NDVI variation tendency under different slopes in Taihang Mountain

Slopes in mountain areas are critical for the management of soil erosion and vegetation growth-a key determinant factor of site conditions of vegetation. Analysis of the variation tendency of NDVI (normal difference vegetation index) at different slope gradients in Taihang Mountain helps not only to understand the response of vegetation to slope gradients under long-term adaptation, but also to deepen the understanding of the variation mechanism of vegetation. Using the 250 m×250 m resolution MODIS data derived from NASA Terra Satellite, the 90 m×90 m resolution DEM data from SRTMDEM, pixel-scale trend analysis and slope-related regression analysis, this study investigated the changes in vegetation conditions in Taihang Mountain for the period 2000-2015. As the most important indicator, average NDVI in growing season was used for vegetation condition. After the selection of about 1 000 typical samples in the study area, the relationship between changes in the greenness of vegetation and slope gradient was explored using regression analysis. Meanwhile, land use transition matrixes were used to explore the transfer and direction of the acreage of land between different land use types under different slope gradients. The results showed that:1) the greenness of vegetation in Taihang Mountain improved for the study period, the vegetation-improving area accounted for 93.5% of the total study area. 2) The most obvious increase trend in NDVI was in the central and western regions (especially in central and east Shanxi Province). Also the most obvious decrease trend of NDVI was in patches of medium and lower elevations in eastern and southern Taihang Mountain, which was mainly concentrated in the eastern hilly regions adjacent to the North China Plain. 3) Average NDVI for the growing season increased with increasing slope. 4) The relationship between variation trend of NDVI (y) and slope (x) was non-linear, and the best represented by a quadratic function (y=-0.311x²+8.098x+28.027). 5) The tendency for increase in NDVI was most obvious for slope gradient within the range of 7°-15°, then for slope gradient of 15°-20°. The tendency of NDVI variation in slope range of 7°-20° was 15.8% and 29.8% higher than that in the slope > 20° and≤7°, respectively. 6) From 2000 to 2010, areas of arable land, grassland and forest land under lower (0°-7°), middle (7°-20°) and higher (> 20°) slopes decreased, and mainly transformed into construction land. However, the NDVI variation trends in the three slope ranges were all positive, and the most obvious was the middle slope area, followed by higher slope area and lower slope area. 7) The trend of NDVI variation was less affected by land use type and area, which was mainly influenced by its biochemical conditions, natural environment and human disturbance (land use intensity). Based on the above results, this paper is of great significance to rational utilization of land resources and ecological and environmental protection under different slopes of Taihang Mountain.