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Forest litter accumulation and decomposition contribute to the nutrient pool and are critical ecological processes in the ecosystem. The Casuarina equisetifolia L.coastal protection forest（CCPF） in Hainan Island was the focus of this study, and a total of 71 temporary sample plots were set up to investigate and analyze the stand structure and litter accumulation in CCPF by means of typical survey sampling, in order to explore the amount and impact factors of litter accumulation on C. equisetifolia. The results showed that six factors affect litter accumulation of CCPF, including the average diameter at breast height（DBH）, basal area, density, canopy density, herb coverage, and pH value; litter accumulation was significantly positively correlated with the average DBH, basal area, and canopy density（P<0.01）, highly negatively correlated with herb coverage（P<0.01）, positively correlated with density, and negatively correlated with pH value. The impact factors and related properties of semi-decomposed litter mass were consistent with those of litter accumulation, but they were negatively correlated with pH value（P<0.05）. There was no significant difference in litter accumulation in different semiarid and wet climate regions of Hainan Island（P>0.05）. In addition, the six impact factors can be divided into direct and indirect factors, and into main and non-main factors according to their different impact modes and degrees on the litter accumulation. Among them, average DBH, basal area, and density were considered direct impact factors, whereas canopy density, herb coverage, and pH value were indirect impact factors. The basal area and herb coverage were the main impact factors, among which herb coverage was an important indicator of litter accumulation. The level of litter accumulation of CCPF was high, with an average of 2.735 kg·m-2 and a maximum value of 10.090 kg·m-2. The average litter thickness was 3.629 cm, and the maximum thickness was 8 cm. In litter accumulation, the semi-decomposed litter mass accounted for 72.249%. Therefore, a large amount of litter accumulation is not only an important nutrient source for C. equisetifolia, but also one of the barriers to the regeneration of undergrowth plants. [ABSTRACT FROM AUTHOR]

Casuarina equisetifolia coastal protection forest(C. CPF)is an important barrier of coastal ecology in Hainan Island, and natural regeneration is of great significance for it to play a role of protection function continuously. It is found that undergrowth natural regeneration is difficult in the most area of C. equisetifolia forests in Hainan Island, however, there is a phenomenon of good regeneration in the local area. In order to explore the cause of the great disparity in the natural regeneration, and to provide certain theoretical basis for promoting the transformation of C. CPF from plantation to near-natural forest, the main factors affecting the natural regeneration of C. CPF were studied through the analysis of the differences of regeneration quality for different forest lands. Seventy-three temporary plots were established in the C. CPF in Hainan Island, and statistical methods, such as variance analysis and correlation analysis, were used to study the effects of climate zones, stand conditions, soil factors and litter accumulation on natural regeneration quality and density. The results were as follows:(1)The regeneration of C. CPF in wet zone was significantly better than that in dry zone;(2)There was significantly negative correlation between C. equisetifolia stand density and regeneration density or herb coverage, but other factors of stand conditions had little effect on regeneration;(3)There was no significant diffe-rences in soil pH value and nutrients in the sample plots with different regeneration qualities, but ammonium nitrogen promoted the regeneration of seedlings and organic matter did the regeneration of saplings to some extent;(4)The accumulation of litter was not conductive to natural regeneration as a whole. The results indicate that the climate factor, density of C. equisetifolia stands and accumulation quantity of C. equisetifolia litters were the main impact factors for the natural regeneration of undergrowth in C. CPF. [ABSTRACT FROM AUTHOR]

Successful natural regeneration can play its ecological protection function with three conditions:(1)Abundant and good seed resources;(2)Micro-habitat suitable for seed germination;(3)Ecological conditions for the sur-vival of seedlings and young trees. However, Casuarina equisetifolia coastal protection forest(C. CPF)in Hainan Island exists unable to naturally regenerate by itself. To explore obstacle factors of unable natural regeneration of C. equisetifolia itself, the ecological conditions affecting seed germination of C. equisetifolia, such as C. equisetifolia allelopathy, soil pH, salinity, temperature, matrix type and moisture were studied. The reason is that ecological conditions affecting seed germination are one of the three conditions that restrict the success of natural regeneration, and the other two conditions are the quantity and quality of provenances, and ecological conditions for survival of seedlings and saplings respectively. C. equisetifolia cones, litters and in-situ habitat soil were collected from the C. CPF near Houwei Village, Haikou City, Hainan Island. The seed germination test was conducted by paper dish method, with 100 seeds per dish and three repeated treatments. The C. equisetifolia litter, humus and sandy soil extracts were diluted into five concentration gradients respectively and a total of 15 extracts were obtained. The pH was set to 5.0, 5.5, 6.0 and 6.5 concentration gradients for the germination solution. Salinity was set to 0.02%, 0.05% and 0.10%. The temperature was set to 25, 30 and 35 ℃. The PEG method was used for water stress test, with five concentration gradients of 0, 50, 100, 150 and 200 g·L-1. The orthogonal test was designed as three factors and four levels. The results were as follows:(1)There was no significant difference of seed germination rate between CK group and the one treated with different water extract concentrations of different extracts.(2)pH, salinity and temperature within the set range had no significant effects on seed germination rate of C. equisetifolia.(3)The germination rate of C. equisetifolia seeds treated with PEG solution with different concentration gradients was significantly different, and the seed germination rate of C. equisetifolia decreased sharply with the increase of PEG solution concentration.(4)Different matrices and watering frequencies also had significant effects on seed germination rate. Combining PEG drought stress, matrix and watering frequency, it can be found that C. equisetifolia seeds have weak drought resistance and are sensitive to water, therefore, moisture is the main limiting factor for the germination of C. equisetifolia seeds. Meanwhile, the poor water retention of litters and coastal sandy soil also restricts the germination of seeds to varying degrees. [ABSTRACT FROM AUTHOR]