Common buckthorn (Rhamnus cathartica) is an invasive shrub that dominates forest understories in the northern US and southern Canada. Its ability to invade is partially attributable to its high shade tolerance and ability to capture light both early and late in the growing season. Competitors that mitigate this phenological advantage by casting shade early and late in the growing season may therefore increase biotic resistance against invasion. Understanding the relationship between phenology and biotic resistance is relevant to assessing which ecosystems are most at risk of invasion and also for designing successful restorations. Here, we compare the spring and autumn phenologies of five shade tolerant, native woody species to those of buckthorn in two common garden experiments around St. Paul, Minnesota and relate these phenologies to buckthorn performance beneath natives. Each experiment took place in an invaded oak-dominated forest where we planted Sambucus canadensis, Sambucus racemosa, Corylus americana, and Cornus racemosa, and Acer saccharum as bare-root seedlings. Buckthorn were grown alongside native seedlings from seed. We evaluated the spring and autumn phenology of individuals of each species according to the following phenophases: presenting intact buds, having at least one bud broken with some leaf tissue visible, having at least one leaf fully expanded, having all leaves fully expanded, having at least one leaf undergo visible senescence, having all leaves undergo senescence, and having dropped all leaves.
Results/Conclusions
Spring phenology of buckthorn was often comparable to the five native species considered. All native species broke bud no later than 7 days after buckthorn, with S. racemosa reaching spring phenophases consistently earlier than buckthorn. In autumn, native species started to senesce up to 20 days earlier than buckthorn, but both Sambucus species senesced slowly and held their leaves equally long as buckthorn. Buckthorn growth and survival beneath woody plantings was reduced by up to 70% on average compared to unplanted areas (both beneath intact oak forest canopy). These findings illustrate that buckthorn does not possess unique phenology among understory species. Forest communities rich in deciduous shrubs or trees phenologically similar to buckthorn therefore likely exert greater biotic resistance against invasion by buckthorn. This is relevant both to our consideration of invasion processes in intact forests and for management. Land managers and property owners in areas where buckthorn is a threat should seek to promote these species, particularly S. racemosa, in order to limit invasion or re-establishment of buckthorn during management.