The Forest Strata-dependent Relationships Among Envirlonment,Biodiversity And Aboyeground Biomass In A Subtropical Forest In Tiantong,Zhejiang Province

The relationships between biodiversity and aboveground biomass in forest ecosystems have been intensively studied in recent decades.However,natural forests are structurally complex and plant species with different functional strategies are generally assembled in different forest strata(i.e.overstorey and understorey).It is not very clear which ecological mechanism(s)underpin the relationships between biodiversity including stand structure and aboveground biomass across forest strata while accounting for the effects of environmental factors.This thesis focused on the linkage among environmental factors,biodiversity and stand structural attributes,and aboveground biomass across forest strata and at whole-community level(for comparison)of a subtropical evergreen broadleaved forest.Aboveground biomass,multiple metrics of biodiversity(taxonomic,functional trait and phylogenetic diversity indices),stand structural attributes and environmental factors were quantified or measured for 125 plots in a 5-ha subtropical forest,located in Ningbo city,Zhejiang Province,in Eastern China.We1 mainly used structural equation model and/or multiple linear models for linking environment,biodiversity(including stand structure)and aboveground biomass across forest strata and at whole-community level.The complementary Pearson's correlations and bivariate relationships to the structural equation model and/or multiple linear models were also evaluated.To account for spatial autocorrelation,we performed generalized least-squares models with(accounted for the spatial location of each plot)and without spatial autocorrelation among plots for each of the proposed relationships.In case of using multiple linear models,we also applied the Moran's I test for spatial autocorrelation in the selected optimal linear model residuals.The main finding of this thesis is that no sole and ubiquitous relationship between biodiversity and aboveground biomass exists in a structurally complex forest,but rather that the magnitude and direction of this relationship is greatly dependent on the forest strata where available resources shift substantially.Specifically,we found that:1)Aboveground biomass in overstorey strata was driven by high individual tree size variation and species diversity.In understorey strata,the mixture effects of tree development,high degree of biotic interaction,and increased resource heterogeneity complicated the relationship between biodiversity and aboveground biomass.2)High functional trait diversity of understorey enhanced aboveground biomass through the niche complementarity effect while low functional trait diversity of overstorey enhanced aboveground biomass through functionally-similar big trees effect.3)Intraspecific and interspecific functional trait diversity positively mediated the response of aboveground biomass to species richness at understorey strata through niche differentiation,whereas negligible or negative roles at overstorey strata and whole-community indicated that only few large trees occupy larger niche space in a community.4)Understorey aboveground biomass was great for groups of phylogenetically distant species having high functional richness due to specific functional strategy shared by all the species.By contrast,high overstorey aboveground biomass was related with groups of phylogenetically close species having low functional trait diversity due to the evolutionary relatedness.The mixed effects of overstorey evolutionary relatedness and understorey functional trait diversity on their corresponding aboveground biomass complicated these relationships at the whole-community level.5)With respect to the species' functional strategies,high aboveground biomass was potentially driven by functional identity(community-weighted mean of a trait values)of tree height through making use of plentiful soil nutrients and light at overstorey strata,whereas by conservative strategy at understorey strata through enduring resource-poor conditions.6)In combination,this study found that functional identity of tree height with low functional trait diversity at overstorey,whereas niche complementarity with conservative strategy on resource-poor soils were of great importance for driving aboveground biomass.At whole-community level,aboveground biomass was most strongly determined by soil nutrients,individual tree size variation and functional dominance of big trees.The results in this thesis have highlighted the fundamental roles of forest strata where overstorey and understorey strata contribute to their corresponding aboveground biomass with contrasting functional strategies or ecological mechanisms across a range of soil nutrients.Hence,ecological models for predicting aboveground biomass would be improved by including separate effects of overstorey and understorey diversity.This study concluded that conserving biodiversity and maintaining stand structure at both overstorey and understorey strata in subtropical forests would be the beneficial and sustainable management strategies in the context of global change.