Land Cover Change and its Causes in Yulin Distr ict
陕北榆林地区土地覆被变化分析

Yulin district is located in the most Northern part of Shaanxi province, between 36°57′and 39°34′N, and 107°28′and 111°15′E. Yulin district is one of the areas of the most serious soil erosion in the world and receives a lot of attention from the Chinese government and international societies. After the establishment of the People's Republic of China, the desertification intensity declined because of rehabilitation measures. However, there is lack of knowledge concerning the dynamics of land cover and the complex role of factors in inducing changes in land cover. In the study, by usage of normalized difference vegetation index(NDVI) as an indicator of vegetation activity, seasonal trends of vegetation activity and their dynamic responses to climate changes in Yulin district were explored based on remotely sensed data (NOAA- AVHRR and VEGETATION) from 1981 to 2004. As a result, spatially averaged seasonal NDVI in Yulin showed a pronounced increase during the last two decades. The average autumn NDVI indicated the most significant increase. By analyzing inter- annual changes in seasonal NDVI, it was found that the advance of growing season was a major way for response of vegetation activity to climate changes and that the way exhibited a large regional heterogeneity. From the southeast to the northwest, change amplitude showed a declining trend.