Assessing the Effects of Spartina alterniflora Management on Mangrove Carbon Storage Dynamics in the Zhangjiang Estuary Using Multi-Source Remote Sensing from 2016–2025

Mangroves are critical components of coastal wetland ecosystems, providing essential services such as high-capacity carbon sequestration, biodiversity conservation, and shoreline protection. They play an irreplaceable role in mitigating climate change and maintaining coastal ecological balance. However, the effects of S. alterniflora management on mangrove carbon sinks in coastal wetlands remain poorly quantified, particularly with respect to canopy structural constraints. To address this issue, this study took the Zhangjiang Estuary Mangrove National Nature Reserve in Fujian Province as the study area and constructed a retrieval framework for mangrove above-ground vegetation carbon stock integrating Sentinel-1/2 multi-source remote sensing and ICESat-2 spaceborne LiDAR data. Based on the 10-year time series data from 2016 to 2025, we analyzed the spatiotemporal evolution patterns of mangroves and S. alterniflora. Mangrove area was calculated by counting valid mangrove pixels from 10-m annual land-cover maps generated by the accuracy-validated Random Forest classifier, with pixel area used for areal conversion. We further selected three key years (2021, 2023, and 2025) to explore the phased-variation characteristics of mangrove canopy height and above-ground vegetation carbon stock. The results showed that: (1) the mangrove area in Zhangjiang Estuary increased significantly from 2016 to 2025, with two phases, namely stable at 75.78–81.34 ha (2016–2022) and rapid growth to 183.49 ha by 2025, which is consistent with the expected outcomes of local conservation and restoration practices; (2) mangrove canopy height was retrieved using the random forest model, with a coefficient of determination R2 of 0.75 and a root mean square error (RMSE) of 0.92 m, which reliably characterized the spatial distribution patterns of mangrove canopy height in the study area; and (3) from 2021 to 2025, S. alterniflora coverage dropped by more than 98%, while mangrove carbon storage rose 118.3% and vegetation expanded outward to tidal flats. The enhancement of mangrove carbon sink function was highly consistent with the implementation progress of S. alterniflora governance and associated mangrove restoration practices. The results of this study provide a scientific basis for evaluating invasive alien species management outcomes, guiding mangrove ecological restoration, and optimizing blue carbon resource management in coastal wetland ecosystems.