Abstract: Aims Investigating the species diversity of naturally regenerated understory vegetation in different exotic plantations within the National Park of Hainan Tropical Rainforest, and identifiy the dominant factors affecting variation in species composition. Methods A total of 173 plots (20×20 m) were established in Mt.Jianfengling section of National Park of Hainan Tropical Rainforest, including three exotic plantation types (Eucalyptus urophylla, Acacia mangium, and Pinus caribaea) and natural tropical lowland rainforests. In each plot, all woody individuals with a diameter at breast height (DBH)≥1cm were surveyed and environmental variables were sampled. The non-parametric tests (Kruskal-Wallis and Dunn’s test) were applied to compare stem density, basal area, and species richness across three DBH classes. Dominant species were identified based on importance value, and indicator species were identified using Indicator Species Analysis (ISA). Variation in species composition among forest types were examined by using similarity analysis and nonmetric multidimensional scaling (NMDS). Permutation tests were conducted to assess the effects of different varied environmental factors on variation in community composition. Important findings Eucalyptus urophylla plantations had the lowest species richness (30 ±1 species) but the highest stem density (392 ±13 individuals). In contrast, the species richness (47±1 and 42±1 species) and stem density (291±16 and 262±12 individuals) of A. mangium and P. caribaea plantations did not differ significantly from those of natural forests (54±3 species, 214±31 individuals). Naturally regenerated vegetation understory of plantations generally lacked large-diameter individuals (DBH≥10 cm). The understorey of E. urophylla plantations was dominated by Alchornea rugosa, whereas A. mangium and P. caribaea plantations had no particularly dominant species. Species composition differed significantly among forest types: E. urophylla plantations showed the lowest similarity to natural forests, while A. mangium and P. caribaea plantations exhibited greater similarity but still differed notably from natural forests. Soil pH, total nitrogen, total available phosphorus, available potassium, elevation, stand age, and basal area of planted species were the main environmental drivers to determine species composition . Conclusion For exotic species plantations in the National Park of Hainan Tropical Rainforest, future management should prioritize optimizing stand structure and promoting the growth and succession of small diameter individuals of native species to facilitate their transition into larger size classes. Given the variation in natural regeneration among plantation types, specific restoration strategies should be developed for each forest type.

Key words: species composition. species richness. indicator species, dominant species, soil factor