| Land-use/land-cover change and carbon cycle form two of the most substantial aspects of global climate change on both global and regional scales. It is expected that changing land use / land cover pattern will be one of the driving forces of environment change at regional scale superimposed on the natural changes at the time scale from decade to century. At the same time, the carbon cycle at mid-latitudes of North Hemisphere still remains unknown, which leads people to nowhere in gaining a deep understanding of the mechanism of global change. In Xilin River Basin, Inner Mongolia, which is right located at the mid-latitudes of North Hemisphere, the high speed of social-economic development shows high rate and strong intensity to land use / land cover change in the past two decades.In this paper, remote sensing, GIS and ecological modeling techniques were combined to study the land use / land cover change and carbon cycle of Xilin River Basin. The land use/land cover change of Xilin River Basin in the past two decades was investigated through land use / land cover classification of multi-temporal Landsat TM/ETM+ images. Century model was used to simulate the carbon cycle of the typical grassland in Xilin River Basin. The main conclusions of this study were as follows:1) Land-use/land-cover classification of Xilin River Basin. 4 sets (each set contains 2 scenes) of Landsat TM/ETM+ images acquired on Jul.31, 1987, Aug. 11, 1991, Sep. 27, 1997 and May 23, 2000, respectively, were used to classify land use / land cover of Xilin River Basin, Inner Mongolia. The overall classification accuracy was 81.0% for 1987, 81.7% for 1991, 80.1% for 1997 and 78.2% for 2000. The classification maps were optimized for later land use / cover change analysis.2) Land-use/land-cover change of Xilin River Basin. The main characteristics of land use/land cover change in Xilin River Basin over the past two decades were significant decrease in area of meadow grassland, temperate grassland vs. significant increase in area of cropland, desert grassland, urban area and desertilized land. For the latter, the desert grassland had the biggest increase in area, i.e. 2328 km2, equal to 56% of the total area of desert grassland in 1987. The cropland and urban area had increased from 114.3 km2 and 25.2 km2in 1987 to 332.1 km2'and 43.6 km2in 2000, respectively. The A. lymus + bunchgrass steppe, A. lymus + forbs steppe had the greatest decrease in area, i.e. 2040 km2. The decrease in area of meadow grassland and temperate grassland with high production vs. increase in area of desert grassland with low production indicated a dramatic degradation of the grassland in North China and the overwhelming impacts of human activities superimposed on the natural grassland ecosystem.3) Evolution route of degradation of the grassland ecosystem in Xilin River Basin. The degradation evolution route of the grassland ecosystem in Xilin River Basin was analyzed utilizing GIS techniques from the view of the point of land use/land coverclassification of multi-temporal Landsat TM/ETM+ images. Driven by overgrazing, the grassland ecosystem in Xilin River Basin, Inner Mongolia had undergone and was undergoing degradation evolution; the evolution route was from meadow grassland (F. sibiricum Steppe, S. bacalensis Steppe), via temperate grassland (A. lymus + bunchgrass Steppe, A. lymus + forbs Steppe, A. lymus + S. grandis Steppe, S. grandis + bunchgrass Steppe, S. grandis + forbs Steppe and A. lymus+ Ar. Frigida Steppe) to desert grassland (S. krylavii Steppe and Ar. frigida Steppe).4) Simulating the carbon cycle of the grassland ecosystem in Xilin River Basin with CENTURY model. The carbon cycle of the grassland ecosystem in Xilin River Basin was simulated with CENTURY model, using monthly precipitation accumulator, monthly mean minimum temperature, monthly mean maximum temperature, soil PH, soil texture, etc. as basic input variables. The model was evaluated and validated by comparing one output variable of the model, agcacc, i.e. aboveground live bio...
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