Vegetation Dynamics And Its Driving Factors During 2000 To 2016 In Tian Shan, Central Asia

The Tian Shan plays a key role in the sustainable development of ecological environment in arid and semi-arid regions along the Silk Road Economic Belt as an important ecological barrier connecting the Central Asia and Xinjiang,China.Previous studies indicated that the global warming led to a series of ecological problems of Tian Shan,such as a decrease in snowfall rate,severe glacier melting,and significant grassland degradation.It has become an important and urgent task to monitor the dynamic changes of vegetation and analyze its driving factors in Tian Shan.Based on MODIS NDVI data,this study generated the Vegetation Fractional Coverage(VFC)of the Tian Shan during 2000-2016 using the harmonic analysis of time series(HANTS)algorithm and the dimidiate pixel model.Linear regression and partial correlation analyses are also used to map the tempo-spatial variations of VFC and analyze the correlation results with the precipitation and temperature.In addition,the vegetation changes and its driving factors in typical areas are studied using Landsat images and field trips.Major conclusions were drawn:(1)The average growing season VFC in Tian Shan is 0.36,and it shows significant regional differentiation.The VFC in Middle Tian Shan and western part of Northern Tian Shan is higher,and main vegetation types are farmland,forest,and grassland.The VFC in Sourthern Tian Shan and eastern part of Northern Tian Shan is lower,and main vegation types are shrub-desert,shrubland,and grassland.In addition,the VFC also varies signicantly with different topographic factors.Vegetation in Tian Shan mainly distributes in altitude of 1200-3900 m,slope of 0-40°,aspect of south and north.And the distribution of different types of vegetation changes greatly with geographical factors.(2)67% and 33% of vegetation pixels display as improving or degradation trend.Degradation mainly occurred in areas with high VFC of Middle Tian Shan and western part of Northern Tian Shan,and improvement mainly occurred in areas with low VFC of Southern Tian Shan and eastern part of Northern Tian Shan.The high VFC areas are the most important parts of the plant biomass.Vegetation degradation in these areas may lead to regional climate change and affect the ecological stability.(3)The average growing season temperature and precipitation of Tian Shan is13.62? and 134.76 mm,they all increased during study period and the increments were 5.9% and 1.4% of the average temperature and precipitation,respectively.Therefore,although the climate in Tian Shan showes warmer and more humid changes during study period,but the temperature rises much more significantly than precipitation.The area percentages of VFC that correlated significantly with temperature and precipitation are 18.0% and 42.6%,respectively,and the precipitation is the dominant factor affecting the change of vegetation in Tian Shan.The decrease of precipitation and rapid increase of temperature are main reasons for VFC degradation in Middle Tian Shan and western part of Northern Tian Shan.On the contrary,the moderate increase of precipitation and temperature led to the improvement of VFC in Southern Tian Shan and eastern part of Northern Tian Shan.(4)The vegetation in Bayinbuluke and Bogeda Chaiwopu Lake,which are core regions of Tian Shan Natural Heritage District,was greatly affected by human activities.We studied the NDVI changes of Bayinbuluke in 2011,2013 and 2016,found that the vegetation in western and northwestern parts of buffer zone degenerated significantly in 2013 due to road construction and gazing.In addition,because of the development of tourism,the core region of Bayinbuluke still had degenerated areas in 2016.Chaiwopu Lake showed a continuous shrinkage and improvement of vegetation coverage around the lake during study period,and the over-exploitation of groundwater at the lake was the main cause of shrinkage.In the meantime,a large area of halophytes grew on the salinization soil formed after lake shrinkage,and the shrub-desert around the lake was slowly improving with the increase of precipitation.Therefore,Chaiwopu Lake generally showed an improvement trend in vegetation coverage.