Potential impacts of the invasive exotic shrub Rhamnus frangula L. (glossy buckthorn) on forests of southern New Hampshire

Northeastern Naturalist, 2003 by Frappier, Brian, Eckert, Robert T, Lee, Thomas D

ABSTRACT - This paper investigated the potential for the exotic shrub Rhamnus frangula L. (glossy buckthorn) to alter native plant community composition in southeastern New Hampshire. Stratified random sampling was performed with 2 m x 2 m plots randomly located in 5 m intervals along three 50 m transects in four even-aged Pmwj-mixed hardwood forests, three of which were managed stands. The associations between R. frangula and the measured species abundances and environmental variables were investigated using linear, least-squares multiple regression and Non-metric Multidimensional Scaling Ordination. Plot basal area of R. frangula was inversely related to woody seedling density (p

INTRODUCTION

The United States has seen a dramatic increase in the number of naturalized exotic species over the past 100 years (United States Congress 1993), probably due to exponentially increasing transport opportunities (Di Castri 1989). Within New England, an estimated 30% of the flora is exotic (Ricketts et al. 1999). Impacts from exotic species are now listed as either the greatest (Czech and Krausman 1997) or the second greatest threat to Federally listed "endangered" species (Wilcove et al. 1998). Alteration of native species recruitment due to exotic species can lead to changes in community composition (Macdonald et al. 1989). Such effects have been demonstrated for exotic species such as Mimosa pigra L. in Australia (Braithwaite et al. 1989), Melaleuca quinquenervia (Cav.) Blake in southern Florida (Myers 1983), and Myricafaya Aiton in Hawai'i (Vitousek and Walker 1989). Beyond direct effects on species, some invasive species have the capacity to alter ecosystem functions such as fire frequency, nutrient cycling, and water availability (Ewel 1986, Mack et al. 2000, Ramakrishnan and Vitousek 1989, Vitousek 1986).

Some exotic plant species have exhibited the capacity to invade relatively undisturbed ecosystems (Ewel 1986, Mooney and Drake 1989, Ramakrishnan and Vitousek 1989). Brothers and Spingarn (1991) found 47 exotic species in strictly protected, albeit fragmented, forest reserves in Indiana; three species were able to establish within the reserve centers. Such nature reserves require that both ecosystem structure and function remain intact (Noss and Cooperrider 1994). Thus, the threat of community alteration by the interaction of exotic plant species with the biotic and abiotic environment warrants attention in areas specifically protected for their community composition.

Indiscriminate control of an abundant exotic species, regardless of whether evidence of an impact exists, is not an effective strategy for managing invasions. The limited resources of most conservation efforts necessitates that we prioritize control and focus on those exotic species having an impact and, subsequently, on those having the most dramatic impacts (Macdonald et al. 1989). Control efforts may also have adverse impacts on native species, some of which may be rare or threatened (Usher 1987, Westman 1990). If the individualistic model of community assemblage is assumed, then each invasive species could have different impacts and those impacts may differ with the invaded community considered (Westman 1990). Therefore, in cases where an exotic species attains substantial abundance, direct experimental and/or comparative investigation is both justified and necessary to prioritize control and avoid unnecessary harmful impacts.

The objective of this research was to investigate the potential of the exotic shrub Rhamnus frangula L. (glossy buckthorn; Rhamnaceae) to alter plant community composition in southern New Hampshire Pinusmixed hardwood forests. Originally native to Europe (Gleason and Cronquist 1983), R. frangula has naturalized throughout the northeastern United States. Its current range in North America extends from Nova Scotia south to Pennsylvania, west to Illinois with some spotty distribution in Minnesota and Wisconsin (Converse 1984). The earliest confirmed collections in North America are from London, Ontario in 1898 and Ottawa in 1899. The period of most rapid expansion in Canada appears to be between the 1970s and 1980s (Catling and Porebski 1994). Howell and Blackwell (1977) report the likely establishment of R. frangula in Ohio between 1914 and 1932. Its range will likely continue to expand in North America, the species becoming abundant in open and semi-open wetlands and some upland woodlands (Catling and Porebski 1994).

Rhamnus frangula inhabits a wide range of soil types and soil moisture conditions (Converse 1984) but prefers moist to wet sites that are not fully flooded. In some fens in its native European habitats, plant species diversity below mature stands of R. frangula was found to be extremely low compared to that before stand maturation (Godwin 1936, Godwin et al. 1974). Conspecific seedling density below parent plants has been reported to be on the order of 100 seedlings m^sup -2^ (Converse 1984, Godwin 1943). Growth rates as high as 0.5 cm diameter growth per year (Godwin 1943) and 4 meters of height in 5 years (Converse 1984) have been recorded for this species.