Data from: Positive species diversity and above-ground biomass relationships are ubiquitous across forest strata despite interference from overstorey trees

There is growing concern over rates of global species diversity loss and
its implications on healthy ecosystem functioning. While positive
relationships between tree species diversity and forest biomass production
have been observed, forests are structurally complex, consisting of
understorey vegetation layers that also contribute to ecosystem
functioning as they often account for the majority of species richness.
However, relationships between understorey vegetation diversity and
function are largely unexplored. Further, few studies have simultaneously
assessed how both overstorey and understory vegetation interact and
contribute to overall ecosystem function. By analysing Canada's
National Forest Inventory data base using structural equation modelling,
we explored the relationships between species richness and above-ground
biomass production across forest vegetation strata while accounting for
potentially confounding factors, including climate, physical site
characteristics and forest ageing. We found positive relationships between
species richness and biomass production across all forest vegetation
layers, but the relationship was strongest for the overstorey layer.
Species richness of the understorey tree, shrub and herb layers was
positively related to overstorey species richness. However, overstorey
biomass had a negative effect on the biomass production of all understorey
layers. Our results suggest that resource filtering by overstorey trees
might have reduced the strength of the positive diversity–productivity
relationships in the forest understorey, supporting previous hypotheses
that the magnitude and direction of diversity–productivity relationships
is context specific and dependent on the conditions of the surrounding
environment. Further, heterogeneity in understory resources, as affected
by the overstorey, may promote niche complementarity as the main mechanism
driving diversity–productivity relationships in understorey vegetation.