Biodiversity-ecosystem functioning relationships of overstorey versus understorey trees in an old-growth temperate forest

The results of this study, based on a large dataset with mapped tree locations, suggest that the relationship between biodiversity and ecosystem functioning (BEF) is different between the over- and understorey in the investigated forest community. Neither the functional nor the phylogenetic diversity showed a significant advantage in predicting aboveground biomass and biomass production (aboveground biomass increment) when compared with species richness.

Context Consistent conclusions have still not been reached regarding the relative importance of the different diversity indices in predicting biomass production. Furthermore, the effects of certain forest strata on the BEF relationship remain unclear.
Methods The woody plant species in the study area were classified as two vertical strata and were referred to as the “overstorey” (trees with dbh > 10 cm) and “understorey” (trees with dbh ≤ 10 cm). The relationships between different metrics of tree diversity and biomass production were quantified using linear models. The set of the best predictors for tree biomass and productions were determined using a multi-model selection approach. The differences in diversity-ecosystem functioning relationships between the two forest vertical strata, and the effects of canopy trees on the understorey trees, were investigated using structural equation models.
Aims The relative importance of the species diversity, phylogenetic diversity, and functional diversity in predicting the aboveground biomass production (ABP) was examined to compare the potential of phylogenetic and functional diversity indices and species richness in predicting biomass productivity and compare the influence of niche complementarity, selection effects, or mass-ratio effects on aboveground biomass variables and to assess possible differences in the BEF relationships between the overstorey and understorey layers in the studied ecosystems. Also, this study investigated possible differences in the BEF relationships between the overstorey and understorey layers in the studied ecosystems.
Results The results confirmed a positive diversity-productivity relationship in the natural coniferous and broadleaved mixed forest. However, the relationship in the overstorey stratum was statistically stronger than that in the understorey layer. The combination of diversity indices and functional traits could explain more of the variations in the biomass and productivity than when examined separately. The correlation between the richness of the overstorey and understorey species was found to be positive. However, the aboveground woody biomass of the overstorey layer had negative effects on the understorey biomass.
Conclusion In summary, this study found a stronger positive correlation between the woody plant diversity and productivity in the overstorey than in the understorey stratum. These findings indicate that the BEF relationships are hierarchically dependent, and supported by niche complementarity. Also, a much weaker relationship was observed between the functional diversity and biomass production than between the species diversity and biomass production due to a selection effect. The aboveground woody biomass was found to be enhanced in communities dominated by functional traits correlated to smaller leaf areas, greater maximum tree heights, and higher leaf carbon content. These findings suggest that the BEF relationships were driven by a mass-ratio hypothesis.

Temperate forest, Temperate forest, Tree species diversity, Biomass productivity, Vertical forest stratum

Xu, W., Luo, W., Zhang, C. et al. Annals of Forest Science (2019) 76: 64.

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Data availability
The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

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