Species Composition, Diversity, and Carbon Stock in Different Stand Types of the Urban Mangrove Forest Project, Rayong Province, Eastern Thailand

The decline of mangrove forests, including those in Thailand, poses significant threats to ecosystem functions and human livelihoods. This study assessed species composition, diversity, and carbon stocks within the Urban Mangrove Forest Project in Rayong Province, focusing on three stand types: natural mangrove forest (NMF), restoring mangrove forest (RMF), and non-submerged forest (NSF). We surveyed 36 plots (12 plots of 20 × 20 m per stand) to evaluate species composition, diversity, and carbon stocks. The Rhizophoraceae family exhibited the highest species richness (five species). The NSF demonstrated the highest species richness and diversity, while the NMF contained the greatest carbon stock (215.68 tons C ha−1). RMF showed a carbon stock similar to NMF (181.60 tons C ha−1), indicating that restoration efforts contribute to carbon storage. Based on the importance value index, Avicennia marina was dominant in NMF, Rhizophora apiculata in RMF, and Acacia auriculiformis in NSF. Although the NSF had a substantially lower carbon stock (21.02 tons C ha−1), it supported a diverse assemblage of mangrove, associate, and upland species. Preserving this ecological transition zone is recommended, as it enhances biodiversity, serves as a natural buffer, and may increase resilience to climate change and land-use pressures. These findings highlight the importance of restoring and conserving transition zones to sustain mangrove ecosystems.