Characteristics of Spatial-Temporal Changes in Moderate Resolution Imaging Spectroradiometer Normalized Difference Vegetation Index in the Chinese Xilingol Grasslands from 2000 to 2009

Time-series MODIS NDVI datasets for 2000–2009 were used in this study to investigate the characteristics of spatial-temporal changes in annual NDVI_max for six typical grassland types, i.e., up-land meadows, low-land meadows, temperate meadow steppes, temperate steppes, desert steppes, and steppe deserts of the Xilingol grasslands, in Inner Mongolia, China. The analysis focused on the spatial change trends, the standard deviations and the occurrence time points of annual NDVI_max for 2000–2009. The results indicated that too much utilization pressure has been put on the eastern research area, including up-land meadows, low-land meadows, temperate meadow steppes and the south and north parts of the temperate steppes; the work of vegetation protection and restoration has been remarkable in the central research area, including part of the desert steppe and the temperate steppe, and the changes in different types of grassland had been stable in total biomass. In terms of the changes in grassland areas over different grades, areas with annual NDVI_max values in (0.1, 0.2], (0.6, 0.7] decreased, and areas with annual NDVI_max values in (0.2, 0.6] increased for the study period. Insofar as space is concerned, deteriorating grasslands account for about 6.79% of the total area; restoring grasslands takes up 9.34%, Furthermore, there is about 82.14% of grassland where the annual NDVI_max occurs between the 193rd day and the 241st day in each year, indicating that this period is the most important growing season for Xilingol grasslands.