Abstract: Time-eries MODIS NDVI datasets from 2000 to 2008 were used to study the spatial change trend, fluctuation degree, and occurrence time of the annual NDVI_max of four typical grassland types, i.e., lowland meadow, temperate steppe, temperate meadow steppe, and upland meadow, in Hulunber Grassland. In 2000-008, the vegetation in Hulunber Grassland presented an obvious deterioration trend. The mean annual NDVI_max of the four grassland types had a great fluctuation, especially in temperate steppe where the maximum change in the mean value of annual NDVI_max approximated to 50%. As for the area change of different grade grasslands, the areas with NDVI_max between 0.4and 1 accounted for about 91% of the total grassland area, which suggested the good vegetation coverage in the Grassland. However, though the areas with NDVI_max values in (0.4, 0.8］ showed an increasing trend, the areas with NDVI_max values in (0.2, 0.4］ and (0.8, 1］ decreased greatly in the study period. Overall, the deteriorating grassland took up about 66.25% of the total area, and the restoring grassland took the rest. There was about 62.85% of the grassland whose NDVI_max occurred between the 193rd day and the 225th day in each year, indicating that this period was the most important vegetation growth season in Hulunber Grassland.

Key words: Hulunber Grassland, MODIS, spatiotemporal characteristics, eutrophication, iron, algae growth, microcystin synthesis, ferric uptake regulation protein (Fur).