The Characteristics Of Cycas Hainanensis Population And Its Growth,ecophysiology Of Seedlings Responses To Different Light Environments

Cycas hainanensis C.J.Chen,is one of the National Key first-class Protected Plants in China and one of endemic species in Hainan Island.It was also classified as a Critically Endangered Population by the International Union for Conservation of Nature（IUCN）.The protection and restoration patterns of Cycas hainanensis population has been the focus of local forestry work in Hainan Province,the protection of Cycas hainanensis population has been effective,but the habitat has not been well protected,leading to the reduction of its population,its endangered mechanism and effective protection methods have been explored.Therefore,in this paper,we assessed the distribution characteristics and population dynamics of this population,the relationship between wild population structure and forest canopy,the physiological and ecological responses of seedlings to different light environments.In order to provide scientific basis for the protection and restoration of Cycas hainanensis,the present situation and dynamic change,and relationship between forest canopy density and population structure in different plots.The findings are as follows:（1）The results showed that:the investigated regional distribution of Cycas hainanensis mainly includes Wuzhishan city,Qiongzhong county,Baoting county,Lingshui county,Sanya city,Wanning city etc,they were concentrated in the mountains of the southern Middle Eastwas mainly and the coastal area of the island.Cycas hainanensis mainly distributed in tropical rainforests with a medium and high elevation range of 400～800m,slope range for the semi-sunny and semi-shady slope area,and a medium and higher canopy density range of 40%～60%,the terrain was flat,they grown in moist biotopes such as under story and the bush of forest edge,and the ravine side.The Cycas hainanensis population structure relatively stable because the young trees and small trees were in the majority,and the big trees and seedlings were rare,its renewal ability was not strong;Tt was indicated by the stationary life table that the population mortality was decreasing with age.The survival curve of Cycas hainanensis population mainly belonged to Deevey-Ⅰtype,the spatial distribution pattern of Cycas hainanensis tended to be uniform distribution.The results showed that the distribution of Cycas hainanensis in the investigated area was wide and the population was stable,under the condition of 40%-60%canopy density,it was suitable to survive,and less than 40%or more than 60%canopy density had negative effect;the population structure of Cycas hainanensis was intact,but the distribution of seedlings is few,and the regeneration ability is not strong,they were not conducive to the survival of the population.（2）Endangered Plant Cycas hainanensis in baomeiling,Changjiang county,Hainan,the community structures were vertically divided into two tree layers,one shrub layers and one herb layer,the Simpson index,Shannon-Wiener index and evenness of the communities were on the high side,the species composition in the community was rich.The wild Cycas hainanensis distributed in the shrub layer,there were uneven distribution of Cycas hainanensis in communities,formed clusters in the about 500m and forest canopy density 30%-65%of the rainforest,forest gaps and the rocks.The population dynamics belonged to growth type,there were lots of seedlings,the big trees were rare,and the old trees were no,and the population renewal ability is strong.The Cycas hainanensis population survival curve was between the Deevey-I type and Deevey-Ⅲtype,that the population mortality was decreasing with age,and the distribution pattern of Cycas hainanensis population was aggregated.The results of multiple regression analysis showed that the population size and age classes of Cycas hainanensis had no correlation with forest canopy density and plant diversity.The estimated results of the regression curve showed that the seedlings and saplings could grow well in the light transmittance range of36.11%-58.33%.（3）The growth and survival of Cycas hainanensis seedlings were different under different light intensity.In the natural forest,Cycas hainanensis had a slow growth,a negative growth state,and the leaf regeneration was very little,the seedling had a direct negative growth and no leaf regeneration,the survival rate of Cycas hainanensis seedlings in Natural Forest is higher than that in open field,the results showed that the seedlings of Cycas hainanensis could survive,but their growth was poor,the survival rate of the seedlings was low because of the high light intensity in the open space,the leaves of Cycas hainanensis seedlings in the open space were yellow and short.the net photosynthetic rate of seedlings was the highest in the forest,and the magnitude of PN was 2.66μmol CO₂·m^-2·s^-1,and did not increase with the increase of light.The net photosynthetic rate indicated that the seedlings were suitable for the forest.（4）After different shading treatments,the results showed that Cycas hainanensis seedlings were suitable for growing under moderate shading.High light 100%NS reduced stomatal conductance and net photosynthetic rate,and water use efficiency and plant growth of Cycas hainanensis seedlings.At 0.61%NS,transpiration rate and net photosynthetic rate increased,water was used effectively,but the light transmittance was too low,so that the available photosynthetic effective radiation of seedlings were too low,resulting in less organic production and high plant mortality,plant elongation to obtain light source,that’s why the leaves were elongated.The diurnal variation of net photosynthetic rate also indicated that Cycas hainanensis seedlings were suitable for growth under shading.Therefore,the results showed that the growth of Cycas hainanensis seedlings was inhibited when the light intensity was too high or too low.Inconclusion,seedling of Cycas hainanensis is suitable to grow in semi-shaded forest environment,forest environment with more forest gaps can improve the natural regeneration ability of the population and alleviate the endangered situation of this population.