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

Abstract: Mangroves are crucial ecosystems with significant ecological and economic roles, providing habitat for diverse species, storm protection, and carbon sequestration. While many mangroves worldwide have faced substantial losses, China has seen notable recovery. Understanding the expansion patterns of mangroves necessitates high-resolution, large-scale monitoring of their coverage over time. 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 of the Guangxi Beibu Gulf, we summarized expansion patterns and calculated expansion and degradation rates from area changes. Our findings reveal a 1,413 ha increase in mangrove area from 2016 to 2022. Estuarine mangroves experienced an initial increase followed by a decrease, while gulf mangroves showed an initial decrease followed by an increase. Three distinct expansion patterns were identified: (1) regular mangroves expanding parallel to coastlines; (2) mangroves expanding in estuarine areas where freshwater and seawater meet; and (3) independent mangrove patches expanding into surrounding wetlands. In summary, stable and changing mangrove areas were in a ratio of 7:3, with expansion areas at 70.5% and degradation areas at 29.5%. The loss of mangroves in Guangxi Beibu Gulf is primarily due to intense human activities, with minimal contributions from sea level rise and marine dynamics.