Rhamnus cathartica L. (Common Buckthorn) as an Ecosystem Dominant in Southern Wisconsin Forests

Northeastern Naturalist, 2007 by Mascaro, Joseph, Schnitzer, Stefan A

Abstract -

Recent work on exotic species in island ecosystems has revealed that many exotic woody plants are capable of dominating forests in which they occur, substantially altering forest structure and nutrient cycling. In mainland forests, however, few empirical examples of exotic dominance exist. The invasive shrub Rhamnus cathartica L. (common buckthorn) is reported to infest temperate forest understories in North America and displace native species, but its degree of dominance has been described only anecdotally. We investigated the extent to which common buckthorn can dominate forest ecosystems, and found strong evidence for monotypic dominance in several mesic and wet sites in southern Wisconsin. Among eight forest sites where common buckthorn was dominant, its mean relative density and basal area was 81% and 45%, respectively. Compared to eight native-dominated sites on similar soils, common buckthorn dominance fundamentally altered forest structure: total woody stem density at Rhamnus-dominated sites was more than twice that of native-dominated sites (two-way ANOVA; P 0.3). When considering dominance by size class within only the eight Rhamnus-dominated sites, common buckthorn genets were more abundant than native genets at 5-cm size classes up to and including 20-25 cm diameter at breast height, evidence that common buckthorn dominance can extend well beyond understory size classes. Within Rhamnus-dominated sites, mean relative density and basal area for common buckthorn exceed that reported for four other woody invaders found in the northeastern US, and thus we suggest that common buckthorn is a particularly successful invasive species in eastern temperate deciduous forests of North America and is capable of acting as an ecosystem dominant.

Introduction

Invasion by exotic plants may cause a myriad of community- and ecosystem-level changes, notably alterations in local diversity, trophic structure, and nutrient cycling (Elton 1958, Mack et al. 2000, Vitousek et al. 1997). Dominant species play a central role in community and ecosystem dynamics (e.g., Crooks 2002, Ellison et al. 2005), and thus alterations caused by exotic plants are likely to be most substantial when successful invaders become dominant (i.e., in terms of relative abundance, basal area, or both; Denslow and Hughes 2004). Forest ecosystems in which the dominance of exotic species exceeds 50% of importance value (mean of density and basal area) have been recently described on islands (e.g., Puerto Rico and Hawai'i; Hughes and Denslow 2005, Lugo and Helmer 2004) or geographically isolated areas (e.g., the fynbos biome in South Africa; Macdonald et al. 1986); however, only anecdotal accounts of such high levels of exotic dominance occur in large mainland ecoregions, such as eastern temperate forests in the US (e.g., Knight 2005). If they are abundant, exotic-dominated temperate forests may already be causing substantial changes to the productivity and nutrient cycling of the region.

Rhamnus cathartica L. ("common" or "European" buckthorn) is a small tree or shrub native to Europe and Asia that was introduced to North America in the 1800s as an ornamental hedge and windbreak (Barnes and Wagner 2004, Knight 2005). The species is naturalized throughout temperate forests of the Midwest, northeastern US, and southeastern and central Canada, commonly invading old fields, roadsides, powerline corridors, and fencelines. Although it may reach heights of several meters, common buckthorn is typically considered a shrubby invader of forest understories (Archibold et al. 1997, Gill and Marks 1991, Harrington et al. 1989, Heneghan et al., 2006, Leitner 1985, Stover and Marks 1998, Zipperer 2002). For example, Leitner (1985) found that common buckthorn was widespread in southern Wisconsin, but largely confined to a shrubby growth form on open, upland sites. Similarly, Stover and Marks (1998) found that buckthorn composed i 20% of basal area at just one of 21 secondary forests on abandoned agriculture and pastureland in New York.

The potential effects of common buckthorn on temperate-forest ecosystem dynamics are substantial, and are likely to be exacerbated if the species is capable of dominating these ecosystems. For instance, several authors have noted the tendency for common buckthorn to exclude woody and herbaceous plants, presumably due to its ability to cast dense shade at levels only conspecifics can tolerate (e.g., Barnes and Wagner 2004, Harrington et al. 1989, citations in Skinner 2005). Furthermore, common buckthorn may increase decomposition and nitrogen turnover where it invades (Heneghan et al.,2006; K.S. Knight, US Forest Service Northern Research Station, Delaware, OH , pers. comm.). Despite the demonstrated importance of common buckthorn for the understory dynamics of temperate forests, little is documented about the extent to which the species is able to dominate temperate forest ecosystems. Thus, we surveyed 16 forest stands in southern Wisconsin to determine the general extent to which common buckthorn is a dominant tree in the area, and the structural characteristics of buckthorndominated stands.