Temporal And Spatial Variation Characteristics And Influencing Factors Of NPP In The Northeast Forest And Grass Interlaced Area

Human activities have caused a significant increase in the concentration of greenhouse gases,which leads to global warming.Vegetation can absorb the greenhouse gas?carbon dioxide,etc.?,and the net primary productivity of vegetation can show the ability of vegetation to fix CO₂ in the atmosphere.We must estimate the net primary productivity of the vegetation in regional or global scale timely and accurately,and analyze the temporal and spatial variation.More detailed studies are needed to fully understand the linkages between the net primary productivity of the vegetation and other related factors.It is very important to understand carbon cycle in regional or global scale.The studies can provide important scientific basis in developing and utilizing natural resources in regional or global scale reasonably.The northeast forest and grass interlaced area is one of the eight ecologically fragile areas,and it is an important area of ecological protection.It is a country with prominent ecological problems,relatively backward economy and poor people's living areas.Quantitative analysis of the spatial and temporal variation characteristics of vegetation NPP and climate change and human activities in the wood-grass ecotone of Northeast China from 2000 to 2015,in order to provide a scientific basis for the evaluation of the ecological capacity of ecological construction projects such as vegetation production capacity evaluation and returning farmland to forests in the region.Based on MOD17A3 datasets in the wood-grass ecotone of Northeast China from 2000 to2015,the methods of GIS,Zhou Guangsheng model and residual analysis are employed to analyze variability of the vegetation.The results are as follows:?1?From the time scale analysis,the precipitation and relative humidity showed an increasing trend,and the temperature,biological temperature,sunshine hours and potential evapotranspiration decreased in the wood-grass ecotone of Northeast China from 2000 to 2015.From the spatial scale analysis,the spatial distribution of temperature,biological temperature and potential evapotranspiration gradually increases from north to south,and the relative humidity gradually decreases from north to south.The precipitation is from north to south more-less-more,sunshine hours is from north to south less-more-less.?2?The actual average NPP value of the average annual vegetation in the wood-grass ecotone of Northeast China from 2000 to 2015 was 185.37-297.19 g C/?m²?a?,with an average of 245.03 g C/?m²·a?,which showed an overall upward trend.The trend from north to south is gradually decreasing,and the area showing an increasing trend?91.84%?is significantly larger than the area showing a decreasing trend?8.16%?;The average annual potential NPP of vegetation is 213.86 g C/?m²·a?,and the variation range is 188.23-248.05 g C/?m²·a?,which is on the rise.From the time scale analysis,the actual and potential NPP occurrences of the vegetation are consistent with the minimum?2001?and maximum?2013?years,and the trends are basically the same.In 2001,2007 and 2010,the annual average vegetation NPP is smaller than the vegetation potential NPP;From the spatial scale analysis,there are three types of 0-141 g C/?m² a??actual NPP is less than potential NPP,5.68%?,precipitation,temperature,biological temperature and relative humidity are higher than the average;141-281 g C/?m²·a??actual NPP equals potential NPP,66.84%?,temperature,biological temperature,sunshine hours and potential evapotranspiration are higher than the average;281-735 g C/?m²·a??actual NPP is greater than potential NPP,27.48%?,precipitation,relative humidity above average.?3?The annual average vegetation NPP is positively correlated with annual precipitation,annual mean annual average relative humidity.The annual average precipitation of vegetation NPP in the study area is greater than the annual average relative humidity.The area of the vegetation in the study area?92.15%?is much larger than the area where the vegetation is degraded?7.85%?.The area where human activities promote vegetation restoration accounts for 0.02%of the total vegetation area,while the area where climatic factors promote vegetation restoration accounts for20.01%of the total vegetation area,and the area where vegetation changes cause vegetation degradation?0.01%of total vegetation area?is less than human activity?accounting for 7.04%of the total vegetation area?.Under the combined effect of climate change and human activities,72.12%of the vegetation is in recovery and 0.8%of the vegetation is degraded.In general,climatic factors?temperature,precipitation,etc.?are the main driving force for vegetation restoration in the study area and play a leading role.Human activities such as ecological construction?returning farmland to forests and grasslands?are secondary factors for vegetation restoration.