Vegetation evolution in arid land of western China

Abstract

NDVI(Normalized Difference Vegetation Index) values range from -1.0 to +1.0, areas with NDVI < 0 rarely contain any vegetation, while NDVI >0.6, vegetation cover reaches 100%. The FVC (Fractional Vegetation Cover) values range from 0 to +1.0. Above this threshold the fractional vegetation cover increases approximately as the square of the NDVI, and reaches 100% at an upper NDVI threshold, which is considerably less than 1.0. In arid and semi-arid regions, FVC is superior to NDVI. In this article we got the classification rules by using the resolution 1km FVC combining with DEM (Digital Elevation Model) to classify the vegetation types in arid parts of western China. This classification system was used to classify all FVC images of the resolution 8km*8km from 1982a to 2000a, in this way, we got the classification maps of vegetation for every year. From these classification maps we calculated three majority maps from 1982-1989a, 1990-1995a and 1996-2000a. Supported by software ARC/INFO allowed us to decided wheter cells were degraded or improved after comparing every cell value at same location in three serial maps and then getting the evolution map of vegetation. The main results are as follows: 1) the vegetation in all of arid west China has increased; 2) the vegetation was increased and the rate is higher than in all study area; cells showing improvement in mountainous areas are almost equal to cells showing degradation3) there were two types of improvement patches; the oasis patch increases around old oases and the plain sparse desert patch has also increased; 4) degradation patches appear more frequently in the lower reaches of the Tarim River, in all reaches of Cherchen River, the Jungger basin and to the south of Alxa Right County of Inner Mongolia; 5) in mountainous areas, most of the improvement cells took place in the south of the Tianshan mountains and Pamir high plain, degradation cells are located on the slope of the north Kunlun mountains as a long narrow region and big patch assembled in the Aljin Mountains and the north slope of the Qilian Mountains.