverage area,which is sporadically distributed south of Yinshan Mountains,indicating that human activities further increase the ecosystem water resource pressure. |
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