Pattern Dynamics Of Total Primary Productivity Of Salt Marsh Vegetation On The Yangtze Estuary And Its Influencing Factors

Salt marsh is one of the ecosystems with the highest productivity,plays an important role in carbon sequestration,plays an important role in the global carbon balance,and is an important part of"blue carbon".Gross primary productivity（GPP）is the key flux of ecosystem carbon sink process.It plays an important role in regulating ecosystem functions and mitigating global warming and is also an important indicator of ecosystem health.However,under the joint influence of human activities and climate change,the structure of salt marsh ecosystem is prone to rapid evolution,which will affect the ecological service function of salt marsh ecosystem,including GPP.Therefore,quantitative research on the pattern dynamics and influencing factors of GPP of salt marsh vegetation in the Yangtze Estuary under the background of human activities and climate change is of great significance for in-depth understanding of the process of salt marsh carbon cycle and providing scientific basis for scientific management of salt marsh ecosystem and carbon sequestration,and for achieving the goal of"carbon neutralization"as soon as possible.This paper took the Yangtze Estuary Salt Marsh as the research object,and used remote sensing image data,field observation data and historical data and other multi-source data to interpret and analyze the temporal and spatial pattern and evolution of the Yangtze Estuary Salt Marsh vegetation from 1989 to 2020.The Leaf Area Index（LAI）inversion model is coupled with the vegetation model,and a dynamic vegetation model was established to simulate the spatiotemporal dynamics of the GPP of salt marsh vegetation in the Yangtze Estuary in the past 30 years.On this basis,the main factors and their influencing processes on the vegetation and GPP pattern of the salt marsh wetlands in the Yangtze Estuary were further quantitatively analyzed.The main findings are as follows:（1）From 1989 to 2020,the vegetation area of salt marshes in the Yangtze Estuary first decreased and then increased.The lowest value was 222.11 km²in 2007,and it rose to 354.97 km²in 2020,still a decrease of 1.6%compared with 1989.Enclosure engineering,mudflat silting and Spartina alterniflora invasion were the three main factors affecting the dynamic changes of the salt marsh vegetation pattern in the Yangtze Estuary.Since the introduction of the alien species S.alterniflora in the1990s,its area and area ratio in the salt marsh plant community have increased rapidly,and its proportion has reached 49.0%（173.94 km²）by 2020;The surrounding project led to a cumulative reduction of 649.46 km²in the area of salt marshes in the Yangtze Estuary,equivalent to 1.80 times the total area in 1989;the siltation of the tidal flats also promoted the increase in the vegetation area of salt marshes in the Yangtze Estuary（643.81 km²in total）;（2）From 1989 to 2020,the total GPP of the salt marsh vegetation in the Yangtze Estuary also decreased first and then increased,and the variation range was from 1.79×10¹¹to 6.28×10¹¹g C·a^-1,the highest in 2020（equivalent to 1.7 times that of 1989）;the periphery of Chongming Island,Jiuduansha and Nanhui Peninsulah were the main contributors to the total GPP of salt marshes in the Yangtze Estuary;In addition to the influence of the change in GPP per unit area(664.20～1767.99 g C·m^-2·a^-1in the last30 years),the enclosing project,siltation of tidal flats,and S.alterniflora invasion were also factors in the Yangtze Estuary.Three main factors influencing the dynamic change of total GPP of salt marsh vegetation.From 1989 to 2020,the total GPP reduction of the Yangtze Estuary salt marsh vegetation caused by the enclosure project accumulated to 6.01×10¹¹g C·a^-1（equivalent to 1.86 times the total in 1989）.The total amount of newly added GPP caused by the siltation of tidal flats and the invasion of S.alterniflora,the total amount of GPP in the salt marshes of the Yangtze River estuary changed from decline to growth around 2000,and the growth rate gradually accelerated,and increased to 6.28×10¹¹g C·a^-1（equivalent to 1.94 times the total in 1989）by 2020,which S.alterniflora community contributed 68.9%;（3）In summary,the Yangtze Estuary is rich in salt marsh vegetation resources and has a high GPP and total GPP,which has shown an overall upward trend from1989 to 2020.Salt marsh plants play an important role in"blue carbon".GPP per unit area,enclosure works,flats siltation and S.alterniflora invasion are the main factors affecting the dynamic change of total GPP in salt marsh vegetation in the Yangtze Estuary.Based on the objectives of conservation,restoration and sustainable development of estuarine wetlands,it is suggested to continue to control large-scale coastal enclosure activities,make scientific use of sediment resources,prevent and control Invasive Species S.alterniflora,and use natural based solutions to improve the health of the salt marsh ecosystem in the Yangtze Estuary,improve a series of ecological service functions including GPP,and enhance the carbon sequestration capacity of the salt marsh ecosystem.