Multi-Source Data-Based Study On The Evolution And Driving Mechanism Of Vegetation Carbon Sink In Fenhe River Basin

The Fenhe River Basin is the second tributary of the Yellow River Basin.The landform in the basin is complex,the terrain is highly undulating,and the ecological environment is fragile.With the impact of economic development,climate change and human activities,the vegetation ecosystem in the basin has been degraded and reduced.Enhancing vegetation carbon sinks can stabilize the ecosystem and is also one of the starting points for achieving carbon peaking and carbon neutrality goals.Net Primary Productivity（NPP）can reflect the quality and capacity of vegetation carbon sink,which plays an important role in evaluating the structure and function of vegetation ecosystem.Taking the Fenhe River Basin as an example,this thesis used the improved CASA（Carnrgie Ames Stanford Approach）model to to measure the NPP of vegetation in the Fenhe River Basin based on MODIS NDVI data,meteorological data,DEM data and vegetation type data.The difference analysis method,trend analysis method and correlation analysis method are used to analyze the spatial and temporal variation characteristics of vegetation NPP in the Fenhe River Basin from 2000 to 2020,and reveal the driving mechanism of the change.The conclusions are as follows:（1）In this thesis,the improved CASA model was used to estimate the NPP of vegetation in the Fenhe River Basin.The measured carbon storage,MODA17 NPP product data and other scholars’research results were used to test the accuracy of the measurement results.The results showed that R² was greater than 0.62,and Pearson correlation coefficient was greater than 0.75.（2）From the spatial distribution,the NPP distribution in the Fenhe River Basin is characterized by’low in the middle and high on both wings.The average NPP value of each year is between 321.3gC·m^-2a^-1 and 457.5gC·m^-2a^-1,and the average NPP is 380.08gC·m^-2a^-1.The average annual NPP of each partition from high to low is:Upstream zone,Linghuoshanxia zone,Taiyuan Basin zone,Linfen Basin zone,Downstream Valley zone.The average annual NPP of each vegetation type from high to low is:deciduous broad-leaved forest,coniferous and broad-leaved mixed forest,shrub,evergreen coniferous forest,grassland,cultivated land.The NPP values of each year in the basin have a very significant spatial autocorrelation,and there is an aggregation phenomenon in space.The global Moran index is higher than 0.6,but the aggregation shows a decreasing trend.（3）From the perspective of change trend analysis,NPP in Fenhe River Basin showed a fluctuating increase trend from 2000 to 2020,with an average annual growth rate of 5.163gC·m^-2a^-1.The NPP deviation value fluctuated and gradually approached the average level.The change trend of NPP within the year was first increasing and then decreasing.The maximum value of NPP in the whole year was in August and the minimum value was in January.The Fenhe River Basin showed a significant growth trend,which was basically consistent with the whole basin,and the growth of Linghuo shanxia zone was the most significant.Among the vegetation types,grassland and cultivated land have the highest annual growth rate.（4）From the analysis of driving factors,the natural driving factors are NDVI,temperature,precipitation and solar radiation,and the non-natural driving factors are latitude and longitude,altitude,slope,GDP and population.Based on the random forest method,the factor contribution rate statistics showed that the correlation between natural factors and NPP was in the following order:NDVI,precipitation,temperature and solar radiation.For non-natural factors,the correlation between NPP and factors was in the following order:population,latitude,slope,altitude,GDP,longitude and aspect.NPP is positively correlated with latitude,longitude and altitude,and negatively correlated with slope,GDP and population.The overall correlation coefficient between NPP and annual average NDVI in the Fenhe River Basin is 0.65,which shows a moderate positive correlation.The area with a positive correlation accounts for 87.69%of the whole basin.The correlation between NPP and NDVI in each partition is consistent with the whole basin,which is a moderately positive correlation.The correlation coefficients from large to small are:Linghuoshanxia zone,Upstream zone,Downstream Valley zone,Linfen Basin zone and Taiyuan Basin zone.The overall correlation between temperature,precipitation and solar radiation and NPP is not significant.Only in the Upstream zone,Taiyuan Basin zone and part of the Downstream Valley zone,do the precipitation and NPP show a low positive correlation.（5）From the perspective of the impact of land use type changes on NPP,the land use types in the Fenhe River Basin are mainly grassland,cultivated land and forest land.The average NPP of each land use type from large to small is:forest land,wetland,cultivated land,shrub,grassland and construction land.The contribution value of NPP of each land use type is:grassland,cultivated land,forest land,construction land,shrub and wetland.The main characteristics of land use conversion in the basin are grassland to cultivated land and grassland to forest land.The conversion of grassland to woodland and cultivated land is the main way of NPP growth in the Fenhe River Basin;the conversion of grassland and cultivated land into construction land is the main reason for NPP loss in Fenhe River Basin.