Abstract: [Objective] Based on three periods of remote sensing images of TM (1989, 2000 and 2012) and digital elevation model of ASTER GDEM, the temporal and spatial variation of vegetation coverage in recent 23 years in upper Dadu River in Sichuan province were analyzed by RS and GIS technology in this paper. [Method] Firstly, the vegetation coverage based on normalized difference vegetation index (NDVI) and the map of terrain factors including altitude, slope and slope aspect based on ASTER GDEM digital elevation were extracted through RS software in three periods. Then, the vegetation coverage map, altitude map, slope gradient map and slope aspect map were ranked through ArcGIS software. Finally, the vegetation coverage level map was analyzed together with the altitude level map, slope gradient map and slope aspect level map in each year, and obtained the vegetation coverage condition and temporal variation characteristics in three periods by statistical analysis in the study area.[Result] The results showed that the vegetation coverage f_c≥0.5 in the study area accounted for more than 58% of the whole study area. The vegetation coverage area of grade Ⅰ (f_c≥0.7) and Ⅱ (0.5≤f_c<0.7) decreased by 2.98% and 4.72% respectively, while the vegetation coverage area of grade Ⅲ (0.3≤f_c<0.5), Ⅳ(0.15≤f_c<0.3) and Ⅴ (f_c<0.15) increased by 10.88%,1.22% and 9.39% respectively. The vegetation coverage in the study area exhibited variation characteristics in different periods. During 1989—2000, the vegetation coverage decreased due to anthropogenic deforestation and overgrazing by human, and during 2000—2012, the vegetation coverage increased due to project and policies about Returning Cropland to Forestland and Natural Forest Protection. The distribution and variation of vegetation coverage were different according to elevations, slope gradient and slope aspect. The areas with high vegetation coverage were mainly distributed in the altitude of 2 500-4 500 m and the slope gradient of 30°-45°. The vegetation coverage was relatively low in the areas of altitude <2 500 m or > 4 500 m and slope gradient <30°. The vegetation coverage in different slope aspect showed as semi-sunny>sunny>semi-shady>shady. The significant variation of vegetation coverage mainly concentrated in the areas of altitude <2 500 m and slope gradient <30°. And no obvious changes were found for vegetation coverage in the areas of altitude > 4 500 m and slope gradient > 45°, where relatively less the intervention of human activities. [Conclusion] The vegetation coverage conditions in the study area were good in 1989—2012, but exhibited a decreasing trend in recent 23 years. Furthermore, the vegetation coverage in the study area was mainly affected by human activities and altitude, slope gradient and slope aspect, indicating that the protection of forest resources should not only strengthen the measures such as returning cropland to forestland and natural forest protection, but also use some advantage terrain to improve the vegetation coverage, so as to avoid the vegetation coverage gradually decreased in the study area.

Key words: RS, GIS, vegetation cover, temporal and spatial variation