Seaward dynamic changes in mangrove forest over a tide-dominated estuary between 1986 and 2023

While most of the world's mangrove ecosystems have suffered severe degradation and continued retreat due to human activities and climate change, those located in tide-dominated estuaries have received much less attention. Here, we conducted a comprehensive and systematic exploration of mangroves in the tide-dominated Jiulongjiang Estuary using 1808 Landsat images and regional water and sediment records through the Continuous Change Detection and Classification method. Our results indicate that the mangrove area within the Jiulongjiang Estuary increased substantially by 275 ha, with an average annual expansion rate of 19.55 % from 1986 to 2023. Between 1986 and 1999, the rate of mangrove increase was 3.68 ha/yr, surging to 9.46 ha/yr from 1999 to 2023, despite a brief stagnation from 2006 to 2010. Before 2010, mangrove expansion was mainly concentrated in estuarine bays, while in later years mangroves rapidly colonized the filled estuarine channels. Meanwhile, mangroves have expanded seaward dramatically, with the mangrove shoreline advancing at a rate of 4.36 m/yr. This spatiotemporal pattern of mangrove expansion reflects the combined influence of human interventions and natural estuarine processes. Human activities are the primary driver of mangrove expansion in the study area, with 67 % of the mangroves being planted and predominantly located near the tidal limit. The flood tidal current plays a critical role in fostering a favorable environment for mangrove growth by resuspending sediments and transporting sediments from the outer bay into the channels, both of which promote siltation. Furthermore, the decline in fluvial sediment discharge, wave action, and rising sea levels have not impeded the seaward expansion of mangrove forests. Episodic typhoons have significantly contributed to temporary stagnation in mangrove growth. Human development activities have also caused a portion of mangrove loss. Looking forward, our findings highlight that prioritizing mangrove planting near the tidal limit can better leverage natural sedimentation processes, lower restoration costs, and minimize ecological conflicts, offering valuable insights for mangrove management in tide-dominated estuaries worldwide.