中文摘要：

基于2000—2008年的MOD IS NDVI数据,研究了呼伦贝尔草原4种主要草原类型（低地草甸、温性草原、温性草甸草原和山地草甸）年NDVImax的空间变化趋势、波动程度、出现时间.结果表明：2000—2008年,呼伦贝尔草原植被趋于恶化的形势较严峻;研究区4类草原NDVImax平均值的年际波动明显,特别是温性草原NDVImax平均值的年际最大波动接近50%;在各等级草原面积的变化上,尽管NDVImax在（0.4,1]之间的草原平均面积约占研究区总面积的91%,说明呼伦贝尔草原植被状况总体较好,但NDVImax值在（0.2,0.4〗、（0.8,1]之间的草原面积趋于减少,NDVImax在（0.4,0.8]之间的中高盖度草原面积趋于增加,总体形势不容乐观;66.25%的研究区草原植被状况趋于恶化,33.75%趋于好转;62.85%研究区的NDVImax出现在每年第193天至第225天（即7月中旬至8月初）,该时期是呼伦贝尔草原植被最重要的生长季节.

英文摘要：

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 NDVImax 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 NDVImax of the four grassland types had a great fluctuation, especially in temperate steppe where the maximum change in the mean value of annual NDVI approximated to 50%. As for the area change of different grade grasslands, the areas with NDVIm,x 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 NDVImax values in （0. 4, 0. 8 ] showed an increasing trend, the areas with NDVImax 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 NDVImax 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.