Vegetation change and its response to climate in the hill and ravine region of the Loess Plateau in Northern Shaanxi.

The hill and ravine region of the Loess Plateau in Northern Shaanxi is the worst area of water and soil conservation in China.Vegetation plays a very important role to hold water and soil in this region.Inter-annual variation of vegetation coverage and its relationship with climate in the region between 1982 and 2007 were examined by using the monthly GIMMS (Global Inventory Modeling and Mapping Studies) and SPOT VEGETATION NDVI (Normalized Difference Vegetation Index) data and local meteorological records.Vegetation coverage of the region was generally improved in the past 26 years but with substantial variations,which had three phases that NDVI was (1) tardily increasing in 1982～1998,(2) relative lowest values in 1999～2001,and (3) rapidly increasing 2003～2007.All seasons NDVI was increasing,and autumn NDVI increase was the most remarkable.Standard deviation of NDVI decreased year after year.Area of lower vegetation coverage decreased in contrast to the increase of higher vegetation coverage area.The annual temperature had a notable increase trend,and the annual precipitation had a decrease trend in the last 26 years.The spring NDVI showed positive correlation with the spring temperature.However,there was a significant negative correlation between the summer NDVI and summer temperature.The summer NDVI showed positive correlation with the summer and spring precipitation.The monthly precipitation was of deferred effect on monthly NDVI in summer.This implies that summer temperature increase and the project of replace farmland with meadow and forest were the main reason of increasing vegetation coverage.Summer water condition was the restrictive factor of vegetation.