极小种群植物天目铁木的濒危机制

Mechanism of Endangerment of Ostrya rehderiana with Extremely Small Populations

我国的极小种群植物多数为我国特有物种,对物种多样性的维持具有重要意义。天目铁木作为浙江省特有的极小种群野生植物,野生植株仅存5株,分布地域狭窄,其濒危机制的揭示是种群得以保护和恢复的重要基础。本文从气候变化、人为干扰和病虫危害等外部因素,以及天目铁木自身的光合能力、生长节律、种子活力和遗传多样性等内部因素2个角度综述了天目铁木濒危机制的研究进展:1)外部因素:第四纪冰期以来的气候变化,导致天目铁木有效种群数量急剧减少;人为干扰导致天目铁木生存压力增大,种群规模进一步减小;病虫害对其幼苗的存活也存在一定影响。2)内部因素:天目铁木较窄的光适应范围和较低的光合速率使其在群落光照资源竞争中处于劣势,环境适应能力较弱;雌雄花授粉时间短,花粉萌发率低、寿命较短等生长节律特点导致其种子产量低、品质差;种子结构特点及内源营养物质、植物激素的限制造成其种子萌发条件苛刻,萌发率低,自然更新幼苗数量十分有限;极小的种群规模、低水平的遗传多样性引发天目铁木近交严重,子代个体杂合度降低,遗传力、生殖力、生活力、适应力减弱。综上,人为干扰和气候变化应是天目铁木种群规模急剧减小的直接原因;极小的种群规模导致其近亲交配更为频繁,遗传多样性降低,进而导致其生殖生长发育存在潜在的缺陷;加之天目铁木所处群落中其他种群的资源竞争,幼苗自然更新存在很大困难,种群规模难以恢复。然而,针对天目铁木这样遗传多样性低的极小种群植物,早期保护工作多集中于濒危因素的去除或控制,对其特有的遗传学问题则缺乏考虑。因此,为有效保护天目铁木,应当科学设计人工杂交策略、结合无性繁殖技术,增加野生个体繁殖力,在有限基因库中尽可能维持其遗传多样性,结合就地保护、迁地保护、野外回归等多种手段,最终使这一物种得以保护和恢复。

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.