Trend of Vegetation Evaluation and Its Responses to Climate Change over the Source Region of the Yellow River

The characteristics of vegetation and its response to climate change were explored using NOAA/AVHRR(NOAA/Advanced Very High Resolution Radiometer) (1982-2001) and EOS/MODIS (Earth Observation System/Moderate Resolution Imaging Spectroradiometer)(2000-2008) NDVI (Normalized Difference Vegetation Index) products, monthly air temperature and precipitation of Maqu, Madoi, and Xinghai meteorology stations over the source region of the Yellow River. The results showed: The vegetation of Madoi region generally revealed a decreasing trend in the time and space scale; the vegetation of Maqu region, which is the source region of the upstream Yellow River, revealed an increasing trend before 2000 and appeared to degenerate after that; The vegetation revealed a decreasing (increasing) trend before (after) 2000 in the north of the source region of the upstream Yellow River. The NDVI appeared to decrease during 1982-1990 in the spatial scale, this occurred in the east region of the Ngoring Lake. The areas with decreasing trend of vegetation further enlarged during 1991-2000. During 2000-2008, an evident degeneration occurred in Maqu area, while the vegetation of Xinghai region generally increased. The responding relationship of the vegetation to climate change implied: The air temperature was the most sensitive factor to vegetation variation in the source region of the Yellow River, an exponential relationship existed between them, the NDVI had a little variation when the air temperature was lower than 0.0 ℃, and the NDVI increased fast with the air temperature when the air temperature was greater than 5.0 ℃. Local precipitation had great contribution to the early growing of the vegetation, when the vegetation saturated and the NDVI reached maximum, local precipitation was not sensitive to vegetation variation anymore.