Impacts of Land Use Change on Net Primary Productivity in the Taihu Basin, China

Net primary productivity (NPP) is a key component of the terrestrial carbon cycle, which reflects not only the productivity of a community of vegetation in its natural environment but also its carbon sequestration ability. This study examined spatial and temporal changes in NPP during the period 2000-2007 in the Tanhu basin, China’s vital economic core area, which is helpful for formulating sustainable development policy and low-carbon economy development in the Taihu basin. NPP was estimated with the Carnegie Ames Stanford Approach (CASA) terrestrial carbon model combined with a major modification. The relationships between Fractional Photosynthetically Active Radiation (FPAR) and Normalized Difference Vegetation Index (NDVI) values for different vegetation types from the lookup table of NDVI-FPAR in the Moderate-resolution Imaging Spectroradiometer product (MOD15) were utilized to replace the original algorithm of FPAR in the CASA model. The regional moisture stress coefficient was estimated by monthly meteorological data and a regional evapotranspiration model. Due to small areas for forest and grassland in the Taihu basin, the maximal light use efficiency values for different forest types and grassland were specified as the average values of typical vegetation types in China, whilst the value for cropland was derived with field samples and least square functions. Historic datasets included 250 m 16-day MODIS-NDVI (MOD13) data of Feb 2000-Jan 2001, 2007, and Jun 2010-Oct 2010, land use in 2000 and 2007, land cover in the 1980s and 2005, and monthly meteorological data (including precipitation, mean temperature, and total solar radiation) from 2000 to 2010 obtained from the National Meteorological Center (NMC) of China. Results show that total NPP in the Taihu basin decreased by 2.2 TgC from 16.4 TgC in 2000 to 14.2TgC in 2007. The reduction in NPP was equivalent to the amount of CO2 emissions from 1.4×106 tons of standard coal burning, accounting for 11.2% of the total CO2 emissions from energy consumption of the Transport, Storage and Telecommunications Departments in Jiangsu Province in 2009. Except for Changxing and Deqing with increasing NPP trends, NPP changes in other cities or counties all showed a decreasing trend. The decreasing NPP trend was concentrated mainly in the northern and eastern Taihu Basin with rapid urbanization, especially for Shanghai, Suzhou, Wuxi, Jiangyin, Changshu, Kunshan, Changzhou, and Jiaxing. Rapid urbanization was the predominant factor reduced NPP by 2.8 TgC, 1.3 times of the total reduction of NPP in the Taihu basin. Land consolidation campaign resulted in increased NPP by 0.43 TgC. Although the Grain for Green Project (GGP) decreased NPP by 59.2 GgC at its early stage, it would increase NPP in the long term. The descending multiple cropping index would decrease NPP, whilst adjustment of the agricultural structure (e.g. raising the ratio of wheat cultivation and reducing rape cultivation) would increase NPP. These dual impacts on NPP in the Taihu basin resulted in an increase NPP of 0.41TgC. Fishery development resulted in a reduction in NPP of 0.33TgC by encroaching on the farmland. The modified CASA model can meet simulation of spatial and temporal changes in NPP in the Taihu basin, whereas the accuracy of the modified CASA model needs to be further examined.