Density Dependence Effect Of Evergreen And Deciduous Broad-leaved Mixed Forest In Karst Hills Of Guilin,Southwest China

One of the central problems in ecological studies has long been to understand the mechanisms that multi-species coexistence in forest plant communities.There is growing evidence that density dependent theory,originating from the Janzen-Connell hypothesis,is one of the important mechanisms for regulating the structure of plant populations and maintaining diversity of forest species.However,there are many questions that need to be explored further.For example,whether the density dependent effect is common in karst hills forests,and whether there are differences in different growth stages of the same species.In view of this,this study took the evergreen deciduous broad-leaved mixed forest community in the karst hills of Guilin as the research object.A 2 hm~2plot of 200 m×100 m was established in Long Village,Luojin Town,Yongfu County,Guilin,reference the standards of the Center for Tropical Forest Science(CTFS).Exploring tree population distribution patterns spatial are helpful in elucidating the mechanisms underlying species coexistence in forest communities.According to different life forms,the species were divided into three stages:saplings,juveniles,and adults by using DBH instead of age,Then,applied a bivariate pair correlation function g(r)fitted with a random labeling null model and a"case-control"design method to investigate the regulation effect of density dependent on species coexistence in secondary evergreen deciduous broad-leaved mixed forest in karst hills.Also,the correlation between spatial distribution patterns of species and topographical factors was analyzed using the Berman test method.The aim of this research is to infer the underlying ecological processes through spatial patterns,revealing the dominant factors of species coexistence and the mechanisms by which species distributions respond to environmental changes.The main findings of this study are as follows:(1)Composition of evergreen and deciduous broad-leaved mixed forests in karst hills of GuilinA total of 3,809 individuals were recorded in this plant community survey,belonging to37 families and 70 genu,91 species,including 53 evergreens,38 deciduous plants,56 trees,23 shrubs,and 12 woody vines.The community structure of the plot is clear,with Quercus glauca,a species common or dominant in karst hills,being the dominant species.The richness is high and the abundances are significantly different,with rare species accounting for 26.37%of the total.The top three species of importance value were Quercus glauca,Mallotus philippensis and Callicarpa bodinieri,and the top three species in terms of species abundance were Mallotus philippensis(686,18.0%),Quercus glauca(405,10.6%)and Callicarpa bodinieri(341,9.0%),respectively.(2)Spatial distribution pattern of common species in mixed evergreen deciduous broad-leaved forests in karst hills of GuilinThe distribution characteristics of common species in the sample site were analyzed by using the point pattern analysis method,choosing the bivariate g(r)fitted with the completely spatial random null model(CSR)and the heterogeneity Poisson process null model(HP),and found that the species in the sample site were mainly distributed in aggregations.Among them,the CSR null model shows that aggregates decrease with increasing diameter and size,and all transform from aggregates to random distributions,with the overall adult tree exhibiting a stochastic pattern,which may be an adaptation mechanism for karst habitats.After using the HP null model to exclude the effects of first order effects,the aggregation of species is reduced and the pattern of aggregation occurs mainly at smaller diameter classes and scales,while the overall trend of decreasing aggregation effects remains.This indicates that tree species in the sample site were more strongly influenced by habitat heterogeneity,while biological processes(density dependence,etc.)may play an important role.(3)Spacial distribution pattern of species and environmental association analysisBerman test was used to analyze the correlation between species distribution patterns and elevation,slope,convexity and slope direction(sine and cosine values).The results showed that slope direction and elevation were important environmental factors affecting community structure,with almost all species showed significant positive associations with at least one environmental variable,and fewer negative associations.However,the significant associations exhibited by all individuals of the species and their different stages of growth were not exclusive,i.e.,most species showed different habitat preferences for different life history stages.(4)Analysis of density dependent effects of evergreen and deciduous broad-leaved mixed forests in karst hills of GuilinThe spatial differences in community patterns of major tree species at different life history stages in the sample plots were detected using the random labeling null model with"case-control"label design method.Both the self-thinning and thinning effects of density dependence were detected at different life history stages,excluding the habitat heterogeneity interference.Both the self-thinning and thinning effects were strongest at the juveniles tree and at smaller scales for the dominant communities of Quercus glauca and Mallotus philippensis.Although the proportion of species showing density dependence thinning effects was high,most species were regulated by density dependence self-thinning or thinning effects mostly at a few discrete scales,i.e.,the proportion of species affected by density dependence effects was low at all scales.This seems to suggest that the regulation of karst hills community structure may be driven by a combination of density dependence effects,habitat heterogeneity,and dispersal limitation processes.In conclusion,the adult trees in the evergreen and deciduous broad-leaved mixed forests communities in the karst hills of Guilin are randomly or regular distributed,and only juveniles tree show aggregation at small scales,which is consistent with the general pattern of forest community distribution.Habitat heterogeneity caused the spatial aggregation pattern of evergreen and deciduous broad-leaved mixed forests to be significant in the younger age classes of trees.A point pattern analysis to explore the prevalence of density dependence revealed that although density dependence were prevalent in this community,they were not the dominant mechanism for maintaining species coexistence in the evergreen and deciduous broad-leaved mixed forests.This study deepened the mechanism of species coexistence and species diversity maintenance in karst mountain forests,and provided a scientific basis for local biodiversity conservation and ecological reconstruction.