Biodiversity–ecosystem functioning relationships (BEF) have been extensively studied, particularly within the primary layers of producers in terrestrial ecosystems. In multi-layer ecosystems such as forests, the contribution of diversity in the secondary layer, i.e. shrubs, to ecosystem functioning is still largely unknown. Here we used 11-year growth data from a forest biodiversity experiment with factorially crossed manipulations of tree and shrub species richness to assess their effects on forest productivity. We found that shrub species richness had positive effects on tree and total woody biomass (sum of tree and shrub biomass), with effect sizes similar in magnitude to those of tree species richness: increasing tree or shrub species richness from two to eight promoted tree biomass by 73.1% or 53.9% and total woody biomass by 46.7% or 37.1%, respectively. The positive effects of tree or shrub species richness on tree and total woody biomass became larger over time. Shrub biomass was reduced by tree species richness. The effects of tree and shrub species richness can be partially explained by their increased functional diversity. Our study provides the first evidence that understory diversity can significantly increase forest productivity and should not be neglected in forest restoration to promote ecosystem functioning.