Abstract:

Most PSESP (Plant Species with Extremely Small Populations) in China are endemic species, and contribute greatly to the maintenance of species diversity in China. As one of the endemic species in Zhejiang Province, Ostrya rehderiana has only 5 naturally-regenerated mature individuals that occur in the same region. The reveal of its mechanism of endangerment is an important basis for the protection and restoration of the population. We reviewed the studies on potential endangerment mechanism of O. rehderiana and classified them into two categories: the external factors like climate change, human disturbance and pest damage, as well as the internal factors such as photosynthetic capacity, growth rhythm, seed vigor and genetic diversity. 1) External factors: Climate change since Quaternary Glaciations led to a sharp decrease in the number of the effective population of O. rehderiana; man-made disturbance made O. rehderiana greater survival pressure and smaller population size; damage from diseases and insects also affected the survival rate of seedlings. 2) Internal factors: The narrow range of light adaptation and low photosynthetic rate of O. rehderiana made it inferior in the competition for light resources in the community, which impaired its environmental adaptability. The features of growth rhythm, such as short pollination time, low pollen germination rate and short life span, resulted in low seed yield and poor quality. The seed structure and the restrictions of endogenous nutrients and phytohormones led to the low seed germination rate and very few naturally-regenerated seedlings. The extremely small population size and the low level genetic diversity made the inbreeding of O. rehderiana very serious, which led to decreased heterozygosity, bringing about the weaker heritability, fertility, vitality and adaptability in offspring. In summary, human disturbance and climate change should be the direct reasons for the sharp decrease of the population size. The frequent inbreeding and lower genetic diversity might have led to potential defects in development and reproduction. In addition, due to the competition for resources with other populations in the community, it might be too difficult for O. rehderiana seedlings to naturally regenerate, which also makes the recovery of its population size improbably. However, the past protection of O. rehderiana, one of the PSESP with very low level genetic diversity, only focused on the removal or control of endangered factors, but not its unique genetic problems. Therefore, in order to protect O. rehderiana effectively, multiple method should be used, such as designing artificial hybridization strategy, together with vegetative propagation, increasing fertility of wild individuals, sustaining genetic diversity in the limited gene pool, combined with in-situ conservation, ex-situ conservation and reintroduction to the wild, the species of O. rehderiana might be finally protected and recovered.

Key words: extremely small population, Ostrya rehderiana, endangerment mechanism, external factors, internal factors, protection