| The sustainability of plant community in old-growth temperate forests has motivated many ecologists to investigate the mechanisms by which species coexistence is maintained in a local-scale habitat.Environmental heterogeneity is among the most fundamental factors governing community structure and drives ecological process,simultaneously influences population dynamics.Besides,the role of environmental heterogeneity is applied to assess plant traits' response to the environment within communities.In this study,a 9-ha large-scale fixed forest dynamic monitoring plot of a typical mixed broadleaved-Korean pine(Pinus koraiensis)forest in the Liangshui National Nature Reserve,Heilongjiang Province,China of the Xiaoxing'an Mountains was taken as the research site.The habitat classification in the study plot was conducted based on ten soil variables,four topography variables,leaf area index(LAI),and species abundance.The soil organic carbon(SOC)and slope were essential factors shaping species distribution and characterized the study plot into three habitat types.The three habitat types are Hab ?(very high soil organic carbon with a moderate slope),Hab ?(low soil organic carbon with a gentle slope),and Hab ?(low soil organic carbon with a strong slope).To understand the mechanism of community assembly processes through habitat filtering approach,we examined species-habitat association for tree and shrub species.Our results showed that only P.koraiensis was positively associated with Hab?.In the life stages level,saplings of P.koraiensis was positively associated with Hab ?,but the adults of P.koraiensis was positively associated with Hab ?.This result suggested that the role of adults of P.koraiensis as the dominant canopy species occurs in Hab ? together with other saplings of five broadleaf species and six shrub species which was positively associated with Hab I.We furthermore tested whether positive habitat association status influence the demographic parameters(i.e.,growth,survival,and recruitment).Our results indicate that species-habitat association status has a significant influence on variation in demographic rates.The species-habitat association status significantly affected trees and shrubs growth,survival,and recruitment.In addition,performing individual-based analyses of leaf trait variations according to the plant functional types and life stages encourage understanding of the forest dynamics in an old-growth temperate forest.Our results showed that the life stage and habitat type had varied influences on the leaf traits of the six species(i.e.,conifer trees:Abies nephrolepis,Pinus koraiensis;broadleaf trees:Acer mono,Fraxinus mandshurica,Tilia amurensis,and Ulmus laciniata).The life stage was an important determinant for all leaf traits.Plant functional types were included in the best models for specific leaf area(SLA),leaf nitrogen content(LNC),and leaf carbon content(LCC).Habitat type had a more significant influence on the leaf dry matter content(LDMC)than the SLA.Meanwhile,habitat type had a greater influence on the LNC and leaf phosphorus content(LPC)than on the LCC.Study on five shrub species(i.e.,Corylus mandshurica,Acanthopanax senticosus,Euonymus verrucosus,Prunus padus,and Viburnum burejaeticum)showed that the ordination of leaf traits explained 64.7%of the variance.The pattern of moderate species abundance in Hab I was opposite with the pattern in Hab ?.The SLA,LNC,and LCC were influenced by species abundance and habitat types,while the habitat types did not significantly affect LDMC and LPC.The effects of intraspecific leaf traits on growth rates varied along environmental gradients due to the particular differences in the local environmental conditions at an individual site.We found that the LDMC and LCC promoted broadleaf sapling growth in the high-nutrient habitat.On the other hand,the growth of coniferous adults was more significantly influenced by leaf traits and soil variation than that of the saplings.Increasing the LDMC,soil organic carbon and soil available phosphorus may increase the conifer adult absolute growth rate(AGR).The effect of leaf traits on the growth of broadleaf saplings was dependent on the habitat type.Soil variation had positive effects on the growth of broadleaf saplings and coniferous adults in the high-nutrient habitat type.Our results also showed that the AGR and relative growth rate(RGR)of shrub in Hab ? was positively influenced by SoilPC3 which more explained by soil volumetric moisture and pH.Meanwhile,the AGR and RGR of shrub in Hab ? and Hab ? were influenced by leaf trait variation.The LeafPC2(i.e.,SLA,LDMC,and LCC)and SoilPC1(i.e.,soil available nitrogen)had a significant positive effect of RGR of shrub in Hab ? and Hab ?,respectively.By using two integrative structural equation models to link SOC and slope with LAI on overstorey and understorey absolute growth rate through species diversity and stand structure,we show that the SOC directly influence overstorey absolute growth rate,whereas slope directly influence understorey absolute growth rate.The overstorey species diversity had a positive effect on overstorey absolute growth rate,but had negative effect on understorey stem density and absolute growth rate.However,the overstorey basal area had negative direct effects on their growth rate which may indicate the competition effect(i.e.,intra-and interspecific competition)within the stratum.We also show that both overstorey basal area and understorey stem density are important for determining the LAI.Simultaneously,the LAI positively affected understorey tree growth rates.Our results indicate that the complex stand structure of overstorey and stem density of understorey supports species coexistence. |
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