This data release includes data and metadata on tree and shrub basal area as well as bird-mediated and passive seed rain for sites selected to have a range of understory cover under canopy trees (Metrosideros polymorpha and Acacia koa). It also includes seedling germination and survival data for a large-scale seed addition and grass removal experiment that varied both seed rain and grass cover. All sites were within Hakalau Forest National Wildlife Refuge on Hawaii Island. Broadly, this study asks what thresholds of seed rain and native and exotic plant cover are needed for passive forest regeneration.