Reasons of vegetation degradation in the Southwest China Guizhou Province base on MODIS NDVI data

Guizhou Province is located in southwest China, a typical subtropical karst landscape area bordering on Sichuan, Yunnan, Guangxi, Hunan and Chongqing, and is an important ecological barrier in the upper reaches of Pearl River and Yangtze River. Its ecological functions such as water, soil and biodiversity conservations are closely related to the forest ecosystem of the province. The recent years, the forest coverage in Guizhou Province has increased significantly, but in some regions such as Chishui-Xishui area it has decreased drastically. The aim of this study is to examine the major factors that has been driving the forest degradation (expressed in vegetation cover) in the province. In this study, the Mann-kendall change detection method was employed to calculate statistical trends in the Normalized Difference Vegetation Index (NDVI-a spectral index representing vegetation cover), using a 16-year MODIS time series between 2000 and 2015, to identify areas where vegetation cover has significantly decreased (z>2.36，P<0.01). For this purpose, 197 land areas with significant NDVI decrease were selected in the conditions of unchanged forestland type from 2000 to 2015 and each piece of the land size was greater than 10 km2. Pearson correlation coefficients of climate and forest NDVI values for the 197 land areas of interest were calculated to reflect the response of vegetation NDVI to climate change，which include the mean annual and seasonal values, respectively. The 197 Region Of Interest （ROI） are mainly distributed in Chishui-Xishui in the northwest, Mountain Fanjingshan in the northeast and non-karst areas in the southeast of Guizhou Province.The results showed that the NDVI annual mean decrease rate in the ROI is -0.006 per year and the variation rate of NDVI between grids in the all ROI is small. The trend of NDVI change in spring and summer is consistent with the annual NDVI at a rate of -0.01 per year from 2000 to 2015. However, the trend of NDVI change in winter is not significantly increasing. The temperature increases significantly in spring and summer, whereas the precipitation and sunshine time do not change significantly, and the overall climate change has shown a warm and dry trend in the study area during the last 16 years. The core-density analysis of the correlation coefficient shows that the increase of summer temperature is the main driving factor for the decrease of NDVI, which may be due to the increase in temperature that promotes soil evaporation, resulting in the decrease of soil moisture and thus inhibiting the growth of vegetation.