Spatiotemporal Characteristics And Influencing Factors Of Vegetation NEP In Qinghai Province From 2000 To 2020

In order to address climate change and achieve sustainable development,the international community is constantly concerned about greenhouse gas emissions and their impacts.In order to mitigate global temperature rise caused by increasing greenhouse gas concentrations,the international community has successively formulated the United Nations Framework Convention on Climate Change（1992）,the Kyoto Protocol（1997）,the Copenhagen Accord（2009）,the Paris Agreement（2015）,and the Glasgow Climate Pact（2021）.Major global economies have announced their own voluntary emission reduction plans to achieve the "dual carbon" goal（i.e.,peak carbon emissions and carbon neutrality）,which is an important global activity for carbon reduction.The Chinese government has released a plan aimed at achieving carbon neutrality by 2060.Land ecosystem carbon sinks have become a hot topic in research on carbon neutrality.Land ecosystems are important carbon reservoirs,as well as the main absorption and emission sources of carbon in the atmosphere,and have a relatively complex underlying surface.Many uncertainties make the study of land ecosystem carbon cycling more complicated.Nevertheless,studying the patterns and mechanisms of these changes is of great scientific significance for understanding the carbon balance within ecosystems.Net ecosystem productivity（NEP）is commonly used to estimate land ecosystem carbon sources/sinks.As the world’s largest ecological regulation zone,Qinghai Province has enormous potential for carbon sequestration and carbon sinks.Therefore,studying the vegetation Net Ecosystem Productivity（NEP）and identifying its influencing factors is of great significance.Based on an improved CASA model and soil respiration model,and utilizing geographic detectors and other methods,the study analyzed the spatial and temporal distribution patterns and changes of NEP from 2000 to 2020 using MODIS products,combined with terrain data,meteorological data,and human activity data.The contribution rates of various driving factors were explored to reveal the spatial and temporal characteristics and main driving factors of Qinghai Province’s vegetation NEP.The results indicate that:（1）From 2000 to 2020,the spatial distribution pattern of vegetation NEP in Qinghai Province showed a trend of higher levels in the east and south,and lower levels in the west and north,gradually decreasing from southeast to northwest.The average annual value of vegetation NEP in Qinghai Province was 189.18 g Cm^-2a^-1,of which the area of carbon sink（NEP>0）was 403,600 km²,accounting for 77.73% of the total vegetation area,and the area of carbon source（NEP<0）was approximately 145,500km²,accounting for 22.27% of the total vegetation area.The administrative region with the strongest vegetation carbon sink in Qinghai Province is Huang nan Tibetan Autonomous Prefecture,with an annual average vegetation NPE value of 116.75 g Cm^-2a^-1.The average vegetation NPE value in Haixi Mongolian Tibetan Autonomous Prefecture is the smallest,at 26.45 g Cm^-2a^-1.The seasonal characteristics of vegetation NEP in Qinghai Province are obvious,with higher values in spring and summer,lower values in autumn,and the lowest NEP value in winter.The trend of monthly vegetation NEP carbon sequestration changes weakly from carbon sink to strong carbon sink,and then back to weak carbon sink from January to December in Qinghai Province.Carbon sink activity is mainly shown from May to September,and the most significant carbon sink activity occurs in July,with a value of 55.98 g Cm^-2a^-1.From November to February,vegetation NEP shows weak carbon sequestration characteristics,and the smallest vegetation NEP value occurs in January,at-0.44 g Cm^-2a^-1,with the most obvious carbon source characteristics.（2）From 2000 to 2020,the annual NEP in Qinghai Province showed a trend of gradual increase,with an average trend coefficient of 3.43.The area where annual NEP increased accounted for 41.34% of the total vegetation area,of which the area where annual NEP increased significantly accounted for 2.17% of the total vegetation area,and the area where annual NEP slightly increased accounted for 39.17% of the total vegetation area.Overall,the spatial changes in vegetation NEP in Qinghai Province were mainly stable,slightly improved,accounting for 58.3%,39.17% of the total area,respectively.From 2000 to 2020,the monthly value of vegetation NEP in Qinghai Province showed a trend of growth,with a greater increase from June to September,and little change in other months.The month with the smallest change in monthly NEP in Qinghai Province was from December to February of the following year.（3）Climate factors directly or indirectly affect the growth and development of vegetation.In Qinghai Province,the region with a positive correlation between vegetation net ecosystem productivity（NEP）and precipitation accounts for 62.49% of the total vegetation area,with positive correlation being the main trend.The average correlation coefficient between vegetation NEP and temperature is 0.11,with positive correlation being the main trend.Regions with positive correlation account for 67.35%of the total vegetation area,while regions with negative correlation account for 32.65%of the total vegetation area.Correlation and partial correlation analysis of vegetation NEP and climate factors show that both temperature and precipitation promote vegetation NEP accumulation,with temperature having a greater impact on vegetation NEP than precipitation,and their coupling effect being more significant.Terrain affects local climate and soil,which in turn affects vegetation growth and NEP.Vegetation NEP in Qinghai Province tends to increase first and then decrease with increasing slope and altitude.Different land use types have different types and amounts of vegetation.From2000 to 2020,the order of average NEP among different land use types in Qinghai Province was: forestland > cropland > grassland > construction land > water area >unused land.The average NEP of each land type has shown a generally increasing trend,with cropland NEP growing the fastest and unused land NEP growing relatively slowly.（4）The influence ranking of 9 factors on vegetation NEP in Qinghai Province is as follows: land use type > annual radiation > GDP > annual average precipitation >altitude > annual average temperature > slope > population density > aspect.This indicates that land use type has the strongest impact on vegetation NEP in Qinghai Province and is the most important factor affecting NEP.In interaction detection,the q values of the annual average precipitation factor interacting with the other factors pairwise were relatively high,and the interaction q value of land use type with the other factors ranked second.In 2020,the q value of the interaction between land use type and annual radiation was the highest,reaching 0.78,and the interaction was a two-factor enhancement.