VARIATIONS IN GRASSLAND VEGETATION COVER IN RELATION TO CLIMATIC FACTORS ON THE TIBETAN PLATEAU

To advance our understanding of the effects of climate change on grassland ecosystems, we used a time series (1982-1999) data set of the Normalized Difference Vegetation Index (NDVI) together with historical climate data to analyze interannual variations in grassland vegetation cover and explore the relationships between NDVI and climatic factors on the grasslands of the Tibetan Plateau. The NDVI increased significantly by a ratio of 0.41% a -1 and a magnitude of 0.001 0 a -1 during the growing season (p=0.015). An increase in NDVI during the growing season resulted from both the advanced growing season and accelerated vegetation activity. The largest NDVI increase was in the spring with a ratio of 0.92% a -1 and a magnitude of 0.001 4 a -1 . The NDVI increase in the summer was a secondary contributor to the NDVI increase during the growing season with a ratio of 0.37% a -1 and a magnitude of 0.001 0 a -1 . In the spring, the NDVI increased significantly in the alpine grasslands (alpine meadow and alpine steppe) and temperate steppe (p 0.01 ; p= 0.001 ; p=0.002). During the summer, a significant NDVI increase was found in alpine meadows (p= 0.027 ). However, the NDVI increase in alpine and temperate steppe was not significant (p=0.106; p=0.087 ). In the autumn, no significant increase was found in the three grasslands (p=0.585; p= 0.461 ; p=0.143). In the spring, the NDVI increase in three grasslands was corresponded to an increase in temperature. In the summer, the NDVI was related to temperature and sensitive to precipitation in the spring in the alpine grasslands (alpine meadow and alpine steppe). However, no significant statistical relationship was found between NDVI and climatic factors in temperate steppe. Significant lagged correlations between precipitation and NDVI were found for alpine grasslands (alpine meadow, alpine steppe).