Revegetation of Elymus grasses suppresses invasive Rhamnus cathartica in deciduous forest understories

Common buckthorn (Rhamnus cathartica) is one of the most widespread invaders of temperate forest and woodland understories in North America. Eradication of buckthorn is a common management goal, but one that is rarely realized since buckthorn quickly re-establishes in managed communities with low biotic resistance. In the years following management, pre-existing native propagules that could potentially re-establish and resist buckthorn re-establishment are scarce whereas buckthorn propagule pressure remains high. Thus, invaded understories often return to or remain in a buckthorn-dominated state when efforts focus only on treating buckthorn over a short period of time. Here, we test the hypothesis that post-management seeding of native graminoids and forbs increases competition and reduces buckthorn re-establishment. We compared pairs of seeded and unseeded plots (n = 35) across 7 sites undergoing buckthorn management near St. Paul, Minnesota, USA from 2017 to 2022. Over that time, we monitored herbaceous cover and light availability as well as the germination, growth, and survival of four cohorts of buckthorn seeds planted into each plot. We found that seeding increased cover of native herbaceous species (especially Elymus grasses and other graminoids) in areas with >7 % tree canopy openness, which in turn reduced light availability to germinating buckthorn by one third. These changes in herbaceous cover and light availability reduced buckthorn growth and survival in complex ways. Effects of seeding on buckthorn were strongest immediately following initial management and in areas with greater tree canopy openness. By the end of the experiment, buckthorn germinating one- and two-years after initial management were 47 % less abundant and 81 % smaller in seeded areas compared to unseeded ones with more than 10 % tree canopy openness. Our findings indicate that post-management seeding of Elymus grasses and other graminoids can partially suppress buckthorn re-establishment in semi-open forests and woodlands following initial management (a critical period wherein buckthorn propagule pressure is high and ambient biotic resistance is low). By augmenting biotic resistance through post-management seeding, managers can effectively limit future invasion and reduce demands for additional management.