三江源地区植被指数下降趋势的空间特征 及其地理背景

利用8km分辨率的Pathfinder NOAA/AVHRR-NDVI数据,结合1km分辨率的DEM,1 ∶ 250000道路、居民点、水系数据以及野外调查数据,分析了植被指数变化总体态势、植被指数变化与海拔及与距道路、水源和居民点距离之间的关系,探讨了三江源区1981~2001年间植被指数变化趋势和空间分异特征。结果表明:①三江源地区植被指数变化以下降趋势为主,下降区域占源区总面积的18.92%,增加区域占13.99%;②不同植被和冻土类型下的植被指数下降特征:灌丛区和森林区下降率最高,下降率与各类型区的居民点密度、生计方式有关;植被指数下降程度与冻土类型关系不明显;③植被指数下降的区域差异明显:下降率各区域分别为长江源区13.56%、黄河源区32.51%和澜沧江源区18.1%;④植被指数下降率随着距道路、河流的距离增加而逐渐减小;下降率在距居民点18~24km的缓冲带上达到最高后随着距离增大而下降;植被指数下降率随着海拔高程的升高呈"低－高－低－高"态势,下降率与居民点的分布高度相关。

Spatial characteristic of vegetation change in the source regions of the Yangtze River, Yellow River and Lancang River in China

The source regions of the Yangtze River, Yellow River and Lancang River are located in the middle east of the Tibetan Plateau with a total area of about 198496 km². In recent years, the risk of vegetation degradation in the regions has been keeping increasing, which led to many negative effects. This study assessed the trend and temporal-spatial distribution of vegetation index change, which is related with vegetation degradation, in the source regions of the three rivers using multi-temporal NOAA AVHRR NDVI data (8 km spatial resolution) from 1981 to 2001 provided by NASA, DEM data (1 km resolution), roads, residents, rivers data (1 ∶ 250000) and collected field data. Data types converting and reclassifying and zonal statistic analysis are completed in Arcgis 9.0 software. Results show that: ①The decrease, improved and the invariant vegetation index occupied 18.92%, 13.99% and 67.09% of the whole study area respectively. ②The decrease rates of brush, needle-leaf forest, alpine meadow, alpine sparse vegetation and alpine steppes are 22.15%, 20.75%, 18.83%, 18.73% and 18.6% respectively; on the different frozen soil types, the decrease rates differ insignificantly.The vegetation index decrease rates on permafrost and seasonal-frozen ground are 19.78% and 19.41% respectively; the decrease rates are relative to the population density and livelihoods. ③The decrease rates are different in space, which in the source regions of the Yangtze River, Yellow River and Lantcang River are 13.56%, 32.51% and 18.1% respectively. ④The nearer to the roads and water sources is, the higher the decrease rates are; but to the distance buffers of the residents, the decrease rates rise with distance expending, and reach the top when arriving at the 24 km. ⑤The trend of decrease rates is according to the settlements density in different elevation zones with the correlation coefficient of 0.78. ⑥The vegetation decrease is highly related with the activities of human beings in the source regions of the Yangtze River, Yellow River and Lancang River.