Phenological niche overlap between invasive buckthorn (Rhamnus cathartica) and native woody species.
Common buckthorn (Rhamnus cathartica L.) is a prolific invader of forest understories throughout eastern North America. Its ability to invade is partially attributable to its relatively high shade tolerance and ability to capture light both early and late in the growing season because of its phenological differences from native species. Competitors that mitigate this phenological advantage by casting shade early and late in the growing season may therefore increase biotic resistance against invasion. However, controlled comparisons between buckthorn and other woody understory species are scarce and to what extent buckthorn's phenology is truly exceptional is incompletely known. Here, we compare the spring and autumn phenologies of five shade tolerant, native woody species (Sambucus canadensis, Sambucus racemosa, Corylus americana, Cornus racemosa, and Acer saccharum) to those of buckthorn in two common garden experiments. 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 contrast to spring, buckthorn fall phenology was distinct in comparison to some natives but not all. 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. These findings illustrate that buckthorn does not possess unique phenology amongst understory species. Forest communities rich in deciduous shrubs or trees that are phenologically similar to buckthorn (particularly S. racemosa) likely limit buckthorn's critical spring and fall carbon gains and exert greater biotic resistance to invasion by buckthorn.