Drivers And Environmental Impacts Of Vegetation Greening In The Semi-Arid Region Of Northwest China In Recent Twenty Years

Vegetation dynamics in semi-arid regions have important implications for the carbon cycle,water cycle,and energy exchanges at local,regional,and global scales.In the last 20 years,China has led the global vegetation greening through land use management,with forests and agricultural land contributing 42% and 32% of the vegetation greening in China,respectively.Are there similar characteristics of vegetation change in the semi-arid Northwest China? What are the roles of climate change and human activities in vegetation dynamics? What factors drove the vegetation dynamics in the region and what were the environmental impacts? Answering these questions will not only provide a better understanding of the interactions between climate change and vegetation dynamics in the semi-arid region but also provide scientific references for the formulation and improvement of environmental conservation policies in China.This study investigated the vegetation greenness trends in a semi-arid region around Lanzhou of Northwest China in the period from 2000 to 2019 using the Normalized Difference Vegetation Index(NDVI)obtained from the Moderate Resolution Imaging Spectroradiometer(MODIS)and analyzed the drivers and environmental impacts of the vegetation dynamics by considering the precipitation,air temperature,land surface temperature,and vapor pressure deficit data in the same period.The main findings of this research are as follows:(1)84.10% of the vegetation in the study area greened up significantly,and only0.43% of the vegetation browned down statistically significantly due to human activities or other factors.The greening rates of grassland and cropland(2.84%/year,2.88/year)were significantly higher than that of forests(1.26%/year),which indicates that non-forests might be more sensitive to climate change than forests.The greening rates of cropland and grassland were very close,indicating that agricultural practices such as fertilization and irrigation might contribute little to vegetation greening in this semi-arid region.(2)The increasing trend of temperature and precipitation as well as the decreasing trend of vapor pressure deficit in the study area were not significant.This study analyzed the correlations between vegetation greenness and temperature,precipitation,as well as vapor pressure deficit,and found that: precipitation was significantly positively correlated with 69.02% of the vegetation greenness in the study area;vapor pressure deficit was significantly negatively correlated with 51.27% of the vegetation greenness in the study area,and the temperature was significantly negatively correlated with 33.17% of the vegetation greenness in the study area.Per-pixel analysis and the analysis at the nine meteorological stations both indicate that the interannual dynamics of vegetation in the study area were dominantly regulated by water conditions and that the vegetation greenness correlated most with precipitation,and then vapor pressure deficit and air temperature.(3)The greening of vegetation had an impact on the temperature in this study area.The growing season daytime surface temperature in the study area decreased at a rate of-0.0827°C/year for the last 20 years,and the growing season nighttime surface temperature increased at a rate of 0.0303°C/year.The negative correlation between the interannual variation of daytime land surface temperature and vegetation greenness indicates that temperature in the study area is dominantly regulated by the cooling effect of enhanced vegetation evapotranspiration.The cooling effect of vegetation greening can mitigate the impacts of climate warming to a certain extent.