RESEARCH ON THE VARIATION CHARACTERISTICS OF VEGETATION CHANGE AND ITS TERRAIN INFLUENCE FACTORS IN ALPINE GRASSLAND IN THE SOURCE REGION OF THE YELLOW RIVER
The source region of the Yellow River is an important ecological barrier in China. Since the 1990s, under the influence of various factors, its runoff has decreased and the ecological environment has deteriorated, which has attracted great attention of governments. Vegetation is the general term of plant communities covering the surface, which is the natural link between the atmosphere, water and soil. It plays an important role in air regulation, soil conservation and maintaining the stability of the whole ecosystem. Therefore, the study on the differential characteristics and topographic influencing factors of alpine grassland change in the source region of the Yellow River can provide reference for the ecological environment protection in -this region in the future. Based on the remote sensing data of MODIS NDVI, we studied the spatial distribution pattern and temporal variation characteristics of vegetation coverage in the source region of the Yellow River from 2000 to 2009 by means of maximum synthesis method, pixel dichotomy model method and trend analysis method. Furthermore, combined with DEM topographic elevation data, we discussed the influence of topographic factors such as elevation, slope and aspect on fractional vegetation cover (FVC) change. The results showed that: (1) From 2000 to 2009, the FVC in the source region of the Yellow River fluctuated upward, showing the spatial distribution characteristics of the southeast region > the central region > the northwest region; (2) During the ten years from 2000 to 2009, the almost constant of FVC accounted for the largest area, and the areas with increasing FVC mainly concentrated in the northeast and northwest of the studied region, the areas with reduced vegetation coverage were scattered in various places in the studied region, mainly in the form of minimal patches or spots; (3) Different terrain factors had different effects on FVC and the effect of altitude on FVC changes was significantly greater than that of slope and aspect. With the increase of altitude, the proportion of significant reduction in FVC increased firstly and then decreased, while the proportion of significant increase in FVC was decreasing and the proportion of almost unchanged in FVC was increasing; With the increase of slope, the proportion in the above three cases (significant reduction, significant increase and almost unchanged in FVC) was increased, decreased gradually and fluctuated respectively. Under different aspect, the proportion in the above three cases varied little, fluctuating only within 1%.