A large-scale systematical investigation of the influence of Pleistocene climate oscillation on mangrove population dynamics could enrich our knowledge about the evolutionary history during times of historical climate change, which in turn may provide important information for their conservation. In this study, phylogeography of a mangrove tree Sonneratia alba was studied by sequencing three chloroplast fragments and seven nuclear genes. A low level of genetic diversity at the population level was detected across its range, especially at the range margins, which was mainly attributed to the steep sea-level drop and associated climate fluctuations during the Pleistocene glacial periods. Extremely small effective population size (Ne) was inferred in populations from both eastern and western Malay Peninsula (44 and 396, respectively), mirroring the fragility of mangrove plants and their paucity of robustness against future climate perturbations and human activity. Two major genetic lineages of high divergence were identified in the two mangrove biodiversity centres: the Indo-Malesia and Australasia regions. The estimated splitting time between these two lineages was 3.153 million year ago (MYA), suggesting a role for pre-Pleistocene events in shaping the major diversity patterns of mangrove species. Within the Indo-Malesia region, a subdivision was implicated between the South China Sea (SCS) and the remaining area with a divergence time of 1.874 MYA, corresponding to glacial vicariance when the emerged Sunda Shelf halted genetic exchange between the western and eastern coasts of the Malay Peninsula during Pleistocene sea-level drops. Notably, genetic admixture was observed in populations at the boundary regions, especially in the two populations near the Malacca Strait, indicating secondary contact between divergent lineages during interglacial periods. These interregional genetic exchanges provided ample opportunity for the re-use of standing genetic variation, which could facilitate mangrove establishment and adaptation in new habitats, especially in the context of global climate changes. Phylogeogrpahic analysis in this study reveal that Pleistocene sea-level fluctuations had profound influence on population differentiation of the mangrove tree S. alba. Our study highlights the fragility of mangrove plants and offers a guide for the conservation of coastal mangrove communities experiencing ongoing changes in sea-level.

Acanthus is a unique genus consisting of both true mangrove and terrestrial species; thus, it represents an ideal system for studying the origin and adaptive evolution of mangrove plants to intertidal environments. However, little is known regarding the two respects of mangrove species in Acanthus. In this study, we sequenced the transcriptomes of the pooled roots and leaves tissues for a mangrove species, Acanthus ilicifolius, and its terrestrial congener, A. leucostachyus, to illustrate the origin of the mangrove species in this genus and their adaptive evolution to harsh habitats. We obtained 73,039 and 69,580 contigs with N50 values of 741 and 1557 bp for A. ilicifolius and A. leucostachyus, respectively. Phylogenetic analyses based on four nuclear segments and three chloroplast fragments revealed that mangroves and terrestrial species in Acanthus fell into different clades, indicating a single origin of the mangrove species in Acanthus. Based on 6634 orthologs, A. ilicifolius and A. leucostachyus were found to be highly divergent, with a peak of synonymous substitution rate (Ks) distribution of 0.145 and an estimated divergence time of approximately 16.8 million years ago (MYA). The transgression in the Early to Middle Miocene may be the major reason for the entry of the mangrove lineage of Acanthus into intertidal environments. Gene ontology (GO) classifications of the full transcriptomes did not show any apparent differences between A. ilicifolius and A. leucostachyus, suggesting the absence of gene components specific to the mangrove transcriptomes. A total of 99 genes in A. ilicifolius were identified with signals of positive selection. Twenty-three of the 99 positively selected genes (PSGs) were found to be involved in salt, heat and ultraviolet stress tolerance, seed germination and embryo development under periodic inundation. These stress-tolerance related PSGs may be crucial for the adaptation of the mangrove species in this genus to stressful marine environments and may contribute to speciation in Acanthus. We characterized the transcriptomes of one mangrove species of Acanthus, A. ilicifolius, and its terrestrial relative, A. leucostachyus, and provided insights into the origin of the mangrove Acanthus species and their adaptive evolution to abiotic stresses in intertidal environments.

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.

The endangered mangrove Scyphiphora hydrophyllacea is found in China only in Hainan’s intertidal zones. Its populations are declining severely due to anthropogenic disturbances and regeneration failure. To clarify its environmental adaptation mechanisms, we investigated the effects of temperature, light intensity, photoperiod, salinity, soil, and flooding cycle on seed germination, seedling growth, and physiological traits, revealing that (1) the optimal germination conditions for seeds were 30–35 °C, 24 h continuous illumination at 25,000 lux, and 0‰ salinity, with soil type showing no significant effect (p > 0.05); (2) seedlings at 1–2 months post-germination achieve maximal growth at 30 °C in non-saline conditions, with salinity suppressing growth and light intensity affecting only crown expansion; and (3) flooding responses are age-dependent: seedlings at 1–2 months post-germination show optimal growth at 8 h per day (100% survival), while 12 h (h) per day reduces survival by 13.3%. One-year-old seedlings exhibit distinct strategies: 4 h per day flooding induces escape responses (peak growth, chlorophyll, sugars), 8 h per day shows photosynthetic compensation despite metabolic trade-offs, and 12 h per day triggers tolerance mechanisms (biomass maximization via structural reinforcement). These findings demonstrate S. hydrophyllacea’s multifactorial adaptation to intertidal conditions, providing critical physiological benchmarks for conservation strategies targeting this threatened ecosystem engineer.

Dolichandrone spathacea is a rare and endangered semi-mangrove plant species with an extremely small population, naturally distributed only in Zhanjiang City and the east coast of Hainan Island, China. Regrettably, the present situation of the D. spathacea population in China has not yet been carefully investigated. In this study, we aim to evaluate the current resource status and population dynamic characteristics of D. spathacea on Hainan Island, China, in order to propose sustainable conservation and management strategies for future ecological restoration of its natural population. Our results show that D. spathacea on Hainan Island can be divided into four populations. All the D. spathacea populations present a state of overall dispersion, local concentration, and occasionally sporadic existence, and they exist in geographical isolation. The young, middle, and old D. spathacea plants account for 20.42%, 66.20%, and 13.38%, respectively, indicating that the overall age class of the D. spathacea population on Hainan Island belongs to a declining type. Furthermore, instead of temporal structure, we chose diameter at breast height (DBH) to establish a static life table, draw a population survival curve, and quantify the future development trend through population dynamic analysis and time-series prediction. These results directly point out that the D. spathacea population on Hainan Island is sensitive to external disturbances and possesses two main increases in mortality rate—namely, in its juvenile and mature stages—due to weak vitality, competition, and anthropogenic interferences, which are the most important reasons for its endangered status. Depending on the current conditions of the D. spathacea population, we should conserve and expand mature trees in situ, preserve their germplasm resources, rehabilitate their habitats to promote provenance restoration, and conduct artificial cultivation and spreading planting in order to realize the sustainable conservation and management of D. spathacea.