Comparison Of Species And Functional Trait Diversity Between Forest Strata Along The Gradient Of Forest Succession In Eastern Zhejiang Province

Successional dynamics of plant species have received long-lasting research interest for understanding community assembly.Shift pattern of species diversity along the successional gradient has been extensively studied at the community level in the past decades.However,in structurally complex natural forest species are generally assembled in different forest strata(i.e.overstorey and understorey layer)according to differences in their functional strategies.Therefore,understanding the distribution patterns of taxonomic and functional diversity across forest strata and the underlying ecological mechanism becomes increasingly important due to the increasing extension of secondary forests globally.This thesis mainly focused on the strata-dependent shifts of taxonomic and functional trait diversity,their successional convergence and divergence,and the role of demographic properties on these patterns along a secondary successional series in subtropical evergreen broadleaf forests in the Ningbo region,Zhejiang Province.Successional index and multiple diversity patterns(i.e.taxonomic and functional trait diversity)were quantified across 70 successional forest plots in the Ningbo region,Zhejiang Province.I quantified the successional stage index based on the proportion of the stem of pioneer species within each plot.Nine plant functional traits,including mean leaf area,specific leaf area,leaf dry matter content,leaf nitrogen concentration,leaf phosphorus concentration,specific stem length,Huber value,wood density,and plant height were measured for each species within each of the 70 successional plots.Based on the above dataset,I have used linear regression models to explore the variation in taxonomic and functional diversity patterns across forest strata during secondary succession.Jaccard and Gower’s indices were used to measure the pairwise taxonomic and functional dissimilarity indices(i.e.beta-diversity patterns)between successional plots.The null model approach was used to test the convergence and divergence in taxonomic and functional traits diversity through community succession.In addition,effects of plant functional traits on species demographic rates between early-and late-successional stages were quantified using multiple linear regression.In this thesis,I aim to answer the following questions:1)whether the temporal patterns of species diversity vary or remain consistent between the overstorey and understorey strata in relation to successional series?and what are the temporal patterns of functional trait diversity with respect to the difference in leaf and wood traits between the overstorey and understorey strata?2)how do shifting degree of species composition and plant functional traits of overstorey and understorey strata vary with forest succession?3)whether the convergence degree in taxonomic and functional traits diversity with forest succession is stronger in the understorey than the overstorey strata?and4)how do functional traits influence species demographic rates across forest strata between early-and late-successional communities?The main finding of this thesis is that the understorey stratum plays a strong role,compared to overstorey,in shaping temporal dynamics in both taxonomic and functional trait diversity through secondary forest succession.The specific findings are detailed below:1)Temporal patterns in plant diversity significantly differ between overstorey and understorey strata in relation to successional series.Through forest succession,species richness significantly decreased in the overstorey stratum but increased in the understorey stratum.Species evenness significantly increased in the overstorey stratum but did not statistically change in the understorey level(R~2=0.07,p>0.05)through succession stratum.Multi-trait-based functional richness and evenness significantly increased in the overstorey stratum but did not change in the understorey stratum.In contrast,multi-trait-based functional divergence and dispersion but did not significantly change in the overstorey stratum but significantly decreased in the understorey stratum.The successional difference in the univariate trait-based functional diversity was trait-specific.2)The degree of temporal shift in both species composition and multivariate trait diversity significantly increased with succession at both overstorey and understorey strata,with the plot-wise dissimilarity of species composition being much stronger in understorey than overstorey strata through forest succession.Successional variation in the pairwise dissimilarities of univariate traits was strata-dependent and trait-specific,with overstorey stratum increasing in plant maximum height only,while understorey stratum increasing in mean leaf area,specific leaf area,and plant maximum height.3)Successional changes of taxonomic and functional trait diversity did not statistically differ from the random assemblage.However,taxonomic diversity in the understorey stratum and multi-trait-based functional diversity in both the overstorey and understorey decreased along the successional series,indicating convergence of taxonomic and functional traits through forest succession.For univariate traits,functional diversity of Huber value in both forest strata was apparently inclined towards functional convergence through succession.In contrast,functional diversity of specific leaf area,mean leaf area,leaf dry matter content,leaf nitrogen concentration,leaf phosphorus concentration,specific stem length,maximum height,and wood density converged significantly in the understorey stratum only with forest succession.4)Effects of functional traits on species’demographic rates varied within forest strata and successional stages.Early-successional species with high relative growth rates were characterized by low leaf nitrogen and phosphorus contents across forest strata,and by low mean leaf area but high plant maximum height in the overstorey stratum.In contrast,early-successional species with high mortality rates were characterized by high leaf nitrogen and phosphorus contents in the understorey stratum.Comparably,in the late-successional stages,species with great relative growth rates were characterized by high wood density in the overstorey stratum.Species with high mortality rates were characterized by high specific leaf area across forest strata,and low mean leaf area in the overstorey stratum.In short,the findings of this thesis provide strong evidence that successional shifts in species composition and functional trait diversity are more robust in the understorey than in overstorey strata.Moreover,this study demonstrates that successional variation in plant functional traits plays a crucial role in shaping forest strata-dependent species’demographic rates for driving temporal shifts in species composition through forest community succession.Hence,these differences in successional patterns of taxonomic and functional trait diversity between overstorey and understorey strata highlight the important functions of forest strata in not only shaping community structure but also assembling taxonomic and functional compositions through forest succession.I concluded that understanding the forest strata-dependent patterns of species distribution through forest succession would be beneficial for the long-term management and conservation of plant diversity,and further for accurately predicting ecosystem functioning in structurally complex subtropical forests.