[Classification and dynamic changes of grasslands in northern Tibet based on recent 20 years satellite data].

Based on the NOAA/AVHRR 10-day composite NDVI data from 1982 to 2000 and by the methods of principal component analysis and unsupervised classification, the vegetations in Naqu District of northern Tibet were classified, and the annual and inter-annual variations of NDVI on the pixels selected from different grassland types were analyzed. The regions where the mean NDVI values during the main growth season of grass were equal to or great than 0.1 were defined as vegetation regions, while those where the mean NDVI values were less than 0.1 were defined as non-vegetation regions. The spatiotemporal changes of NDVI in each pixel in vegetation regions were analyzed, and the results showed that the grasslands in northern Tibet could be classified into four types, i. e., high-cold meadow, high-cold meadow steppe, high-cold steppe, and high-cold desert. This classification was accorded with practical status. The annual changes of the NDVI in the four grassland types showed one peak value, with the maximum in August. In recent 20 years, the mean NDVI from July to August in vegetation regions decreased gradually from southeast to northwest, ranging from 0.1 to 0.6, and the variation coefficient changed between 0.05 and 0.40, which was smaller in the place of high NDVI than in the place of low NDVI. The annual variability of NDVI changed between -0.005 and 0.008. In recent 20 years, the vegetations had no observable change, but the vegetation development in 20% of vegetation regions, which mainly located in Nima in western Naqu and in Jiali, Biru, Suoxian and Baqing in eastern Naqu, was weakened.