The Changing Pattern Of Vegetation EVI And The Analysis Of Responses To Climate Change In The Source Region Of The Yellow River During The 2000 To 2018 Period

As an important component of terrestrial ecosystem,vegetation is the main pathway in the exchange of water,energy and materials between terrene and atmosphere.There is a very complex relationship between climate system and ecosystem.Thus,the vegetation is sensitive to the climate change;in response to the climate change,the evolution of vegetation also shows significant seasonal and regional differences.By using remote sensing data to monitor the spatio-temporal evolution of vegetation,this study could not only reveal the changing pattern of regional ecosystem,but also identify the response of vegetation to climate change.The source region of the Yellow River is both an important component of Qinghai-Tibet Plateau and the most important source of water in the Yellow River basin.Due to the human activity and climate change,the study area is facing various eco-environmental problems,such as the grassland degradation and soil erosion.This study will be beneficial to investigate the changing pattern of ecosystem on the plateau,and assist the government in making corresponding regulations to protect regional ecosystem according to local conditions,which will be important for ecological protection in Qinghai-Tibet Plateau.The MODIS EVI(Enhanced Vegetation Index)data and DEM data were used in this study,and these data were collected for a 19-year period from 2000 to 2018.The spatio-temporal evolution of vegetation had been analyzed in the study area,and the relationship between vegetation and elevation was investigated.Besides,observed climate data were collected from 23 meteorological stations to further illustrate the influence of climate change on vegetation.The main achievements are summarized:(1)During the past 19 years,the annual mean EVI in the source region of the Yellow River showed an increasing trend and was fluctuated between 0.19 and 0.23.EVI also showed significant regional differences in the source region of the Yellow River,and EVI was increased from northwest to southeast.The highest EVI could be detected in the eastern regions,such as the Aba,Hongyuan,Maqu,and Ruoergai.The changing patterns were varied in different seasons.The growth of EVI in the growing season was similar to that in summer,which has the highest EVI value in all seasons.The annual and seasonal EVI presented similar inter-annual changing trends at different altitudes,while different changing rates were observed.With the increase of elevation,there were increases in the annual and seasonal EVI,followed by decreases after 4000 m.(2)The annual mean EVI showed insignificant trends in most of the study area,accounting for 61.66%.The increase of vegetation coverage was most significant in the northern valleys of the study area,the northern and eastern parts of Zhaling Lake and Eling Lake.The decrease of vegetation coverage could only be detected in few regions,which were scattered in the western,southern,and eastern regions.The growing season had the largest area that showed increasing trend of EVI,while the summer season had the largest area with decreasing trend.The autumn EVI was basically constant during the past 19 years.In general,the vegetation coverage was increased in the study area.(3)There were time-lag effects of vegetation responses to climate change in the study area.The lag between temperature and vegetation is 16 days in spring,summer and growing season.The lag between rainfall and vegetation is 1 month in summer and growing season.Positive correlation between annual mean EVI and temperature could be observed in the following regions: Zhaling Lake,Eling Lake,Zeku,Dari,and Maqu counties.The annual mean EVI had positive correlation with precipitation can be detected in Guinan county,north of Zhaling Lake and Eling Lake,the central and eastern study areas.The multiple correlation between EVI and climate showed seasonal differences.(4)Precipitation and temperature were not the main driving forces for vegetation changes in most of the study area.For regions affected by climate change,temperature was the main factor that affected the growth of vegetation.The growth of vegetation could be attributed to the temperature in 10.68% of the study area,5.71% of the study area was driven by temperature and precipitation,while only 1.36% of the study area was driven by precipitation.The area affected by climate in the growing season is the largest,followed by spring,summer,and autumn.