Spatio-Temporal Variation Of Vegetation NPP In Beijing-Tianjin-Hebei Region Based On Multi-Source Remote Sensing Data

As source and sink of CO₂,terrestrial ecosystem plays an important role in global carbon cycle.In terrestrial ecosystem,vegetation plays a key role in regulating global carbon cycle,climate change and energy exchange through photosynthesis and transpiration.As an important component of surface carbon cycle,net primary productivity（NPP）reflects the productivity of vegetation in natural environment to a certain extent,and is also an important factor in determining carbon source/sink of ecosystems and regulating ecological processes.Beijing-Tianjin-Hebei Region is an important economic region in China,ecological environment is the basis of its collaborative development.The study of spatio-temporal variation of vegetation NPP and its influencing factors is important for the construction of ecological civilization,ecological environment restoration and management,and sustainable socio-economic development in Beijing-Tianjin-Hebei Region.MOD17A3 vegetation NPP data from 2001 to 2021 are used to reveal the spatiao-temporal distribution and evolution characteristics of vegetation NPP in Beijing-Tianjin-Hebei Region from 2001 to 2021,using trend analysis,variation coefficient,Hurst index,etc.Correlation analysis and overlay analysis are used to analyze the effects of vegetation factors（NDVI and LAI）,meteorological factors,soil factors,topographic factors,and land use changes to vegetation NPP,Geographic probe is used to analyze quantitatively the driving factors influencing the spatio-temporal variation of vegetation NPP in Beijing-Tianjin-Hebei Region.The following conclusions are as follows.（1）Spatio-temporal distribution of vegetation NPP.The average value of vegetation NPP from 2001 to 2021 in Beijing-Tianjin-Hebei Region is 333.75g C/（m²·a）,and the change trends of maximum and average NPP values are similar,increasing on the basis of maintaining stability.The average NPP values of different vegetation types are forest>grassland>scrub>farmland.The spatial distribution of vegetation NPP in Beijing-Tianjin-Hebei Region is high in the north and low in the south,with high value areas mainly distributed in the north of the study area and near the Yanshan and Taihang Mountains,and the low value areas mainly distributed in the northwestern Bashang plateau and the southeastern plain area.（2）Spatio-temporal evolution characteristics of vegetation NPP.The vegetation NPP in Beijing-Tianjin-Hebei Region mainly shows increasing trend from 2001 to 2021,accounting for 97.91%of the study area.The spatial stability of vegetation NPP in Beijing-Tianjin-Hebei Region from 2001 to 2021 shows coexistence of high and low fluctuations,and mainly low fluctuations,accounting for 42.80%of the study area.Although vegetation NPP in Beijing-Tianjin-Hebei Region shows increasing trend in the past 21 years,the future evolution trend is unsatisfactory,there will be risk of degradation,and the area with degradation risk accounts for 77.71%of the study area.（3）The effect of each factor on vegetation NPP.Vegetation NPP from 2001 to 2021 in Beijing-Tianjin-Hebei Region mainly correlates negatively with temperature,and correlates positively with precipitation,sunshine hours,NDVI and LAI.Vegetation NPP in Beijing-Tianjin-Hebei Region shows"increase-decrease-increase"trend with the increased of elevation with the highest NPP value of 409.76g C/（m²·a）at greater than 1500 m.The average vegetation NPP value is the highest with 340.21g C/（m²·a）on the southwest slope,and vegetation NPP value on drained soil is the highest with 441.54g C/（m²·a）.The proportion of land use types in Beijing-Tianjin-Hebei region is arable land>grassland>building land>forestland>unused land>scrub,and the transfer of land use types in Beijing-Tianjin-Hebei region mainly occurs between arable land and grassland,built-up area and forestland,and between grassland and arable land,forestland and scrub.Land use transfer types in Beijing-Tianjin-Hebei Region are characterized by the transfer out of grassland and cropland and the transfer in of forestland,built-up area and scrub.The transfer out of forestland from 2001 to 2011 and the transfer out of grassland from 2011 to2021 cause the greatest loss of vegetation NPP.The transfer of scrub to grassland and forestland from 2001 to 2011 causes the increase of vegetation NPP and the transfer of cropland to grassland from 2011 to 2021 increased vegetation NPP in Beijing-Tianjin-Hebei region.（4）The driving factors of vegetation NPP in Beijing-Tianjin-Hebei region are detected according to geographic probe model,and the results show that the explanatory power of factors of vegetation NPP spatial differentiation in Beijing-Tianjin-Hebei region is annual mean air temperature>annual sunshine hours>soil type>slope>elevation>population density>annual precipitation>vegetation type>aspect,among which the explanatory power of annual mean temperature,annual sunshine hours,elevation,slope and soil type is more than 30%,which are the main factors affecting the spatial distribution of vegetation NPP in Beijing-Tianjin-Hebei region.The interaction detection results show that the effect of two-factor enhancement is more obvious in the study area,and the interaction of any two factors has greater effect on vegetation NPP than that of single factor,with the strongest interaction between annual mean temperature and soil type.