| Horsfieldia hinanensisis a peculiar evergreen tree species that belong to Myristicaceae and Horsfieldia,and mainly distributes in Guangxi,Yunnan and Hainan of China.It has beenclassified as Second-CIassNational Protectedspecies because the populations are decliningwith islanddisjunction,point-like,even individual scattered distribution.Therefore,it is necessary to figure out the endanger mechanismand make conservation strategy to protect and recover H.hainanensis.This study aimed to investigatethe biological properties,community characteristics,population dynamics,population genetic diversity,seedling regeneration and seed germination,and to figure out the endangered mechanismof H.hainanensis for providing strategies of conservation.The main results are presentedas follows:(1)H.hainanensis is monoecious with the flowering periodfrom April to June and the fruit period from April to May of the next year.The fruits are yellow,oval,and fleshy with red aril.The leavesof H.hainanensis are typical bifacial leaf with well-developed main veins.The stomatal apparatus is a double ring and slightly sunken with3-4 layers palisade tissue and 4-6 layers spongy tissue.In the primary structure of the stem,8-10 ectophloic vascular bundles align in one circle around the pith.In the secondary structure,the vascular column is closely arranged into a ring.In the primary structure of the root,the endodermis has Casparian strips,and the primary xylem is polyarch with a radial arrangement.In the secondary structure,the number of cork layer,cork cambium and phelloderm is 5-6 layers,1 layer,3 layers,respectively.Therefore,H.hainanensis has certain characteristics of shade-enduringand drought-enduring.(2)Species richness and diversity index of H.hainanensis community arerelatively high.The floristic composition has obvious tropical element,so pantropical distribution and tropical Asia distribution are predominant in family and genus.According to the important value of family,the communityofH.hainanensisshows transitional characteristics between tropical and subtropical elements.The species and diversity of trees and shrubs in the community are obviously related to habitat type,and the species and quantity of herbaceous plants show negative correlation with the canopy density.The stability growth and development of the community may be related to frequent human disturbance.(3)The species composition of H.hainanensis population is abundant,and its floristic composition is obviously tropical,competitive advantage in community is not obvious.There is an obvious stewed distribution in size-class structure of H.hainanensis.The number of young seedlings at age-class 1 and age-class 2 is large,accounting for 80.49%,and the population tends to be stable.Under different habitat conditions,the size-classstructure of population and the stock of seedlings were obviously unbalanced,and dioecious tree species were more obvious.The survival curve of H.hainanensis is stable Deevey-II type,but the seed germination rateexists obvious unbalance in light of the influence of habitat conditions.The high competition,low resistance and Environmental Seize environmental filteringresultin high mortality and lead to the discontinuity and instability of population age class in the seedling stage on natural succession process.(4)The seed storage and vigor of the soil seed bank of H.hainanensis were low,storage and vigor are 16.93～24.74 pellets/m2 and 25.7%～33.3%,respectively.The distribution pattern of seed and seedling was concentrated,mainly in the section within the center of 2-3 m of the mother plant.The survival and competitiveness of 1-3 a seedlings in Hainan have been disadvantaged in the habitats,and are greatly influenced by the surrounding herbaceous layer and shrub layer coverage.It takes at least 4 years for seedlings to survive through environmental screening,and the probability of surviving and developing into young trees is very small,the highest value is only 9.1%.Therefore,due to the characteristics of the soil seed bank in H.hainanensis and the influence of the factors such as the herb layer and the shrub layer under the forest,the seed population of H.hainanensis is difficult to develop into seedlings.and the mortality of the seedlings was high.These resulted in the slow renewal and even stop of H.hainanensis,and might be one of the important reasons thatH.hainanennsisis endangered.(5)Fruits ofH.hainanensis,whichareimmature and falls off,do not germinate due to their own aril and pests.Seed germination rateof immature fruit without cracking is extremely low with only 13.33%～22.58%,and the germination rate with seed coat removal can reach 88.57%～92.86%.It isshowed that the germination of uncoated seeds or covered seedsfall sharply by stimulation of high temperature and high humidity environment for 7 days.The germination rate of uncoated seed was 15.6%,while germination rate of covered seeds is only 3.33%.Therefore,under natural conditions,the seed coat,high temperature and humidity environment and pestsare the main factors limiting seed germination.(6)A relatively high genetic diversity.was found for H.hainanensis.Percentage of polymorphic loci(PPB),Shannon’s diversity index(I)and Nei’s genetic diversity index(He)of specieswere79.41%,0.4787 and 0.3314,respectively;PPB,I and Heof average population were 40.36%,0.2615 and 0.1843,respectively.Genetic differentiation was significant between populations and coefficient of genetic differentiation(FsT)was0.4509.The Nei’s genetic identity and genetic distance between population were0.7387～0.8637,and 0.1466～0.3029,respectively.UPGMA cluster analysis indicated that nine tested natural populations ofH.hainanensis were clustered into 2 groups.To sum up,the endangered processof H.hainanensismainly results from the interaction of internal factors(including low fruit set of population,low seed germination rate,seedlings with poor adaptability and resistance are difficult to develop into saplings.)and the external environment(including the ecological environment and artificial damage).On this basis,protection of H.hainanensis natural resources should be insitu conservation,and populations with higher genetic diversity will be protectedin advance with moderate artificial interference,in order to strengthen the management of nature reserves.Meanwhile,suitable growth environmentof H.hainanensisshould be chose for plantationestablishment by artificial breeding,and ex-situconservationshould also be conductedto expand and update the existing population,protect gene pool and prevent the loss of genetic resources. |
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