| The influences of LUCC (Land Use and Land Cover Change) mainly caused by human activities is an important issue of global change study. As a major driver of LUCC, urbanization results in simualtanuosely the changes the natural landscape of the urban land surface and composition and structure of the urban ecological ecosystems. Net primary productivity (NPP), the key parameters of the terrestrial ecosystems process, is an important component to terrestrial carbon cycle. It is also an indicator for assessing the terrestrial support ability and evaluating the sustainabity of terrestrial ecosystems. Therefore, it is of great significance to study the impact of urbanization on NPP of terrestrial ecosystems.In this study, Landsat TM/ETM+Images were used to generate land cover map of Nanjing, Jiangsu Province in each year for the period from2001to2010. These land cover map were used in conjunction with MODIS reflectance data to drive the leaf area index (LAI) inversion algorithm remote sensing data developed on the basis of the4-scale geometric optical model to retrieve8-day LAI at a resolution of250m for the study period. With the land cover maps and LAI data generated in this study, meteorological data, soil texture data, and DEM data., the remote sensing driven process-based BEPS mode was run at daily time steps to calculate NPP at a resolution of250m for the period from2001to2010. Then, spatial and temporal patterns of NPP and impacts of land cover change on of NPP were analyzed. Following conclusions can be drawn from this study:(1) The characteristics of Landcover changesThe urban area is constantly expanding especially in the downtown areas and Jiangning District during the period from2001to2010. The area of vegetation areas, especially the areas of croplands, decreased continuously. The urban area increased from684.4km2in2001to1236.9km2in2010, while the areas of croplands, evergreen coniferous forests and closed deciduous broadleaf forests decreased by689.2km2,86.5km2,85.2km2, respectively. The area of shrub decreased by50%. However, the area of mixed forests and grasslands changed marginally.The area of water body increased by287.8km2due to the expanding of artificial lakes and aquaculture.(2) Impacts of land cover change on LAILAI invsersed using the algorithm on the basis of the4-scale geometrical optical model exhibited obvious temporal and spatial distribution patterns. LAI of forests was obviousely higher than the value of croplands. LAI showed significant seasonal variations, with higher values occurring in the growing season from May to October and lower values during December to February in next year. With the expansion of urban areas, the number of pixels with LAI equal to0continuosely increased. Annual mean LAI showed decreasing trends extensively in downtown area, Jiangning and Gaochun districts (above0.2m2m-2y-1). In contrast, annual mean LAI showed increasing trends in Pukou, Liuhe and Lishui District at rates from0to0.1m2m-2y-1.(3) Impacts of land cover change on NPPThe seasonal dynamics of NPP simulated by BEPS showed double peaks. NPP in November and December was below1g C m-2d-1, and it increased gradually in the period from March to May with the increasing temperature. Particularly, the NPP of forests was higher than that of croplands. In August, the NPP of croplands reached the peak value.The temeporal trends of NPP differed spatially, mirroring the temporal patterns of land cover change. The decreasing rate of NPP was above60g C m-2y-2in most areas of downtown, Jiangning and Gaochun districts. NPP increased at rates above4g C m-2y-2in some areas of Pukou, Liuhe and Lishui Districts.The total of NPP in Nanjing city showed a downward trend form2001to2010. With the expanding of built areas in downtown and Jiangning district, the totals of NPP of forests and crops obviosley decreased. The decreaseing rates of total NPP of forests were3.370×103t C m-2y-2,4.244×103t C m-2y-2,in downtown and Jiangning district, respectively. The corresponding values are6.594X103t C m-2y-2and9.779X103t C m-2y-2for croplands. Total NPP of forests decreased obviously at rates1.410×103t C m-2y-2,1.155×103t C m-2y-2in Pukou and Liuhe districts, respectively. In constract, total NPP of croplands in these two districts increased at rates of4.047X103t C m-2y-2and1.175×103t C m-2y-2. |
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