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Because vegetation affect several processes including water balance, absorption and reemission of solar radiation, latent and sensible heat fluxes, and carbon cycle, the variations in the composition and distribution of vegetation represents one of the most main source of systematic change on local, regional, or global scale. To monitor and better assess natural or man-made change in vegetation of the earth is desirable for modeling and predicting interactions between land surface and atmosphere. The temporal evolution of decadal NDVI composition is regarded as an effective time window able to show the natural seasonal variations. This paper investigates vegetation change between 1998 and 2006 in the west Liao River watershed, North China, which is the east fringe of agro-pasture transitional zone in northern China and highly sensitive to global change. Time series of SPOT-VEGETATION Normalized Difference Vegetation Index (NDVI) data are used to detect the vegetation cover change during last 9 years. Results show that the yearly maximum value composite mean NDVI over the study area increased slightly from 0.277 in 1998 to 0.287 in 2006, which indicated the increasing trend of vegetation activity. The annual average NDVI value in whole area was steady. Very slight improved and slight improved area reached 113442.32 km2 and 27987.34 km2, taking up 67.81% and 16.73% of the whole study area respectively. The degraded regions occupied about 15.16%. During 1998-2006, the landscape evolution in the western Liaohe River Basin was characterized by two opposite processes, namely vegetation restoration (returning cropland for farming to grassland and close grazing) and desertification (especially land salinization). The increasing amplitude is larger than the decreasing amplitude on the whole. There was obvious decrease of monthly MNDVI in spring months, while increasing tendency of monthly MNDVI in summer and autumn was found. Results will help to provide valuable information for environmental management policies involving biodiversity preservation and rational exploitation of natural and agricultural resources in this vulnerable ecotone.
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