Rapid Expansion of Coastal Mangrove Forest in Guangxi Beibu Gulf: Patterns, Drivers and Impacts

Mangroves are crucial ecosystems with ecological and economic importance, providing habitat for diverse species, storm protection, and carbon sequestration. In recent years, many mangroves worldwide have suffered significant losses, contrasting with significant recovery observed in China. Understanding the expansion pattern of mangroves requires high-resolution large-scale monitoring of their coverage over time. Additionally, mangrove changes vary significantly across coastal zones. In this study, we divided coastal zones based on geomorphic and sedimentary environments and generated annual mangrove coverage from 2016 to 2022 using multi-source high-resolution data and a U-Net model. By analyzing morphological changes of mangrove patches in different coastal zones in the Guangxi Beibu Gulf, we summarized mangrove expansion patterns and calculated expansion and degradation rates from area changes. Finally, we identified factors influencing mangrove expansion, including landscape indices, policies, and ocean dynamics. The results reveal a 1413ha increase in the mangrove area within the Guangxi beibu gulf from 2016 to 2022. Mangroves in estuary areas experienced an initial increase followed by a decrease, while those in gulf areas underwent an initial decrease followed by an increase. Three distinct expansion patterns were observed: (1) regular mangroves expand parallel to coastlines towards water bodies; (2) mangroves expand in estuarine areas where freshwater and seawater meet, and (3) independent mangrove patches expand into surrounding wetland areas. In summary, stable and changing mangrove areas were in a ratio of 7:3, with expansion areas comprising 70.5% and degradation areas 29.5%. The mangrove forests are mainly distributed in the estuary and gulf, with the estuarine area experiencing the fastest changes. The loss of mangroves in Guangxi Beibu Gulf is primarily attributed to intense human activities, while sea level rise and marine dynamics contribute minimally to mangrove loss. This study provides comprehensive data for mangrove conservation and restoration.