藏北地区草地退化的时空分布特征

根据草地退化国家标准和藏北地区草地退化实际情况以及遥感数据特征,选择草地植被盖度为草地退化的遥感监测指标,建立藏北地区草地退化遥感监测和评价指标体系,并对藏北地区近24年的草地退化进行遥感监测和评价。结果表明:藏北地区草地退化现状(2004年)十分严重,重度和极重度退化草地面积分别占草地总面积的8.0%和1.7%,区域草地退化指数(GDI)为1.86,接近中度退化等级;其中藏北地区冰川与雪山及其周围等气候变化较为敏感区域和交通要道沿线等人类活动较为频繁区域的草地退化相对严重;从1981年到2004年的近24年以来,藏北地区及其各个区域草地退化较为严重,其草地退化等级分布比例和草地退化指数年际波动较大,草地退化等级在轻度退化至重度退化等级之间波动;近几年藏北全地区总体草地退化情况及中部、东部和北部地区的草地退化具有更加严重的趋势,而西部地区草地退化状况则略有减缓趋势。

Temporal and Spatial Distribution of Grassland Degradation in Northern Tibet

Ranging from 83o41' to 95o11'E and from 30o27' to 35o39'N, northern Tibet in the highest of Tibet is known as the ridge of the roof of world with a mean elevation of 4500 m. Northern Tibet is the headstream of the Yangtze River, Nujiang River and Lancang River. It is both the "sensitive area" of climate system and the "water tower" of China. The environmental condition of this area has significant effect on the main rivers, climate and eco-environment of Tibet and the whole country, even on the globe for its rigorous natural condition and fragile ecosystem.This study selected vegetation cover as the main evaluation index, calculated the grassland degradation index (GDI) and established the remote sense monitoring and evaluation system for grassland degradation in northern Tibet, according to the national standard (GB19377-2003), based on the remote sensing data such as NDVI data derived from NOAA/AVHRR with a spatial resolution of 8 km of 1981-2000 and from SPOT/VGT with a spatial resolution of 1 km of 2001 and from MODIS with a spatial resolution of 0.25 km of 2002-2004 respectively in this area, in combination with the actual condition of grassland degradation. The grassland degradation processes and their response to climate change during 1981-2004 were discussed and analyzed systematically in the study.The result indicated that grassland degradation in northern Tibet is very severe, and mean value of the GDI in recent 20 years is 2.54 which belongs to the severe degradation grade. From 1981 to 2004, the grassland degradation fluctuated evidently with great interannual variations in proportion of degradation degree and GDI but the total tendency turned to severe during this period with the grassland degradation grade changed from light to severe in northern Tibet. The extremely seriously degraded and seriously degraded respectively occupied 1.7% and 8.0% of the study area, the moderately and lightly degraded grassland accounted for 13.2% and 27.9% respectively and un-degraded grassland occupied 49.2% of the total grassland area in 2004. The grassland degradation was severe especially in the conjunctive area of Naqu, Biru and Jiali counties, the headstream of the Yangtze River that is Galadandong snow mountain, the areas along the Qinghai-Tibet railway and highway, and areas around the Tanggula and Nyainqentanglha snow mountains and glaciers. So the snow mountains and glaciers as well as their adjacent areas in northern Tibet which were sensitive to climate change and the areas along the vital communication line with frequent human activities experienced relatively severe grassland degradation.