The Relationship Between Biodiversity And Ecosystem Functioning In Temperate Secondary Broadleaf-conifer Mixed Forests

The study of the relationships between biodiversity and ecosystem functioning(BEF)has emerged as a central issue in ecology and conservational biology,particularly in connection with specific global scenarios involving the continuing extinction of species,as well as the increasing threats posed by climate changes.Forests are among the most important repositories of biodiversity and components of the global carbon cycle.Therefore,examining the BEF relationships in forest ecosystems is a key topic in the field of BEF relationships.Hundreds of studies have been carried out all over the world to test the BEF relationships in forest ecosystems,which have substantially improved our knowledge about BEF relationships.However,the underlying mechanisms that regulating BEF relationships in temperate secondary forests remain highly debated.One of the most intensely debated questions in BEF relationships is whether purely species-based diversity indices,which neglect the function dissimilarity and evolutionarily relatedness of species,such as species richness,can appropriately assess the biodiversity of ecosystems.Actually,biodiversity is a multi-faceted assembly,including species diversity,functional diversity,and phylogenetic diversity.However,which facets of biodiversity account most for variations in ecosystem functions are still not understood very well.On the other hand,forest ecosystems can be influenced by harvesting significantly,but how functional and phylogenetic diversity response to forest harvesting is poorly understood.In this study,based on the observations from a 21.12-hm~2 temperate secondary broadleaf-conifer mixed forest plot in northeastern China,we explored the BEF relationships,and assessed how different biodiversity mechanisms regulate ecosystem functioning.The variable"biodiversity"includes species,functional and phylogenetic diversities.Based on the observations from four 1-hm~2 secondary broadleaf-conifer mixed forest plots with different harvesting intensity,we also explored the influences of forest harvesting on biodiversity,community structure,and ecosystem functions.We focus on two main ecosystem functions,aboveground biomass and productivity,which represent the stock and the flux of the ecosystems,respectively.The main results of the study are as follows:1.Habitat heterogeneity influence forest species growth and diversity pattern.Determining the relative contribution of habitat heterogeneity on forest species growth and diversity pattern can help to understand some underlying ecological mechanisms that may affect BEF relationships.In this study,analysis of variance(ANOVA)was used to test the influence of habitat heterogeneity on forest species diversity pattern,while species-habitat association analyses were employed to test the significance of habitat heterogeneity on species radial growth.The results showed that,in this temperate secondary broadleaf-conifer mixed forest,habitat heterogeneity played a significant role in determining species radial growth and diversity pattern.Species have their habitat preferences,in which species can have a high growth rate and survival rate.Furthermore,the radial growth of tree species was highly sensitive to habitat heterogeneity when compared with that of shrubs and sub-trees.Our findings highlight the fundamental role of habitat heterogeneity and niche differentiation in regulating biodiversity pattern and species radial growth in the secondary broadleaf-conifer mixed forests.Thus,to promote forest ecosystem functioning,forest management and biodiversity conservation should not only protect species per se,but also the specific habitats that conducive to plant growth and survival.2.Biodiversity determine ecosystem functioning in temperate secondary forests.The study of the BEF relationships has been a central issue in ecology and conservational biology.However,which facets of biodiversity account most for variations in ecosystem functions are still not understood very well.This is especially true with regard to temperate secondary forests.Aboveground biomass and productivity are two of the main forest ecosystem functions,which represent the stock and the flux of the ecosystems,respectively.In this study,based on the plant phylogeny and functional traits,we explored the direct and indirect causal relationships between the multi-faceted biodiversity(including species richness,functional diversity,and phylogenetic diversity)and ecosystem functions(including forest biomass and productivity)using structural equation models(SEMs).The results showed that,in this forest,functional and phylogenetic diversity significantly affected forest biomass and productivity,while species richness had only indirect effects via increasing functional and phylogenetic diversity.Functional diversity had a positive effect on forest productivity,while a negative effect on forest biomass.Besides,functional diversity had a negative indirect effect on forest productivity via biomass.Phylogenetic diversity had a positive direct effect on forest biomass,and a positive indirect effect on forest productivity via biomass.When these results were taken together,the total effects(direct and indirect)of phylogenetic diversity were determined to be significantly positive.However,the total effects of the functional diversity on productivity were canceled out and negligible.The results highlight the direct and indirect effects of multi-faceted biodiversity on ecosystem functions in temperate secondary forests.Our findings emphasize the significance of functional and phylogenetic diversity,which play direct roles in determining forest ecosystem functioning.3.Biodiversity drives forest biomass and productivity through multiple mechanisms in temperate secondary forests.The aims of the BEF studies are not only to determine the relationships,but also to elucidate the underlying mechanisms that regulating the BEF relationships.Niche complementarity,mass-ratio,and vegetation quantity effects have been identified as major drivers of the BEF relationships.However,their relative contribution to biomass and productivity is not yet clear in temperate secondary forests.In this study,we assessed how these mechanisms regulate ecosystem functions in a temperate secondary broadleaf-conifer mixed forest.Niche complementarity effect was quantified by functional diversity metric,while the mass-ratio effect was described as functional trait composition using community weighted mean trait values.Vegetation quantity effect was evaluated using vegetation biomass.We performed structural equation modeling to test the three alternative mechanisms.Our results provide evidence for all three mechanisms.Functional diversity increased forest productivity,in line with the niche complementarity hypothesis.Fast-growing and acquisitive traits(e.g.greater specific leaf area and leaf nitrogen concentration)enhance productivity,while slow-growing and conservative traits(e.g.greater wood density)enhance the long-term accumulation of biomass,demonstrating the mass-ratio hypothesis.Furthermore,we observed a significantly positive relationship between biomass and productivity,confirming the vegetation quantity hypothesis.We conclude that functional traits drive biomass and productivity through multiple mechanisms.Both niche complementarity and the mass-ratio effects play roles in this temperate secondary forest.In addition,we emphasize the importance of preserving sufficient biomass stock to ensure maximum productivity in secondary forests.Our study contributes to the identification of the mechanisms underlying BEF relationships,and has practical significance for guiding temperate secondary forest management and conservation.4.Forest harvesting influence forest functional and phylogenetic diversity and structure.Forest harvesting can have a significant impact on forest biodiversity,community structure,and ecosystem functioning.However,the influences of harvesting on functional and phylogenetic diversity and structure are still poorly understood.In this study,based on four 1-hm~2 stem-mapped forest plots with different harvesting intensity,we explored the potential effects of harvesting intensity on forest functional and phylogenetic diversity and structure at three different spatial scales.The influences of harvesting intensity on radial growth of the residual trees were also assessed.The results showed that forest harvesting could reduce functional and phylogenetic diversity.However,the influences of harvesting on the functional and phylogenetic diversity were scale-dependent.Low and moderate harvesting intensity showed significant effects only at the small study scale(10 m×10 m),but high harvesting intensity at all study scales(10 m×10 m,20 m×20 m,and 50 m×50 m).Furthermore,the results showed that the variations of functional and phylogenetic structures depended on the studied scales.At the small scale,forest harvesting enhanced the degree of dispersion but reduced the degree of aggregation of community structure.However,at the middle and large scales,harvesting enhanced the degree of aggregation but reduced the degree of dispersion.The radial growths of residual trees are from top to bottom moderate harvesting intensity,high harvesting intensity,low harvesting intensity and controls.Therefore,from the perspective of forest management,low or moderate harvesting intensity is the suitable way to regulate community spatial structure,conserve the biodiversity and accelerate the tree growth,while high intensity harvesting will damage forest biodiversity and ecosystem functioning.