Study of dynamic changes in fractional vegetation coverage in Hechi City in Guangxi karst regions based on TM images

Hechi City is located in northwest Guangxi Province, one of the most famous karst regions in southwest China with high temperature and precipitation. The area's upper Paleozoic Devonian, Carboniferous, Permian and Triassic carbonate sedimentary rocks have a total thickness of nearly 10,000 meters. The karst topography in Hechi is well developed, concentrated and morphologically diverse. Three periods of the Landsat TM images were selected (1900, 2000 and 2010) to investigate space-time dynamic changes in the fractional vegetation coverage and changes in the karst and non-karst regions, delineation of the different vegetation cover and their change in area according to the dimidiate pixel model based on NDVI. It was shown that during the 20 year period from 1990 to 2010, the area of high vegetation coverage increased from 30.81 % to 53.66 % of the total study area. However, the medium-low, medium and medium-high fractional vegetation coverage areas decreased from 9.63 % to 8.54 %, 17.25 % to 9.88 %, and 31.97 % to 26.74 % respectively during the same period. This shows that, over nearly 20 years, returning farmland to forests in karst regions has been an effective rocky desertification control measure. The change in the area of low fractional vegetation coverage was greater in the karst districts than in the non-karst districts, with change rates of 37.74 % in karst and 3.28 % in non-karst over nearly 20 years. But changes in the area of medinm-low, medium, medium-high and high fractional vegetation coverage were greater in non-karst than in karst, with change rates of 54.3 %, 57.47 %, 26.75 %, 75.77 % in non-karst and 34.84 %, 43.07 %, 16.34 %, 71.55 % in karst. The change in the area of medinm-low and medium fractional vegetation coverage from 1990-2000 was greater than that from 2000-2010, with the change rates of 4.68 %, 0.11 % and 5.68 %, 1.79 % for these two periods. But the change in the area of low and medium-high fractional vegetation coverage from 2000 to 2010 was greater than that from 1990 to 2000, with the change rates of 1.75 %, 5.07 % and 1.64 %, 3.59 % in these two periods. In non-karst, the change in the area of high vegetation coverage from 2000 to 2010 was greater than that from 1990 to 2000, with change rates of 8.38 % from 1990 to 2000 and 16.04 % from 2000 to 2010. The differences in the change of vegetation fraction between karst district and non-karst district are mainly due to differences in lithological conditions.