Based on TM Remote sensing of 6 stages during the year of 1978-2009, Binary analysis method combined with Normalized Difference Vegetation Index (NDVI) is used to determine the threshold value of vegetation coverage in order to research the change of forest vegetation of Beijing yongding river basin in the past 32 years, it Reveals driving forces and action mechanism of the change of yongding river basin forest vegetation. The result shows that vegetation coverage is high in the upriver of yongding river and vegetation coverage is low in the the middle and lower reaches of yongding river, it also shows that vegetation coverage fluctuated during the whole study period. Vegetation coverage dropped sharply from 1978 to 1987 and slightly improved and then dropped rapidly during 1987-1995. In 2000, vegetation coverage reached the lowest value. From 2000 to 2004 vegetation coverage shows ascendant trend and from 2004 to 2009 vegetation coverage reached a plateau. Driving force analysis shows that the vegetation cover changes mainly caused by man-made factors, including both the damage and protection of human . The second factor is the climate including rainfall and temperature, and the rainfall is dominant factor.%利用1978 ～2009年间共6期TM遥感数据,采用归一化植被指数(NDVI)结合二值化分析方法确定植被覆盖度的阈值,对北京市永定河流域32年来的森林植被覆盖变化情况进行研究,揭示永定河流域森林植被覆盖变化的驱动力因子与作用机制.结果表明,永定河流域上游植被覆盖度较高,中下游覆盖度较低,整个研究时期内植被覆盖度呈现波动变化,1978～1987年间间植被覆盖度急剧下降,而1987 ～1995年间间被覆盖度略有提高,但之后又迅速下降,2000年植被覆盖度处于历年最低值,2000～2004年间植被覆盖度呈上升趋势,2004～2009年间间被覆盖度趋于平稳.驱动力分析表明,引起植被覆盖变化的主要驱动因素是人为因素,包括人类破坏和保护2方面,其次是气候因素,包括降水和温度,其中降水占主导地位.
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