Spatial And Temporal Changes And Driving Factors Of NDVI Eight Provinces In Western China Over The Past 30 Years

The western part of China is one of the important birthplaces of Chinese civilization and an important ecological barrier.Historically,the western region of China was once lush and shady,but now the problem of soil erosion and desertification is serious.The restoration and construction of vegetation is one of the key issues in improving the western ecological environment.Therefore,scientific research on vegetation cover change and its driving factors in the western region is of great significance for the rational evaluation of the ecological environment quality and the formulation of ecological protection policies.This paper takes eight regions(Shaanxi,Gansu,Ningxia,Inner Mongolia,Xinjiang,Yunnan,Guizhou and Sichuan)distributed in a "T" shape as the research object,which are located in the second terrain ladder of China.Based on GIMMS NDVI3 g long time series data set from 1986 to 2015,spatial-temporal evolution and regional differentiation characteristics of vegetation are revealed by using spatial statistics and statistical models.Spatial and temporal trends of vegetation cover in Western China are analyzed by Sen+Mann-Kendall method and relative change rate model.The dominant driving factors are screened through principal component analysis.Least squares regression model(OLS),eigenvector spatial filtering model(ESF)and geographically weighted regression model(GWR)are constructed to explore the driving factors of vegetation cover change.Based on the GWR model,the impacts of different dominant factors on vegetation cover change and the heterogeneity of spatial distribution were discussed,and the relevant policies and suggestions are put forward.The main conclusions have been achieved.1)The spatial and temporal characteristics of vegetation cover in the eight western provinces are significant.The variation of NDVI value with time is characterized by single peak oscillation during the year and rising fluctuation in different years.Evolution law of spatial differentiation characteristics with time and region is as follows.The NDVI values of each year and in different years show the characteristics of high in Southeast and low in Northwest.The former exhibits obvious directional characteristics,while the latter exhibits repeated gravity transfer,and the variation of elliptic parameters with standard deviation is relatively stable.The NDVI value increased gradually from west to east,and showed a trend of "high-low-high" from low to high latitude.The vegetation growth in each vertical zone is stable,but the difference between the zones is significant.2)The change of vegetation cover in the study area showed an overall improvement trend,and there are serious degradation phenomena in some areas.The increasing trend of NDVI from 2001 to 2015 is more obvious and spatial distribution wider than that from 1986 to 2000.NDVI of all kinds of land use types showed an increasing trend in general,and the highest proportion was cultivated land area(50.17%).From the perspective of different administrative divisions,the most significant increase of NDVI is in Guizhou Province(70.77%).3)Three regression models have their own advantages and disadvantages in the analysis of vegetation cover change factors.The GWR model fits the data more than the OLS model and the ESF model,which better explains the spatial differentiation of the drivers of vegetation cover change in the western region.ESF model can effectively extract the spatial autocorrelation of data to reduce the estimation bias of the model,and the fitting effect is better than OLS model.4)Factors of climate,soil texture,topography,socio-economic conditions,land use types,afforestation area and linear geophysical have significant effects on vegetation cover change.Taking districts and counties as research units,the spatial influence factors and intensity of vegetation cover vary with geographical location,which fully explains the complexity of the mechanism of changes in human-land relationship on vegetation cover change.A relatively complete ecological environment information system should be established,taking into account regional differences in driving factors for changes in vegetation cover,tailoring to local conditions,and long-term planning.