Region character of seasonal change in vegetation activity in response to climate change in the West of China in recent 20 years

Using NDVI data of NOAA-AVHRR in recent 20 years and the temperature and precipitation data of west China, the vegetation activity is discussed by adopting the EOF and REOF decompose functions. The result shows that it is increasing of overall trend of vegetation activity in different seasons, which reflects the growth period of vegetation is advanced and prolonged under the circumstance of climate warming, but the vegetation envelopment has much inconsistency between different regions and seasons. There are 4 notable regions, 8 sub-areas for vegetation envelopment in spring and summer, and 9 sub-areas in autumn. The vegetation activity in the most sub-areas is increasing. The most notable region is represented by Lazi station in Tibet plateau. Two other marked stations are represented by Aletai station in Xinjiang province and Penshui station in Sichuan province. But the trend through the time series analysis of NDVI of the other two sub-areas, Tulufan station in Xinjiang province and Huashan station in Shanxi province, are descending. It is an important reason of vegetation envelopment that temperature ascends in the most regions and descends in the east region in some seasons. But another important reason for vegetation envelopment is that precipitation is ascending in the west and descending in the east of the region.