Response of vegetation cover to temperature and precipitation in the source region of the Yellow River

The source region of the Yellow River is located in the hinterland of the Qinghai-Tibet Plateau. The evolution of its alpine vegetation system not only determines the local ecological environment and livestock production,but also significantly influences the ecological safety of the Yellow River basin. Under the influence of climate change and human activities,grassland degradation over upstream headwaters of the Yellow River is increasingly serious. Examining the spatial-temporal characteristics of vegetation cover and its response to climate change in the study area is important for understanding vegetation ecology and changes in water yield across the Tibetan Plateau. Here,based on NOAA/AVHRR NDVI(1982-2006) and MODIS NDVI(2000-2012),monthly air temperature and precipitation of stations,spatial-temporal characteristics of vegetation cover and its response to climate change were investigated in the study area. To analyze the long time series of vegetation variation in the study area,NOAA/AVHRR NDVI were extended by a correction algorithm based on the two kinds of data sets during the overlapping period(2000-2006). The results show that vegetation cover decreased from the southeast to the northwest of the study area. Vegetation cover was poor in areas where elevation was lower than 3000 m and higher than 4 500 m,but better between 3 000 m and 4 500 m. Annual vegetation cover revealed an increasing trend from 1982-2012 but there was an abrupt point in 2000,and the increasing rate after 2000 was about two times before. Vegetation cover change varied in different regions. On the whole,vegetation cover increased from 1982 to 2012 but there was also sporadic degradation. In low vegetation cover regions mainly located in the western high altitude regions and northern arid regions,vegetation cover increases were characterized by large interannual fluctuations. The results of correlation analysis between vegetation cover and climate factors showed vegetation cover was more sensitive to temperature compared with precipitation in the growing season. This indicates that the study area is a typical calorie restriction ecological region.