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The genetic diversity and structure of nine natural populations of Horsfieldia hainanensis Merr., an endangered plant endemic to China, were studied using inter-simple sequence repeat markers. Nine primers were selected from 100 primers to evaluate 126 individual plants, from which a total of 136 bands were amplified and 108 bands were polymorphic. Our results demonstrated that the genetic diversity level of H. hainanensis was high with a percentage of polymorphic bands, Shannon's diversity index and Nei's genetic diversity index at the species level of 79.4%, 0.4787 and 0.3314, respectively, and correspondingly, averages of 40.4%, 0.2615 and 0.1843 at the population level. Significant genetic differentiation was observed among populations, showing that the coefficient of genetic differentiation among populations calculated using Nei's genetic diversity was 0.4509. The ranges of Nei's genetic identity and genetic distance among populations were 0.7387-0.8637 and 0.1466-0.3029, respectively. The unweighted pair group method with arithmetic mean clustering based on Nei's genetic distance indicated that nine natural H. hainanensis populations could be classified into two lineages. Collectively, we speculated that habitat fragmentation and disturbance from human activities could be considered the main reasons for the endangerment of H. hainanensis, and we propose in situ conservation for the existing natural H. hainanensis populations, especially at Mengtun, Niandou and Tongbiguan, where the genetic diversity is relatively high.

Three well-conserved Horsfieldia hainanensis Merr. populations were used to investigate their soil seed bank and seedling regeneration characteristics and their relationship to environmental factors. The results showed that the seed reserves were low in the H. hainanensis soil seed bank (16.93~24.74 seed/m2). The distribution pattern for the seeds and seedlings in the H. hainanensis populations was aggregated, and they were mainly found around 2–3 m from the mother plant. The seeds in the litter layer and the 5–10 cm soil layer showed no vigor, and only 25.7%~33.3% of the total seeds in the 0–5 cm soil layer were viable affected by the high temperature and humidity, the animals’ eating and poisoning. Affected by the height and coverage of the surrounding herbaceous layer and shrub layer, the seedlings of H. hainanensis could not obtain enough light and nutrients in the competition, resulting in the survival competitiveness of 1- to 3-year-old (1–3a) seedlings in the habitat had been in a weak position and a large number of seedlings died. It would take at least four years for seedlings to develop under the current environmental constraints. It can be concluded that the low seed reserve in the soil seed bank and high mortality of seedlings of H. hainanensis lead to slow or even stagnation of population regeneration, which was an important reason for the endangered of H. hainanensis. Therefore, the next research focus is to explore the influence mechanism of environmental factors on seed germination and seedling growth of H. hainanensis.

decreased in the 10-20 cm and 20-40 cm soil layers, and MBN increased firstly and then decreased in all three soil layers. As the soil depth increased, both MBC and MBN gradually decreased for all three forests. The MBC and MBN basically had the same seasonal variation in all three soil layers of all three forests, i.e., high in the summer and low in the winter. Correlation analysis showed that MBC was significantly positively correlated with soil organic matter, total nitrogen, and soil moisture, whereas MBN was significantly positively correlated with soil total nitrogen. It showed that soil moisture content was the main factor determining the variation of soil microbial biomass by Redundancy analysis. The results showed that the soil properties changed continuously as the young forest grew into the middle-aged forest, which increased soil microbial biomass and enriched the soil nutrients. However, the soil microbial biomass declined as the middle-age forest continued to grow, and the soil nutrients were reduced in the mature forest.

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