Abstract: Hopea reticulata Tardieu belongs to the genus Hopea from Dipterocarpaceae, which is only naturally distributed in Ganza Ridge of Hainan Island in China. The slash-and-burn agriculture and logging have resulted in the degradation of the original lowland rainforest to secondary forest in Ganza Ridge. After the establishment of Ganza Ridge Provincial Nature Preserve, vegetation in lowland rainforest gradually recovered and developed into a tropical rainforest community with H. reticulata as the sole dominant species. To evaluate conservation efficacy and genetic dynamics, 102 individuals of H. reticulata were divided into three age classes (youth, adult, and post-adult) based on diameter classes. Genome-wide single nucleotide polymorphisms (SNPs) were obtained via double-digest restriction site-associated DNA sequencing (ddRAD-seq) to assess nucleotide diversity and fine-scale spatial genetic structure. The results showed that H. reticulata maintained a high nucleotide diversity (π=0.25527), surpassing two endangered species, Vatica guangxiensis (π=0.21780) and Horsfieldia hainanensis (π=0.17200). The structure analysis, NJ tree and PCoA analysis revealed no significant genetic differentiation among age classes. All the three age classes showed a significant fine-scale spatial genetic structure, the strength (Sp=0.0283-0.0240) of which was negatively correlated with ages, probably due to the limited seed dispersal and self-thinning within the population. In summary, H. reticulata population in Ganza Ridge maintained a relatively high level of genetic variation due to the large population size. The nuts of H. reticulata are wingless and fall around its parent trees after maturity. The limited seed dispersal distance leads to a strong fine-scale spatial genetic structure. With increasing population ages, intraspecific competition intensifies, and the strength of fine-scale spatial genetic structure would decrease in elder age classes. Based on these findings, we suggest that the distance between individuals for seed collection and for cross-pollination should be at least more than 260 m to reduce genetic similarity, and preserve offspring genetic diversity in the work of provenance collection and human-assisted pollination.

Key words: Hopea reticulata Tardieu, double-digest restriction site-associated sequencing (ddRAD-seq), genetic diversity, fine-scale spatial genetic structure, age class