Laguncularia racemosa is widely planted in coastal mudflats as a pioneer species of mangrove afforestation in China; however, whether it exhibits invasiveness as an exotic species has been a topic of debate. In this study, we investigated the distribution, community structure, and population characteristics of L. racemosa on Hainan Island, China, to discuss its potential invasiveness. The results showed that Hainan Island has become an important distribution area for L. racemosa in China, covering a total area of 64.90 ha. In this investigation, a total of 11 true mangrove species, belonging to six families and 11 genera, were discovered in the L. racemosa community throughout Hainan Island. Furthermore, L. racemosa exhibited a strong adaptive range to seawater salinity and demonstrated a high capacity for natural dispersion. L. racemosa exhibited high importance values in the tree, shrub, and seedling layers of this artificial community, indicating its dominance and potential exclusion of other mangrove species. Over time, the importance values of L. racemosa in the tree and shrub layers initially decreased and then increased, while the opposite trend was observed in the seedling layer. Overall, the diversity indexes, including the Simpson index, Shannon–Wiener index, and Pielou community evenness index, were low in the planted L. racemosa forest on Hainan Island, suggesting a relatively simple species composition within the L. racemosa community. Additionally, based on the height levels of the L. racemosa population, we found that it mainly belonged to the first level, characterized by a large number of seedlings on Hainan Island. The number of L. racemosa seedlings gradually decreased in subsequent levels, and higher height levels of L. racemosa appeared with longer planting times, indicating a growing trend of potential invasiveness of L. racemosa on Hainan Island over an extended period. We suggest that the government should be able to list L. racemosa as an invasive species that can be harvested in the future. Based on this, this study provides fundamental data and theoretical references for the sustainable prevention, control, and management of the exotic species L. racemosa on Hainan Island, and also in other mangrove wetlands in China.

Nypa fruticans, commonly known as the Nipa palm, belongs to the true mangrove plants of the Arecaceae family. In China, it is naturally distributed only on Hainan Island and designated as a second-class National Key Protected Wild Plants List. Field research and indoor simulation experiments were systematically employed to study the resource status of N. fruticans and the adaptation of seed germination to environmental factors. The results showed that: (1) Four natural populations of N. fruticans, approximately 9319 trees within a total area of 3.96 hm², were distributed in Haikou, Wenchang, Qionghai, and Wanning on Hainan Island. Only the Wanning population was developed in small patches, while other populations were scattered sporadically. (2) A total of 23 mangrove species belonged to 19 genera in 13 families, which were recorded in all study sites, of which 18 were true mangroves and 5 were semi-mangrove species. The vertical structures of 4 N. fruticans communities exhibited the consistent pattern, characterized by distinct layers including the tree, shrub, and herb layers. However, notable differences in species composition and dominant species were observed among the layers of each community. (3) The population dynamics of N. fruticans in Haikou, Qionghai, and Wanning were declining, while the population in Wenchang was growing. (4) Seed germination of N. fruticans was not resistant to strong light and required some shade treatment with an optimal light intensity of 60%. The suitable salinity range for seed germination was 0‰ to 10‰. With the increase of salinity, the germination rate and seedling rate showed an increasing and then decreasing trend with maximum values of 63.3% and 50.0% at 5‰, which showed the sensitivity of seed germination to salinity, with low salinity promoting germination whereas high salinity inhibiting germination. Around 8 h/d of flooding time was most suitable for the seed germination, and 10 h/d was a critical flooding time. This study provides a theoretical basis for population recovery, resource utilization, and other further research of N. fruticans.

Coastal pond-to-mangrove restoration has become a prominent Nature-based Solution, yet its short-term ecological effects on waterbird communities remain unclear. We assessed taxonomic, functional, and compositional responses of waterbirds to large-scale restoration in Bamen Bay, Hainan Island, using BACI-style comparisons between restored and unrestored aquaculture ponds in 2021 and 2023. Restored areas exhibited higher taxonomic α diversity and functional richness (p < 0.001), coinciding with rapid habitat diversification following hydrological reconnection. Species richness (p < 0.001), Shannon diversity (p < 0.01), and functional richness (p < 0.01) were consistently higher in restored areas than in aquaculture ponds. In contrast, β diversity patterns diverged between habitats: restored areas remained relatively stable, whereas aquaculture ponds showed greater between-year compositional change (p < 0.05). Guild-specific responses revealed contrasting patterns: herons showed higher diversity in restored habitats (p < 0.05), whereas shorebirds exhibited no significant changes (p > 0.05), consistent with their dependence on open mudflats that were only partially retained. Although no significant declines were detected, functional richness tended to be lower in 2023 (p > 0.05), and ongoing mudflat loss suggests potential long-term risks for mudflat specialists, warranting extended monitoring. Taken together, our findings suggest that effective pond-to-mangrove restoration in Bamen Bay should balance mangrove expansion with the retention of tidal flats and managed shallow-water habitats to support diverse waterbird assemblages.

Accurate mangrove canopy height mapping is essential for assessing blue carbon and protecting coastal communities. Yet, existing methods face dual challenges: (1) spectral saturation causing systematic bias (underestimation of tall trees and overestimation of short trees), and (2) spatial heterogeneity of mangrove ecosystems impairing model generalizability across large areas. This study develops an Automatic Ecological Zoning and Geo-balanced Optimization (AEZGO) approach integrating Sentinel-2 imagery, UAV lidar data, and mangrove ecological knowledge to address the challenges. The approach involves: (1) Automatic Ecological Zoning via an Adaptive Weighted Partitioning Algorithm based on four ecological knowledge types (environmental factors, spectral characteristics, canopy height-spectral correlations, and administrative boundaries) to reduce spatial heterogeneity, and (2) Geo-balanced Optimization integrates sample balancing, feature selection, and spatial covariates to resolve spectral saturation in spectral-height relationships. Applied to Hainan Island, China, the approach achieved significantly higher accuracy (R² = 0.74, RMSE = 1.12 m) compared to traditional random forest (R² = 0.59, RMSE = 1.34 m). Notably, it reduced balanced mean errors (aME) by 42% for tall trees (>7 m, 2.57 to 1.49 m) and 37% for short trees (<3 m, -0.76 to -0.48 m). The resultant mangrove canopy height map revealed an average height of 3.77 m across Hainan's mangroves. Compared to the existing mangrove canopy height maps from other studies, our approach yielded the most accurate results for Hainan Island. The proposed approach shows potential for broader application, thereby supporting large-scale precision management of mangrove-related tasks such as carbon sink accounting and the evaluation of coastal protection services like windbreak and wave dissipation.