Spatiotemporal dynamics of grassland degradation in Yili Valley of Xinjiang over the last 15 years

Based on data of fractional coverage extracted from MODIS NDVI data and a dimidiate pixel model, and with the assistance of digital elevation model data and Getis-Ord cold/hot spot analysis method, spatiotemporal variations of grassland degradation during the 2001-2015 period in Yili Valley were investigated. The results showed: 1) Affected by persistent overgrazing and variation in climatic conditions, grassland degeneration in Yili Valley exhibited a steady declining trend from 2001 to 2015. The proportion of degraded grassland rose to 46.18% within 15 years, but was primarily lightly degraded. 2) Spatially, degraded grassland expanded to the high-altitude area gradually, where the expansion of the degraded grassland located in the middle and sub-high mountain areas within 1 500-3 000 m above sea level was the most obvious. 3) The policies of ecological conservation implemented in the grassland slowed down the rate of grassland degradation. Spatial differences between degraded grassland and improved grassland became distinct gradually, hence the trends in grassland dynamics caused by degradation were altered. 4) The method of evaluating grassland degradation by utilizing vegetation coverage inversed from NDVI suffered from weak sensitivity to grassland degradation in high vegetation coverage areas.