The forest strata-dependent relationship between biodiversity and aboveground biomass within a subtropical forest

The relationships between biodiversity and aboveground biomass in forest ecosystems have been intensively studied in recent decades. Still, the mechanisms that underlie it remain highly debated. We hypothesized that overstorey species diversity and individual tree size variation contribute to aboveground biomass and understorey species diversity through the niche complementarity effect, while weaken the relationship between understorey aboveground biomass and individual tree size variation due to the mixed effects of tree development, biotic interaction and reduced available resources by overstorey strata. The integrative relationships of species diversity and tree size variation (variation in diameter at breast height-DBH) with aboveground biomass were analysed at overstorey and understorey strata across 125 plots in a 5-ha subtropical forest in Eastern China. For comparison, we tested these relationships at individual strata (isolation modelling), and whole-community level, by using linear structural equation model while accounting for the effects of soil nutrients. The integrative modelling accounted for 35, 31, 16, 12, 4, and 0% of the variation in understorey aboveground biomass, overstorey aboveground biomass, overstorey DBH variation, overstorey species diversity, understorey species diversity, and understorey DBH variation, respectively. Overstorey DBH variation and species diversity had the positive direct effects on overstorey aboveground biomass. Overstorey species diversity significantly promoted the understorey species diversity, but DBH variation and aboveground biomass of overstorey strata had negligible effects on the diversity and aboveground biomass of understorey strata. Soil nutrients had positive direct effect on overstorey DBH variation, but negative direct effects on overstorey and understorey aboveground biomass and overstorey species diversity. These results provide strong evidence for the niche complementarity effect for driving positive relationships of species diversity and individual tree size variation with aboveground biomass at overstorey strata. The strong and consistent negative effects of soil nutrients on overstorey aboveground biomass and species diversity suggest an important mechanism that high species diversity of overstorey strata with great tree size variation on nutrient-poor soils is crucial for driving high aboveground biomass in subtropical forest ecosystems. In conclusion, this study suggests 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. We argue that ecological models for predicting aboveground biomass would be improved by including separate effects of overstorey and understorey diversity.