关键词: 植被覆盖度, 归一化植被指数(NDVI), 植被变化趋势, 地形因子, 乌鲁木齐

Abstract: Based on four periods of remote sensing images of TM/ETM+/OLI (2000,2006,2010 and 2014) and digital elevation model of ASTER GDEM,the vegetation fractional cover distribution maps in the four periods were got by conducting the pixel dichotomy model using NDVI (Normalized Difference Vegetation Index).Further,the vegetation coverage level maps in the four periods were overlaid with altitude map,slope gradient map and slope aspect map respectively.The map were generated using digital elevation model ASTER GDEM to calculate and analyze the distributions and variations of vegetation cover in the horizontal and vertical direction.Finally the temporal and spatial changes of vegetation fractional cover,the vegetation improvement/degradation and the driving forces were analyzed.Massive degradation and significant improvement like the shape of U and the 2006 as a turning point showed the trends of vegetation fractional cover in the four periods.Compared with 2006,the vegetation cover of temperate semi-shrubby and dwarf semi-shrubby desert region in the north of Urumqi County and in the northeast of Midong District were improved to some extent.And the vegetation cover of temperate dwarf semi-arboreou desert in the north of Midong District and temperate dwarf needlegrass,dwarf semi-shrubby desert steppe in Dabancheng District were improved more significantly in 2014.Therefore,the distributions and variations of vegetation cover in Urumqi city was affected by natural factors such as precipitation,terrain factors (elevation,slope,aspect) and vegetation cover type and so on,as well as anthropogenic factors including economic activity and ecological conservation program.Desert grassland coverage had stronger instability,which was coupled by natural and anthropogenic factors,so we should improve the level of protection.Especially,a variety of ecological protection projects of forestry and grass in recent years in Urumqi city were carried out on a large scale,which played a vital role in the promotion of vegetation fractional cover and improvement of the ecological environment of Urumqi city.

Key words: vegetation fractional cover, NDVI, change trends of vegetation, terrain factors, Urumqi