| Productivity is a critical component of the terrestrial carbon cycle. Remote sensing driven light use efficiency models have been widely utilized to calculate the productivity of terrestrial ecosystems. Based on the theory of resources balance, they calculate the gross primary productivity (GPP) and net primary productivity (NPP) as a linear product of maximum light use efficiency εmax, scaling factors of environmental stress, and absorbed photosynthetically active radiation (APAR). The outputs of these models are very sensitive to the values of maximum light use efficiency (εmax), which represents the utilization rate of photosynthetically active radiation underideal conditions without restrictions. It exhibit spatial and temporal variations since it is affected by numerous factors, including plant types and species, canopy nitrogen content, atmospheric concentration of CO2and so on. However, in currently used light use efficiency models, this parameter only changes with plant types. Such simplified parameterization might induce large uncertainties in calculated productivity of terrestrial ecosystems at regional and global scales. Therefore, it is of importance to analyze the temporal and spatial-variations of εmax and its possible causes, which is beneficial to improvethe parameterization scheme of εmax for improving the calculation of productivity of terrestrial ecosystems using light use efficiency models and remote sensing data.In this thesis, the yield census data of differnet types, including rice, wheat, maize, legume, potatoes, cotton, sorghum, millet, peanuts, rapeseed, sugar beet, sugarcane, sesame, sunflower, and barley, was converted into total net primary productivity (NPP) for the whole country, different province in the mainland of China, and differnet cities in main cropd areas of middle and east China over the period from2001to2011in China. In this way, the cropland NPP of each administrative region unit was obtained, and the spatial and temporal variations of cropland NPP were analyzed. According to the NPP/GPP ratio, the cropland GPP of each administrative region unit was estimated. Then, the estimated cropland GPP data was use in comination with MODIS reflectance data, locally observed meteorological data, and the vegetation photosynthesis model (VPM) to εmax in the VPM model on national, provincial and city scales in China for the period from2001to2011. Then, the spatial and temporal variations of cropland εmax and possible driving factors were analyzed. The main conclusions in this study could be drawn as follows:(1) Variations of cropland NPP in China from2001to2011The mean NPP of national cropland derived from statistical yield data varied between358-481g C m-2a-1during the period from2001to2011. The national total cropland NPP averaged over the study period was511.12Tg C. It was higher in main crop production areas including the northeastern China, Huang-huai-hai area, the middle-lower Yangtze area and Sichuan province, and Henan province was the largest (54.09Tg C), accounting for10.58%of national total cropland NPP. The national total cropland NPP increased at a rate of13.79Tg C a-1. It increased in all provinces except Zhejiang, Guangdong, Fujian, Guizhou, Shanghai and Xizang. The change rate of cropland NPP in Heilongjiang was the highest (2.38Tg C a-1), and the cropland NPP in Henan increased at rate of2.14Tg C a-1, while it did not show obvious increased trends in Ningxia, Beijing, Tianjin, Chongqing, Hainan and Qinghai. During11years, the lowest national total cropland NPP was441.45Tg C, which appeared in2003, while the highest was585.12Tg C, which appeared in2011. The total cropland NPP showed obvious interannual fluctuations in Xinjiang, Heilongjiang, Beijing and Inner Mongolia, while it was comparatively stable in Sichuan and Xizang. The cropland total NPP was above5Tg C in Baoding city of Hebei, Zhoukou city, Zhumadian city, Nanyang city, Shangqiu city, and Nanyang city of Henan, Dezhou city and Heze city of Shandong, Yancheng of Jangsu. The change rate of cropland NPP in Xinyang city, Zhumadian city, and Nanyang city of Henan, and Dezhou city and Heze city of Shandong was relatively high.(2) Variations of cropland εmax in China from2001to2011During the study period, the national cropland εmax showed obvious increased trends, but showed obvious interannual fluctuations during the period from2001to2007. It was lowest in year2003, which is1.18g C-MJ-1. The national cropland εmax reached1.49g C·MJ-1in year2011. The province-level means of cropland εmax in31provinces varied between0.57-2.20g C·MJ-1during the period from2001to2011. It was higher in the east and central parts and lower in the northwest and southwest parts. Cropland εmax in Northeast Three Province, Guangxi, Inner Mongolia, Huang-huai-hai area, and most province of the middle-lower Yangtze area increased obviously during the11years. The interannual fluctuations of cropland εmax were normally higher in the north than in the south and higher in the east than in the west, and it was largest in Beijing, Heilongjiang, and Inner Mongolia, while lowest in Yunnan and Xizang. The city-level cropland εmax also showed the spatial and temporal variations obviously in main crop production area of central and east China. It was higher in eastern of Henan, northern of Hebei, western of Shandong, middle of Jiangsu, northwestern of Anhui, eastern of Hunan, and Jiangxi (except Jiujiang city, Pingxiang city, and Shangrao city), and the cropland εmax averaged over the study period was1.8-2.8g C·MJ-1. The increasing rate of city-level cropland εmax was lager in northeastern of Hunan, northern of Jiangxi, northwestern of Hubei, northern of Anhui, northern of Jiangsu, southern of Hebei, and most cities in Henan and Shandong.(3) Possible driving factors causing spatial and temporal variations of cropland εmaxDuring the study period, the annual means of cropland εmax had strong positive correlation with the amount of fertilizer used in per unit area of cultivated cropland on national scale, provincial scale innortheast China, southwest China (except Qinghai), north China (except Beijing and Tianjin), central China, Anhui, and Guangxi, and regional scale in Hebei (except Shijiazhuang city and Tangshan city), Henan, northern of Jiangsu, western and eastern of Anhui, middle of Hubei, northern and middle of Hunan, northeastern and southern of Jiangxi, and Shandong (including Liaocheng city, Heze city, Zaozhuang city, Weifang city, and Linyi city), and it reached significant level (P<0.05), so the increase of the consumption of chemical fertilizer in these regions was one of the main causes of the increase of cropland εmax. Since the interannual fluctuations of εmax were also related to the yield fraction of C4crops (corn), the increase of the yield fraction of C4crops could also induce the increase of cropland εmax. The spatial variations of cropland εmax were possibly caused by the spatial variations of the amount of fertilizer used in per unit area of cultivated cropland and the yield fraction of C4crops. The increasing rates of cropland εmax were positively correlated with the increasing rates of fertilizer utilization in per unit area of cultivated cropland and the yield fraction of C4crops. The underlying mechanisms for driving significant spatial and temporal variations of εmax need further thorough investigation. |
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