From degradation to conservation: Tracking mangrove dynamic changes of Hainan Island in China

Mangroves are vital coastal ecosystems in tropical and subtropical regions, providing critical services such as typhoon wave attenuation and carbon sequestration. However, these ecosystems face increasing natural and anthropogenic threats. Here, we analysed the spatiotemporal dynamics of mangroves within Hainan's protected areas using multi-temporal remote sensing data from 1988 to 2023 and the Random Forest (RF) classifier, dividing them into six study areas. Our results show a net mangrove loss of 129.25 ha, characterized by three distinct phases: degradation (1988-2003), stabilization (2003-2013), and slight expansion (2013-2023). Spatially, Dongzhaigang showed consistent growth, and Huachangwan recovered after early losses, while others experienced net losses. The spatial distribution of mangroves tends to expand seaward, while losses are predominantly concentrated in landward regions. Five of the six study areas are located within coastal lagoons, where sheltered conditions and sediment input driven by tidal currents and waves promote seaward mangrove expansion. Despite a relative sea-level rise of 4.44 mm/year, mangroves continued to expand seaward, demonstrating resilience against inundation pressures. These lagoons also provide ideal conditions for coastal aquaculture, particularly in the landward zones. Although these protected areas were primarily established in the 1980s, significant expanses of landward mangroves between 1988 and 2003 were transformed into aquaculture ponds. Following 2003, strengthened ecological policies successfully halted degradation, enabling widespread mangrove recovery. These findings provide critical insights for enhancing mangrove resilience and guiding global restoration initiatives.