Responses Of Vegetation Coverage To Climate Change And Human Activities In The Tarim River Basin

The northwest arid region of China is located in the hinterland of Eurasian continent, the ecological environment is fragile and sensitive to climate change. Vegetation distribution is sparse in arid area, and it is influenced by climate, water and human activities in the process of growth. Therefore vegetation cover change is considered as an important indicator to reflect the environmental changes in arid area. Numerous studies have pointed out that the climate in arid areas of China has become warmer and drier and the interference of human disturbance has increased in recent years. Whether the climate change becoming warmer and drier is common in the arid area? How did climate change and sustainable human disturbance affect vegetation change in arid regions? How to effectively separate the effects of climate change and human activities? And the positive effects of human disturbance on the ecological construction in arid areas are the key scientific problems to be solved urgently in order to maintain the ecological security and sustainable development of social economy in arid areas.The Tarim River Basin is a typical extreme arid area. Under the condition of climate change and a lot of irrational overexploitation of water and land resource, land desertification developed rapidly and resources of forest and grass appeared significantly reduction, the overall ecological environment of the Tarim River Basin had a serious degradation. With "Recent Comprehensive Management Project in the Tarim River Basin" approved by the Sate Council in 2001, the ecological environment and vegetation coverage of the Tarim River Basin has been improved initially. Based on this foundation, the paper selected the Tarim River Basin as the study area, using MODIS/NDVI data and the meteorological data from 2000 to 2014 to analyze the changing character and trend of vegetation coverage and the relationship between climate change and vegetation coverage. Secondly, the multivariate regression between climate factors and NDVI was constructed based on the residual analysis method for each pixel, in order to differentiate climate- and human-induced drivers of vegetation coverage change. Then, we quantitatively analyzed the response of the vegetation coverage to human activities and differentiate the relative effect of climate- and human-induced drivers of vegetation coverage change. The main results of this study are as follows:(1) Vegetation cover in the Tarim River Basin showed improved in general. Spatially, the vegetation coverage gradually decrease from the oasis agriculture area and grassland in the Southern Slope of Tianshan Mountain located in the northwest of the Tarim River Basin to the desert and gobi located in the southeast. According to the statistic of the area with different trend of vegetation coverage change in the Tarim River Basin, the area of vegetation cover which had improved accounted for 29.18% of the total area, the area of vegetation cover which had degenerated accounted for 8.83% and the other 61.98% of the total area remained unchanged, In space, the oasis agriculture area and alpine desert grassland in Kunlun Mountains and the Green Corri of lower reaches of the Tarim River appeared significantly increasing trend, but the populus euphratica Forest Region with shrub and grassland in the upper and middle reaches of the Tarim River and Bayanbulak Grassland appeared significantly decreasing trend.(2) Climate factors influencing vegetation cover change in the Tarim River Basin during 2000-2014 period are mainly temperature and precipitation. NDVI was negatively correlated with mean annual air temperature, and significantly positively correlated with precipitation. The increase of annual precipitation and the decrease of annual average temperature promoted the improvement of vegetation coverage in the Tarim River Basin, but climate change was not the only driver of the improvement of vegetation coverage. On seasonal scale, vegetation coverage had strong response and time-lag effect to summer precipitation, summer precipitation is one of the main climatic factors that promote vegetation coverage.(3)By using the residual analysis method to quantitatively analyze the impact of human activities on vegetation coverage, the results showed that the residuals of NDVI increased significantly from 2000 to 2014 and the value of residuals experienced the process of transition from negative to positive. The results also indicated that human activities begin to generate positive impact on the vegetation coverage in the Tarim River Basin. Spatially, the area which human activities had improved vegetation coverage accounted for 68.2% of the total area, the area which human activities had destroyed vegetation coverage accounted for 32.8% of the total area. We also combined the TM remote sensing data and the transferring matrix of land use type to test the analysis results of the residual analysis method, the results showed the residual analysis method can accurately differentiate the impact of climate- and human-induced drivers on vegetation coverage change in the arid and semiarid areas and has strong practical operational.(4) By constructing calculation and evaluation methods of assess of the relative effects of human activities and climate factors in the process of vegetation change for different scenarios and dividing the phase of vegetation coverage change based on the Meyer mother wavelet, we quantitatively differentiate the relative effect of climate- and human-induced drivers of vegetation coverage change in 2000-2008 and 2009-2014. The results indicated that human activities are the main drivers on vegetation coverage change in the different period with different trend of vegetation coverage. Extensive human activities in the Tarim River Basin has greatly weakened the impact of climate change on vegetation coverage change.