Biogeomorphology and carbon sequestration in a coastal shoal invaded by Spartina alterniflora in the Yangtze Estuary: 22-year simulation for management implication

Coastal wetlands are key global carbon sinks and are sensitive to human activities, biological invasion, and hydro-sedimentary processes. Spartina alterniflora has invaded China's coastal region for more than 40 years. This study developed a coupled model synthesizing vegetation dynamic, biogeomorphological processes, and carbon sequestration (land-atmosphere CO₂ flux) for coastal salt marshes invaded by S. alterniflora in the Yangtze Estuary. The model was validated against measurements of vegetation community dynamics, topographic evolution, and CO₂ flux, and thus the modeling results were used to characterize the 22-year variation in biogeomorphological development and carbon sequestration in a salt marsh (Jiuduansha Wetland National Nature Reserve) in the estuary. The modeled and measured results from 2000 to 2022 showed that the total vegetation area continuously increased in the salt marsh, whereas S. alterniflora exhibited robust proliferation capabilities and interspecies competitiveness, manifesting a much higher expansion rate than the dominant native species (Phragmites australis and Scirpus mariqueter). The model reproduced the topographic evolution of the coastal shoal, indicating positive feedback between platform elevation and vegetation expansion. The model also captured species-specific net ecosystem exchange at different temporal scales. From 2000 to 2022, contribution rates of gross primary production and net ecosystem exchange by the invasive S. alterniflora increased from 40 % to 74 % and 38 % to 71 %, respectively. As a result, the contribution rate of carbon sequestration by the native species declined to less than 30 %. The invasive S. alterniflora is the predominant contributor to carbon sequestration in salt marshes due to its rapid colonization and high photosynthetic efficiency. Our model is useful for predicting the effects of coastal engineering for S. alterniflora eradication on geomorphology and carbon dynamics and assessing the feasibility of native species restoration.