The Barrier Mechanism Of The Natural Regeneration Difficulty For Casuarina Equisetifolia Artificial Coastal Protection Forest In Hainan Island

Casuarina equisetifolia is the main tree species for coastal protection forests（CPF）in Hainan Island and even in the south of China.It has the characteristics of strong wind resistance,poor soil resistance and fast growth.However,many studies have found that C.equisetifolia forest has many problems,such as thick litter,not easy to decompose,simple forest hierarchical structure,and difficult natural regeneration.However,there is a lack of systematic research on the factors affecting the natural regeneration of C.equisetifolia CPF.Therefore,in this study,C.equisetifolia CPF was taken as the research object,and three sample plots were set up in C.equisetifolia CPF,and the investigation of seed rain,seed aging test,seed germination and seedling growth test were carried out in the original habitat of C.equisetifolia,and the control test and orthogonal design test were carried out in the laboratory.Through laboratory experiments and comparative experiments in the original habitat,using the method of Biostatistics,the provenance,seed aging,and the potential factors affecting seed germination and seedling growth of C.equisetifolia were explored in the paper,so as to reveal the barrier mechanism of natural regeneration of C.equisetifolia itself.At the same time,the study of seed rain also clarified the quantity and quality of provenance of C.equisetifolia.According to the single factor multi-level experiment and multi factor multi-level orthogonal experiment,the suitable germination conditions of C.equisetifolia seeds were selected.This study systematically revealed the interannual dynamics,distribution pattern,germination and aging characteristics of C.equisetifolia seed rain,impact factors of seed germination and limiting factors of seedling growth,etc.,which not only revealed the barrier mechanism of natural regeneration of C.equisetifolia itself,but also more deeply understood the propagation characteristics and ecological habits of C.equisetifolia.This study has a certain practical guiding significance for the management transformation and ecological construction of the C.equisetifolia CPF,and also has an important academic value for enriching the management theory of the C.equisetifolia CPF.（1）In the past two years,the total density of C.equisetifolia seed rain in three plots was1764.63 grain m^-22 a^-11 and 712.60 grain m^-22 a^-1,among which the largest was in plot 1 and the smallest was in plot 3.The change trend of seed rain density is that the density of seed rain increases slowly from the beginning of August,and the peak is concentrated in the beginning and the middle of December,during which the amount of seed falling accounts for 43.87%of the total amount of scattered.Then the density fluctuated slowly over time,and ended by the middle of April of the following year.the cluster distribution of seed rain is basically in the middle of April of the next year.The average germination rate of seed rain is 14.62%.Seed rain showed aggregated distribution,and an average germination rate of seed rain was14.62%.（2）Water is the main factor that affects the germination of C.equisetifolia seeds.Temperature and light can also affect the germination of C.equisetifolia seeds,seedling growth,and litter and others indirectly impact the permination of seeds.The seeds usually germinate smoothly under the condition of sufficient water,and the seedlings grow well under the condition of sufficient light.Other factors,such as salinity,pH value,litter extract concentration and soil extract,may affect the seed germination or seedling growth of C.equisetifolia to some extent,but they have no significant effect in the setting range of this study.（3）The seed of C.equisetifolia aged rapidly,in natural conditions,especially on the top of litters and in the litters,but the aging rate was slower in the laboratory,and on the trees in the original habitat.After repeated water stress,the germination rate of C.seeds was significantly affected.（4）There are many factors affecting the germination of C.equisetifolia seeds,but in the original habitat of C.equisetifolia,although the germination rate of seeds is not high,the seeds can germinate,but because of the weak drought resistance of C.equisetifolia seedlings,if there is no rain for one week,the seedlings will die.In conclusion,the ability of C.equisetifolia seeds and seedlings to resist drought stress is poor,and water is a significant barrier factor for its natural regeneration.In conclusion,the provenance of C.equisetifolia is not an obstacle factor to its natural regeneration,but light,water,litter,aging and so on all affect the germination rate of seeds to varying degrees,of which the ability of C.equisetifolia seeds and seedlings to resist drought stress is weak,so water is the limiting factor to the survival rate of C.equisetifolia seedlings.