Current status and threatening factors of the critically endangered mangrove fern Acrostichum speciosum in China

Acrostichum speciosum , a critically endangered mangrove fern in China, faces severe population decline with poorly understood mechanisms. We combined field quadrat surveys, reproductive experiments, and genotyping-by-sequencing across its Chinese range to investigate its community ecology, population structure, and genetic diversity. The species inhabits species-poor communities, though Zhanjiang supported higher diversity than Wenchang. While population structures were stable, regeneration was limited by scarce juveniles. The Zhanjiang population demonstrated superior density, cover, and vigor. The fern exhibits slow growth, high disease susceptibility, and critically low reproductive efficiency—evidenced by minimal spore germination (3.78%), low seedling establishment (6.38%), and prolonged gametophyte development (240 days). Genetically, populations were significantly differentiated. Despite high overall diversity and heterozygote excess, nucleotide diversity positively correlated with community diversity, and both were reduced by human activities. We conclude that anthropogenic pressures, combined with intrinsically low reproductive efficiency and slow regeneration, drive its endangerment. Conservation should prioritize the robust Zhanjiang population, minimize anthropogenic disturbance in Wenchang, and enhance gene flow through assisted introduction and rapid propagation techniques.