Spatiotemporal characteristics of MODIS NDVI in Hulunber Grassland
呼伦贝尔草原MODIS NDVI的时空变化特征

Time-series 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 2000-2008,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 0.4 and 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 about66.25 % 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.