中文摘要：

本文以藏北地区为研究区域，基于归一化植被指数（NDVI）反演草地植被盖度，以植被盖度作为判断草地退化的标准，综合计算藏北地区2000-2010年逐年草地退化指数（grassland degradation index），定量揭示藏北地区草地退化的时空分布规律。结果表明，截至2010年，藏北地区草地退化现状仍然较为严重，退化草地的面积占全区面积的58．2％。其中，重度退化和极重度退化草地面积的比例分别为19．0％和6．5％，区域草地退化指数为1．97，接近中等退化水平。分区结果显示，藏北地区中部、东部、北部草地退化情况较为严重，均处于中等退化等级，西部地区的退化情况最轻，处于轻度退化等级。分析2000-2010年藏北地区全区及各个分区的草地退化趋势表明，只有北部地区有减缓趋势；2000-2010年间与1981-1985年间对比结果显示，重度退化及极重度退化草地面积比例上升幅度较大。2000-2010年间藏北地区平均草地退化指数为1．84，草地退化等级在轻度退化到中度退化之间波动。

英文摘要：

An expansive alpine grassland ecosystem dominates the landscape in Northern Tibet,and the status of this grassland has significant implications for the major Asian rivers,and for the climate and eco-environ-ment of Tibet and the wider region.The harsh natural environment and fragile ecosystem,together with in-creasing frequency and intensity of human activities such as pastoralism,and climate change,have resulted in increasing reports of degradation of the grassland ecosystem.We used vegetation cover as main indicator of eco-system health,and with remote sensing monitoring,calculated the grassland degradation index (GDI)and e-valuated grassland degradation in Northern Tibet from 2000 to 2010.The analysis was based on the Normalized Differential Vegetation Index (NDVI)data from the Advanced Very High-Resolution Radiometer (AVHRR) with a spatial resolution of 8 km from 1981 to 1985 and from Moderate Resolution Imaging Spectroradiometer (MODIS)with a spatial resolution of 0.25 km of 2000-2010.Then,we have analyzed the temporal and spatial distribution of grassland degradation in Northern Tibet from 2000 to 2010.It was found that grassland degra-dation in Northern Tibet was still severe.The GDI was 1.97 in 2010,near to seriously degraded status.The extremely seriously degraded and seriously degraded categories occupied 6.5% and 19.0% of the study area, respectively.The moderately and slightly degraded grassland categories accounted for 15.1% and 12.1%,re-spectively.Undegraded grassland occupied 41.8% of the total grassland area in 2010.The extent of extremely seriously degraded and seriously degraded categories was greatly increased in the data from 2000-2010,com-pared with the period from 1981 -1985.However,the increase in grassland degradation in Northern Tibet from 2000 to 2010 was slight,with the mean value of the GDI being 1.84 in 2010,representing a change from light degradation to moderate degradation in Northern Tibet from 2000 to 2010.