Abstract: Madhuca hainanensis is an endangered plant species endemic in Hainan, China. We conducted vegetation surveys for both the tree and shrub layers of M. hainanensis communities in Diaoluoshan and Jianfengling to explore species composition, floristic distribution patterns, diversity, community stability, and niche characteristics, which could provide a scientific basis for species conservation and population recovery in these areas. The results showed that Lauraceae was the dominant family among associated tree species in both sites, and the overall species composition exhibited distinct tropical floristic traits. There were no significant differences in α-diversity indices (Margalef index, Simpson index, Shannon index and Pielou index) of tree layer between the two sites, indicating broadly convergent community structural feature. In contrast, shrub layer at Jianfengling showed significantly higher Margalef (7.03) and Shannon (3.08) indices than at Diaoluoshan (4.61 and 2.59, respectively), reflecting greater species richness. Both sites exhibited significant β-diversity in both tree and shrub layers, indicating pronounced spatial differentiation in species composition. Cyclobalanopsis patentilimba and Castanopsis carlesii displayed relatively broad niche breadths (5.79 and 5.51, respectively) and high importance values (4.1% and 3.9%, respectively), jointly as constructive species alongside M. hainanensis and playing critical roles in maintaining community structure and ecological function. Species composition and structure of M. hainanensis communities in the two sites exhibited significant spatial difference, providing irreplaceable ecological value for regional biodiversity conservation and the preservation of germplasm resources of plant species with extremely small populations. On the basis of strengthening in-situ conservation, moderate artificial intervention should be implemented on tree species with high niche overlap with M. hainanensis, such as Schima superba (0.83) and Castanopsis fissa (0.91). When carrying out ex-situ conservation, species with niche complementarity with M. hainanensis, such as Acronychia pedunculata and Diospyros howii, could be preferentially selected to reduce the pressure of interspecific competition.

Key words: extremely small population, associated community, species composition, diversity, stability