Vegetation variation of mid-subtropical forest based on MODIS NDVI data — A case study of Jinggangshan City, Jiangxi Province

Abstract

Vegetation variation is an important topic of global change research, which is of great significance to deeply understand the relationship between vegetation and global change or human activities, and to disclose regional environment evolution and transition. The dynamics of forest vegetation in the mid-subtropical zone have received little attention. Thus, this paper takes the typical distribution area of the subtropical forest ecosystem — Jinggangshan City in Jiangxi Province as a study area. The changes within the year, inter-annual changes trend and spatial variation of the mid-subtropical forest vegetation index during the recent 10years are analyzed based on MODIS NDVI data from 2000–2011 with the spatial resolution of 250m. The Savitzky–Golay filter is used to smooth the original MODIS NDVI data. The forest distribution data is taken as the mask to eliminate the impact of non-forest cover area. The results showed that: (1) The changes of forest vegetation index within the year present a single peak mode with the maximum value in July; in the past 10years, the forest vegetation index fluctuated with a downward trend; NDVI values were high and stable in summer and autumn, but low and unstable in winter; (2) The distribution of NDVI values of forest vegetation had great spatial difference. The NDVI values were low in the area nearby non-forest area in the north, where the non-forest vegetation is widely distributed. The NDVI values were high in the northwestern and southeastern areas. The distribution of NDVI values are comparatively even in the middle area with the NDVI values of more than 0.7; (3) High NDVI values (>0.75) distributed most in the northwestern and southeastern areas with the altitude of 400–600m. Low NDVI values (<0.65) distributed mostly in the northern areas with the altitude less than 400m. As for different altitude zones, NDVI values are high in the area with altitude of 400–800m and low in the area with altitude below 400m or above 1200m. There is an agreement between the spatial distribution of the NDVI value of forest vegetation and regional topography, because topography has great impacts on the distribution of forest types which are different in coverage; (4) The NDVI value of forest vegetation presents a downward trend in the northern area, but an increasing trend in the southern area. The vegetation coverage tends to decrease with high population density and intensive distribution of township and scenic spot.