Population Structures And Natural Regeneration Of Dacrydium Pectinatum In Bawangling, Hainan Island

Dacrydium pectinatum is the only Dacrydium genus of Podocarpaceae family in China. It is one of the symbolic species in tropical mountain rain forest in Hainan Island and one of the key species to maintain the natural communities in that region. Its distribution range is very narrow and it can only be found in the forest region of Bawang Mountain, Jianfengling Mountain and Diaoluoshan Mountain. From the beginning of1960s, the natural forest of D. pectinatum has been severely damaged because of people’s massive logging of tropical forests in Hainan. As a result the distribution area of D. pectinatum has become ever smaller, the quality of D. pectinatum forest has gotten ever worse, and it has become ever harder for the D. pectinatum to naturally regenerate themselves. What’s worse, there has emerged a decline in tree-age structure. Based on this situation, D. pectinatum was listed in China Rare and Endangered Plants Red Data Book as the third degree of endangered plants in1992.The results showed that:(1) The number for each age class of the D. pectinatum population showed that the mortality of the first period of life stage is high, and the population is relatively stable in the middle period, after which mortality increase gradually. The survival curve of D. pectinatum population was with large variation range. The population mortality and disappear rate was higher in I-V age classes, which showed the form of Deevey-Ⅲ, and among the Ⅶ～ⅩⅦ age classes, the survival curve showed the form of Deevey-Ⅰ.(2) At the range of the study scale, there was an obvious difference in the spatial distribution of D. pectinatum in its different life stages. The individuals performed aggregation distribution in small and medium scale at the early and middle period of life stages, however, the aggregation intensity gradually weakened with the increase of scale and age, from a clumped to a random pattern as trees matures from young to middle-aged to old trees. At the later period of life stage, individuals showed random distribution among the entire study scale. The spatial association among different life stages was not significant, most of them performed no correlation. And the paper provided a scientific foundation for better understanding of the relationship between the mechanisms of population maintenance, the spatial distribution pattern of D. pectinatum, and its regeneration dynamics.(3)The paper first defines the three stages of D. pectinatum growth based on its diameter at breast height and tree height. The paper used geostatistics and constrained ordination analysis method to analyze the space distribution characteristics of D. pectinatum in three life stages from the perspective that there is relevance between its space distribution in different life stages and microhabitats factors. Also, the spatial autocorrelation of11microhabitats is analyzed. There was an obvious difference in the spatial distribution of D. pectinatum in its different life stages. The seedlings were in aggregating distribution and lived with significant spatial autocorrelation. However, the aggregating intensity gradually weakened with the seedlings growing into sapling and then to matured trees. RDA analysis showed that there were significant differences in the relationship between the distribution patterns and microhabitat factors in different life stages of D. pectinatum, and the effect of habitat filtering varied with the change in its life stages. The effect of microhabitat variables on the spatial variability of the seedling, the sapling, and the mature trees were78.4%,41.2%, and33.6%, respectively. In the whole life stage, the period from the seedling to the sapling is the key to successful plant colonization. And seedlings have got stronger environmental sensitivity than the adult tree. It follows that the restrictive effect of microhabitat factors on D. pectinatum’s distribution mainly occurred in its seedling stage. In sound conditions,22.2%variation can be interpreted by PCNM variables and microhabitat variables in common. And a typical’induced spatial variation’phenomenon was found, which means that the spatial variation of microhabitat variables can result in the spatial variation of D. pectinatum throughout its seedling, sapling stage and matured stage. The results confirmed the relevance between microhabitats and the distribution of D. pectinatum in different life stages.(4) D. pectinatum population has poor seed quality, the healthy seed collected from the seed accounts for about5.65%, shell seeds account for37.46%, and the seeds eaten by birds account for39.21%. The healthy seeds in the soil seed bank were more low (0.49%), rotten seeds account for44.16%, black seeds account for31.52%, the defect seeds account for23.82%, too little effective seeds that can not satify the needs for natural regeneration; D. pectinatum population has longer seed rain cycle, each year by the end of September or early October began to fall and continued until the following year by the end of June or early July, three seed tree seed rain intensity was (891.9±83.61) seeds·m2·a-1,(693.9±37.67) seeds·m2·a-1,(283.8±38.93) seeds·m2·a-1. The litter layer has the largest number of seeds account for89.57%out of soil seed bank.The paper provided a scientific foundation for better understanding of the relationship between the mechanisms of population maintenance, the spatial distribution pattern of D. pectinatum, and its regeneration dynamics. If there will be any further research on this topic, it is suggested that the monitoring and the study be carried out during a longer time span, and larger size of sample be studied, for a more accurate analysis of the influence of microhabitat factors on seedling regeneration and its survival pattern.