Forest ecosystem multifunctionality and the provision of ecosystem services benefit from high biodiversity, including from the understorey. Hence, it is crucial to get insight into the mechanisms that influence the composition and diversity in this forest layer. Understorey species have specific environmental requirements which are influenced by the overstorey in a species-specific way. Mixing tree species may create a heterogeneous pattern of environmental conditions similar to the component species’ monocultures, or induce new environmental conditions. We quantified overstorey–understorey taxonomic and functional diversity relationships at different spatial scales in an innovative large-scaled European diversity-oriented research platform that allows the separation of tree species identity and diversity effects. We considered our results in relation to the divergent patterns expected, depending on whether environmental heterogeneity or novel environment creation is the mechanism driving overstorey–understorey diversity relationships.

We examined the understorey in mature forests in six European regions. Per region, three to five tree species were selected and 28–43 plots of 30 m × 30 m were established (209 in total), covering a tree species richness gradient from monocultures to three- to five-species mixed plots. Within each plot, we performed vegetation surveys in three vegetation quadrats. We examined the understorey compositional variation among and within regions and the influence of overstorey taxonomic and functional diversity on understorey diversity at different spatial scales.

The understorey composition showed clear interregional differences. We found lower compositional differentiation among quadrats with a higher tree species richness level (Finland, Germany and Italy). These quadrats share more tree species and probably have more similar environmental conditions. Neither overstorey taxonomic nor functional diversity affected the stand-level understorey species richness. At the forest-level, adding two-species mixtures to component monocultures clearly increased the understorey species richness, while adding mixtures with a higher tree species richness level to component monocultures had a negligible or negative impact.

Overstorey–understorey diversity relationships depend on the spatial scale. At the stand-level, our results lacked support for the environmental heterogeneity hypothesis and revealed the possibility of novel environment creation in tree-by-tree mixed stands. At the forest-level, favouring the mixture of two-species mixed stands and their component monoculture stands offers the best prospects for a higher understorey species richness.