| Vegetation covers a large area of land surface, and plays an important role in terrestrial biosphere. Vegetation can absorb CO2, mitigate the rise of greenhouse gas concentrations, adjust the balance of globlal carbon cycle and matain the stability of globlal climate, and it plays an important role in global material and energy cycle. Vegtation can fix CO2 by photosynthesis, and one part of the synthetic organic matter is used by autotrophic respiration, and the remaining part is used for growth and reproduction of vegetation, which is called vegetation net primary production(NPP). NPP plays an important role in carbon cycle under the background of global warming. Therefore, research on NPP is of great singnificance.MODIS MOD17A3 datasets from 2000 to 2013 were used to analyze the temporal and spatial changes of vegetation NPP in Hebei Province. The correlations between NPP and LAI(Leaf Area Index), NDVI(Normal Difference Vegetation Index), and climatic factors were analyzed. By analyzing the relationship between MOD17A3 datasets and NOAA GLOPEM datasets in 2000, the temporal change and spatial distribution of vegetation NPP during the recent 33 years in Hebei were achived by fusion method. The main conclusions were as follows:(1) The temporal changes of vegetation NPP in Hebei Province from 2000 to 2013 were analyzed. The change trends of maximum and average values of NPP in Hebei Province from 2000 to 2013 were similar, which were slightly increasing. NPP values in 2004, 2008 and 2012 were higher due to abundant precipitation, which was beneficial to vegetation growth. NPP values in 2000, 2001 and 2007 were lower because of drought. NPP peak values appeared in the years with much water and suitable heat.(2) The spatial distribution of annual average NPP in Hebei Province from 2000 to 2013 presented a descending trend from east to west and from south to north. The spatial distribution of vegetation NPP was influenced by many factors, such as vegetation types, air temperature, precipitation, and so on. The annual average NPP values of farmland or cultivated vegetation were higher than those of natural vegetation, indicating that vegetation productivity in North China Plain was greatly affected by human activities such as irrigation and fertilization.(3) The annual change ratio(Kslope) of vegetation NPP in Hebei Province from 2000 to 2013 was analyzed. In most part of the study area, vegetation NPP presented an increasing trend from 2000 to 2013. Areas with NPP increase were mainly distributed in some places of North China Plain and northwest of Zhangjiakou city. The increase of vegetation NPP was related to human activities such as afforestation and agriculture irrigation and fertilization. The significant test results of change trend from 2000 to 2013 showed that most vegetation didn’t change significantly in this period.(4) The relationships between MODIS NPP and MODIS LAI, NDVI, SPOT NDVI and climatic factors were analyzed. MOD17A3 NPP in Hebei Province from 2000 to 2013 was mainly correlated positively with MODIS LAI, with medium and low correlations. NPP was mainly correlated positively with MODIS NDVI and SPOT NDVI, with medium correlations. NPP had medium or low correlations with air temperature, precipitation and ground temperature, in which negatively with air temperature and ground temperature, and positively with precipitation.(5) Both MOD17A3 NPP in 2000 and GLOPEM NPP in 2000 could reflect the spatial distribution trend of NPP descrease from south to north and from southeast to northwest. NPP values of cultivated vegetation or farmland affected intensely by human activities were highest in 2000 in the study area. The differences between these two kinds of data were: data range was different, GLOPEM NPP values were commonly higher than MOD17A3 NPP, the spatial distribution and range of higher values and lower values were inconsistent. There was great difference between the minimum, maximum, mean and standard deviation of MOD17A3 in 2000 and those of GLOPEM NPP in 2000, but there was relationship between them.(6) The change trend of NPP in Hebei Province from 1981 to 2013 was analyzed using MOD17A3 NPP and NOAA GLOPEM NPP. The maximum presented a trend of obvious decrease, while the annual average values presented a trend of small decrease. Obvious peak values of NPP appeared in 1990, 1998, 2004 and 2008, while apparent low values of NPP appeared in 1981-1985, 1988, 1992, 2000, 2001 and 2007, owing to the influence of drought.(7) The spatial distribution of NPP from 1981 to 2013 and from 2000 to 2013 was similar, which presented a decrease trend from east to west and from south to north. The temporal change characteristics were different, the former showed that vegetation NPP decrease slightly, while the latter presented a small increase. |
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