Understorey diversity in southern boreal forests is regulated by productivity and its indirect impacts on resource availability and heterogeneity

1. Understanding the relationship between species diversity and productivity is central to linking compositional and functional aspects of terrestrial ecosystems, and little is known about such issues in boreal forests. We used structural equation modelling (SEM) to test several hypotheses about direct and indirect influences of productivity, its correlate basal area, and resources on understorey vascular plant diversity on 2025 plots in 81 southern boreal forests in Minnesota, USA. 2. We first examined the hypothesis that increasing basal area reduces plot‐scale species richness due to competitive exclusion from the most limiting resource, light. As expected, light pre‐emption increased with total basal area, which directly reduced understorey species richness. However, complex relations between basal area, dominant understorey species, and resource supply to the understorey can also influence understorey communities. Hence, we addressed whether plots with low light availability in the understorey were associated with low abundance of dominant understorey species and alleviation of competitive exclusion of other understorey species. SEM results showed that low light decreased total understorey cover, alleviating resource competition from this stratum and thus increasing understorey species richness. Furthermore, the cover of four dominant understorey species was positively correlated with light availability and negatively correlated with plot‐scale species richness. 3. Aggregating data for the 25 plots at each stand, SEM showed that stand‐scale species richness was positively influenced by light heterogeneity, which in turn increased with annual above‐ground productivity. 4. Species richness was positively influenced by litter %N, considered an index of nitrogen availability at the plot and stand scale. 5. Synthesis. These results suggest that understorey species richness in boreal forests is regulated by productivity, but is primarily mediated by the indirect effects of productivity of the dominant producers on resource availability and heterogeneity.