黄土高原水蚀风蚀交错带植被覆盖度动态变化

以GIMMS（global inventory modeling and mapping studies）归一化植被指数（normalized difference vegetation index，NDVI）为数据源，采用像元二分模型，提取1982-2006年黄土高原水蚀风蚀交错带不同时段（1982-1989年、1990-1999年、2000-2006年）的植被覆盖度，并运用转移矩阵模型，定量分析水蚀风蚀交错带植被覆盖变化情况。结果显示：研究区植被覆盖度整体呈增加趋势，时段间先增加后减少；从植被空间分布来看，中低植被覆盖度所占比重最大，其次为低植被覆盖和中植被覆盖类型，高植被覆盖度类型所占比例最小。总体来说，从时段1（1982-1989年）到时段3（2000-2006年）研究区植被是良性发展的，虽然局部环境有一定恶化，但整体环境仍呈改善趋势。分析发现，低覆盖度植被类型重心向西南方向移动，而中高覆盖度类型和高覆盖度类型重心向东移动，反映了黄土高原西南地区宁夏、甘肃中部受降雨减少和城市化等因素的影响，在近25 a植被受到了一定破坏，而中东部吴旗、志丹、安塞、延安等地及毛乌素沙地东胜附近近年来进行的退耕还林等一系列生态建设，产生了较好的效益。

Dynamic Change of Vegetation Coverage in Water wind Erosion Ecotone in the Loess Plateau

NDVI (normalized difference vegetation index) of GIMMS (global inventory modeling and mapping studies) was taken as the data source and the dimidiate pixel model was used to extract the data of vegetation coverage with different grades in water wind erosion ecotone in the Loess Plateau in periods of 1982-1989, 1990-1999 and 2000-2006. The transition matrix model was used to quantitatively analyze the change of vegetation coverage in the study area. The result showed that the vegetation coverage in the study area was holistically in an increase trend, and it was temporally increased at first and then decreased. Viewing from the spatial distribution, it was found that the proportion of moderate low vegetation coverage was the highest, then that of low and moderate vegetation coverage, and that of high vegetation coverage was the lowest. In general, the development of vegetation coverage in the study area was virtuous from the period Ⅰ (1982-1989) to the period Ⅲ (2000-2006), and the environment was holistically improved except in some few regions. Analysis revealed that the center of low vegetation coverage was shifted southwestward, and that of moderate high and high vegetation coverage was shifted eastward, which indicated that the vegetation in Ningxia and central Gansu in southwest region of the study area was degenerated due to the decrease of precipitation and the impact of urbanization in recent 25 years. However, the ecological construction including the conversion of cropland to forest and grassland has brought good ecological benefits in Wuqi, Zhidan, Ansai, Yan[JP8]’a[JP]n and Dongsheng in Mu Us Sandy Land in the eastern part of the study area