The Effects Of Density Dependence In Subtropical Evergreen Broad-leaved Forest Of Baishanzu

Density dependence was considered as an important mechanism to maintain high species diversity in forest. This theory was discussed widely in past40years. There is a long debate among ecologists as to whether density dependence is a community-wide important mechanism in maintenance of species diversity in natural forests. Some empirical and experimental studies supported this theory, but many others not. There are many questions about this theory are waiting for discussion. For example, whether is it prevalent and important in subtropical forest or not? How the effect changes form between species, from stage to stage, and from site to site. In this study, I used the data from5-ha Baishanzu Forest Dynamic Plot (FDP), a part of an evergreen broadleaved forest in the Fengyangshan-Baishanzu National Nature Reserve in eastern China, to investigate the contribution of density dependence to species coexistence in subtropical evergreen broad-leaved forest. The study was divided into three aspects:individual survival analysis, spatial pattern analysis of populations and dynamic analysis of species and phylogenetic diversity.Following density dependence theory, the survival of plants would be impaired by high density (or biomass) of conspecifics or heterspecifics with close phylogenetic relationship. At the same time, herd-protect effects would protect individuals by improving heterospecific density. After controlling effects of self size, habitat heterogeneity and spatial autocorrelation, auto-logistic regression was a good tool to reveal relationship between individual survival and density of conspecifcs/heterospecifics. Results showed that individual survival was strongly influenced by self size; topography somewhat influenced individual survival. Among22analyzed species, only8species exhibited the pattern that individual survival was negatively related to the density (or biomass) of conspecifics. Increase of phylogenetic complexity significantly improved tree survival of2species only, while increase of heterospecific density significantly reduced tree survival of4species. There was no herd-protect effects in the forest. Density dependent effects on trees and seedlings survival did not show significant difference. The effect of density dependence varied among functional guilds, which suggested the strength of this effect depended on species functional traits.If density dependence works in this forest, the clustering should decline and average distance between offspring and adult increase with the age of size classes. In this study, I used point pattern analysis with heterogeneous Poisson process and random labeling to rule out the influence of habitat heterogeneity, and selectively analyze plant-plant interaction. Results showed that spatial distribution of53.3%species we examined did not exhibit any thinning effect of density dependence. For other species that showed thinning effects, the effects were not consistent throughout the whole life history. Few species distributed regularly at adult stage. There were only16.7%species whose non-adults were apart from adults at least at a stage. The distance effect of density dependence did almost not occur in the forest. While63.3%species exhibited the pattern that non-adults were close to adults. Because of coexistence of many processes, the recruitment patterns always change with life stage. Any pattern defined by McCanny can not describe the recruitment pattern of species accurately.If abundant species (or clade) in the community suffer more serious effects of density dependence, species (or phylogenetic) diversity would increase when a group of plants aged. In this study, I compared species and phylogenetic diversity among different size-classes and checked shifts in species and phylogenetic diversity between pre-and post-mortality communities. Results showed that species diversity significantly decreased while phylogenetic diversity kept unchanged after mortality occurring. This suggested that density dependence did not determine species and phylogenetic diversity in short-term, while habitat filtering may work strongly. I also noted that species and phylogenetic diversity increase with ages of size-classes. The finding was consistent with prediction of density dependence, but it was not the result of density dependence, and may be the result of stable processes such as storage effect. However, the stable process was also restricted in ridge.All in all, I believed that the effects of density dependence on structure and dynamics of this subtropical forest were very weak. Relative to other mechanisms, negative density dependence only have little influence on maintenance of species diversity in the subtropical forest.