Variation And The Driving Mechanism Of Density Dependence In Temperate Forests,Northeastern China

Forest community dynamics indicate a series of dynamic process including plant growth,mortality,reproduction and regeneration,these processes are not only the foundation of the forest community regeneration,but also are the important factors that maintaining the forest biodiversity and ecosystem function.Ecologists have long been exploring how species coexist in the community and examining the driving mechanisms of the community dynamics,they have proposed plenty of species coexistence theory to explain the driving mechanisms that influencing species coexistence and community dynamics at local scale.Broadleaved-Korean pine forest and Spruce-fir valley forest is the zonal and non-zonal top vegetation in Xiaoxingan mountains in Northeastern China,examining the relative importance of different ecological processes at the local scale could help us to have a better understanding of the species coexistence and biodiversity maintaining and further could provide science support for revealing driving mechanisms of species coexistence and community dynamics.Therefore,this study conducted in a 9 hm2 Broadleaved-Korean pine forest dynamics plot and a 9.12 hm2 Spruce-fir valley forest dynamics plot,based on data of individual survival and growth data from 2005-2020,species functional traits,topography and soil properties to address the following questions:How the relative importance of density dependence and niche theory varied among species,life stage and the forest type?How plant functional traits influence the processes that mentioned above?What are the potential driving factors that influence the intraspecific variation of these ecological processes?(1)Based on dataset of 4 years interval individual survival of seedlings,saplings and adults in two forest plots,we tested whether individual survival is related to neighborhood effects and habitat variables.The results showed that the relative importance of neighborhood effects and habitat variables to focal individual survival varied among life stages and sites and habitat variables contribute to the patterns of focal individual survival.The strength of CNDD effect decreased as the life stages progressed and varied from a negative effect at the seedling stage to a positive one at the adult stage.However,the strength of PNDD increased as the life stages progressed.The CNDD and PNDD widely varied among species,indicating the importance of including the relative abundance,life history strategies and functional traits of species when investigating elements that affect species coexistence in tree communities.(2)Based on the dataset of stage-specific functional traits and a long-term,spatially explicit data of plant survival including 7971 individuals and 41 species monitored for seedling and adult life stages in BKPF,examined the relative role of these mechanisms influencing individual survival across ontogenetic stages.The results showed that for seedlings,CNDD became weaker for species with larger leaf area.while for adults,the conspecific neighbors had a positive effect on survival and this effect became stronger with decreasing seed mass.Also.the results examining the effects of neighboring heterospecifics suggest a prevalence of niche partitioning at the seedling stage but a prevalence of environmental filtering or hierarchical competition for the adult stage.In particular,the strength of these processes at the adult life stage increased with higher maximum height and smaller specific leaf area.Finally,we found that the effect of traits on individual survival for seedling and adult life stages was dependent on both the prevalence of conspecifics and functional diversity of heterospecifics in the neighborhood.Overall,the results demonstrates that ecological processes tend to have a stronger diversifying effect for seedlings than for adults and these effects depend on stage-specific species traits.Considering trait variation and their interacting effects with biotic factors across life stages provides further insights into ecological mechanisms shaping plant communities and driving its dynamics.(3)Based on the dataset of leaf herbivory,growth and survival data for 2411 seedlings of 10 tree species collected over 3 years in BKPF.to examine the independent and interactive effects of mycorrhizal type,and conspecific density on seedling performance(%leaf herbivory,growth,and survival).The results showed that leaf herbivory increased with conspecific adult density and conspecific seedling density.Seedling growth and survival decreased with conspecific adult density but were not affected by conspecific seedling density.Further,EM tree species experienced weaker CNDD effects via reduced leaf herbivory and enhanced seedling survival than AM tree species;and older EM seedlings showed lower leaf herbivory but higher growth and survival rates than younger EM seedlings with increasing conspecific seedling density,but this age-mediated effect was not significant for AM seedlings.This work suggests that EM-associated seedlings have a performance advantage relative to the AM-associated seedlings potentially associated with an anti-herbivory-effect produced by EM fungi and this effect of EM fungi can alleviate CNDD by reducing the effect of aboveground herbivores on leaf damage.(4)Based on the dataset of seedling survival for 12 years interval and species functional traits including leaf size,leaf economic spectrum and chemical defense in two plots,we explored relationships between species CNDD and leaf traits.The results showed that CNDD have marginally significant effects on seedling survival in the BKPF and the seedling mortality increased with increasing elevation;The strength of species CNDD negatively correlated with seedling leaf area in BKPF indicating that species with larger leaf area experienced stronger CNDD,but this relationship was opposite for seedlings in SPVF;The strength of species CNDD positively and negatively correlated with species leaf thickness and specific leaf area,respectively,indicating that species with less thickness and higher specific leaf area experienced stronger CNDD,however,seedlings in SPVF showed opposite trend with these relationships;The strength of species CNDD positively and negatively correlated with leaf dry matter content and leaf nitrogen content respectively after removing the coniferous species in SPVF,indicating that species with lower leaf dry matter content and higher leaf nitrogen content suffered stronger CNDD;For the leaf chemical defense traits,the strength of species CNDD positively correlated with species total phenolics and tannins in the BKPF,indicating that species with higher defense compounds experienced weaker CNDD,however,this relationships are not significant in SPVF.Finally,the relationship of species CNDD strength between BKPF and SPVF was not significant.These results suggest that species functional traits could influence the strength of species CNDD and chemical defense traits can capture more than common measured leaf economic spectrum traits,furthermore,species might be able to adapt different in various habitat which might be able to drive the intraspecific variation of CNDD.