Understory composition and diversity after invasive shrub removal are determined jointly by canopy openness and revegetation method

Revegetation of native plants can reduce re-establishment of invasive shrubs during forest restoration. However, the consequences of revegetation for understory composition beyond invasive suppression are not well understood. We initiated two experiments (Exp1 and Exp2) in 2017 to test revegetation effects on buckthorn ( Rhamnus cathartica ) reinvasion and understory development in Minnesota, USA. Exp1 compared revegetation seeding against unseeded controls. In a subset of sites, we compared effects of additional revegetation techniques including herbaceous seed additions (with and without fencing), sedge plugs, bare-root shrubs, or bare-root trees (Exp 2). Five years later, we found corroborating results from both experiments: revegetation treatments increased cover of native species, particularly in areas with greater light availability or in fenced plots. Light also drove increased species richness, floristic quality, and total cover with seeding, but decreased species evenness due to high grass dominance in Exp 1. In Exp 2, we found evidence of a trade-off between woody and herbaceous species, such that planting trees or shrubs increased total cover at the expense of grasses and forbs. Overall, these findings suggest two distinct sets of outcomes of revegetation in buckthorn management. Although managers can effectively increase total cover using either herbaceous or woody species, densely seeding a mix of native grasses and forbs also increases native plant diversity whereas planting woody species creates less diverse and structurally distinct communities. These findings build expectations of the consequences of revegetation strategies in Minnesota woodlands and to enable managers to select the best approach based on their management goals. • Understory plant species diversity and composition were driven by canopy openness. • Species richness, floristic quality, and total cover increased with openness. • Seeding led to high Elymus spp. grass dominance, increasing total plant cover. • Transplanted woody natives suppressed buckthorn and herbaceous species. • Fencing reduced cover of non-native species.