The Rhamnacea, or buckthorn family, includes over a hundred North American species (Brockman 1968), but few of these are native to central Wisconsin. However, several invasive Old World buckthorns now threaten to catastrophically alter the composition of midwestern forests. Common buckthorn, an ecological convergent with the genus Alnus, was present in Wisconsin by 1849 (Wisconsin Department of Natural Resources 2002), and glossy buckthorn reached London, Ontario by 1898 (Haber 1997a).
Exotic buckthorn remains available as an ornamental at some commercial nurseries (Wisconsin Department of Natural Resources 2001; Moriarty 2002), and even receives institutional encouragement for public planting (University of North Carolina Arboretum 2001), suggesting one major motivation for its importation. The wood also has had use for shoe supports, veneer, and fine gunpowder charcoal (Dobelis 1986); the bark and berries produce dyes (Haber 1997b); the flowers can be nectar sources for honey making (Risnes 1980); and its distillations formerly had medicinal use as a purgative and emetic (Felter and Lloyd 2002). It serves as important winter forage for moose (Alces alces) in Eastern Europe (Borkowska and Konopko 1994, Dunin 1989), but North America's white-tail deer (Odecoileus virginianus) reject it as a food source (City of Eagan 2002).
The exotic buckthorn species of greatest concern are R. utilis (Asian buckthorn), R. cathartica (common, or European, buckthorn), and R. frangula [syn. Frangula alnus] (glossy, or columnar, buckthorn). These species resemble one another with minor variations in form; buckthorn is a short-stature broadleaf tree rarely exceeding eight meters in height, produces either deep blue or reddish seed-bearing berries approximately one centimeter in diameter, has circular-spotted dark brown bark, and may exhibit multiple stems up to 100 centimeters in diameter. Hairy twigs lack thorns and the crenulated leaves have smooth edges for R. frangula (Figure 1), but R. cathartica can have spiny twigs and its flat leaves have serrate edges (Minnesota Department of Natural Resources 2002; Wisconsin Department of Natural Resources 2002). Further confounding identification, hybridization occurs between some of these species (Gil-ad and Reznicek 1997).
A standard explanation for the rapid dispersal of buckthorn is that its berries attract feeding birds, which then rapidly transport the relatively indigestible seeds across large distances (Converse 1985). However, many buckthorn berries are infertile, although those from the hybrids are not (Cox 1999). Alternatively, buckthorn may spread into new habitat by suckering, either from extensive shallow root systems or from stumps (Moriarty 2001). Typically it penetrates and thrives, often at the expense of less competitive native plants, into hydric to mesic soils; it has the reputation of being water-loving, but it is not a wetland plant (Gourley and Howell 1984).
Buckthorn is an aggressive pioneer competitor. It has frequently obliterated native vegetation, especially the understory species (Osmund 1997). Buckthorn may have allopathic tendencies (Boudreau and Willson 1992, Heidorn 1991), is a prolific seed producer of up to 770 berries per square meter, and root-colonizes at up to 200 stems per square meter (Moriarty 2002). Land managers have come to recognize buckthorn as a serious compromise of native ecosystems, and invest substantial labor and budget to mechanically and/or chemically remove buckthorn. Control efforts have also included burning, grazing, and pre-emptive planting with native competitors, and in Canada land managers have tried the European insects Scotosia vetulata and Triphos dubiata as biocontrol agents. Such efforts have rarely proven effective, and perhaps may so further challenge the already-stressed native vegetation as to encourage reinfestation (U.S. Geological Survey 2002). A suggestion also exists that flooding might eliminate infestations from low-lying terrain (Haber 1997b), but the effectiveness and durability of this approach lacks demonstration.
Schmeeckle Nature Reserve came into existence in 1977 through a private land donation and cooperative planning between local businesses, governments, and the University of Wisconsin at Stevens Point, Wisconsin. It is the site of the Wisconsin Conservation Hall of Fame, with a museum, conservation laboratories, and classrooms. The Reserve provides multiple-use benefits to the surrounding community, with emphasis on preservation, restoration, education, recreation, and flood control.
Including more recent additions, the reserve currently consists of some 485 hectares of forests and wetlands. Red oaks (Quercus spp.) and maples (Acer spp.) occupy mesic ground, especially in the higher northeastern portion of our study area. White pine (Pinus strobus) and red pine (Pinus resinosa) groves occur on drier sandy soil on the west, and jack pine (Pinus banksiana) prevails near 1977 road construction at the center of our study area. Raspberries (Rubus spp.), strawberries (Fragaria spp.), sweetferns (Comptonia spp.), and various grasses dominate more recent disturbances in drier soils.
Wetlands cross the western side of the study area from north to south, and an artificial lake occupies the southeast corner. Mixed stands of quaking aspen (Poplus tremuloides) with birches (Betula spp.) cover seasonally-flooded wetland borders, while alders (Alnus spp.) and willows (Salix spp.) form thickets within shallower portions of the wetlands. Cattail (Typha latifolia) and sedges (Carex spp.) occupy deeper and more permanently flooded wetlands.
Invasive buckthorn has visibly increased as a forest understory presence within Schmeeckle Reserve during the past decade, so much so that for the past several years there has been considerable investment of finances and labor for annual attempts to remove or contain this small tree. Glossy buckthorn (R. frangula) was the most common woody exotic invader that we observed within the Reserve. How buckthorn arrived remains uncertain; perhaps visiting birds brought seeds, or possibly humans may have discarded trimmings that rooted along roadsides. However, control efforts appear have been futile, as buckthorn has vigorously resprouted almost as fast as land managers have removed it (Figure 2).
Despite growing and subjective concerns,
no surveys or mapping of buckthorn distribution within Schmeeckle Reserve were
available as of September 2001 (Zimmerman 2001). We therefore undertook a collaborative
undergraduate research project to make a baseline determination of this plant’s
distribution within the reserve. Specifically, we
sought to:
establish permanent observation sites for long-term habitat monitoring
map patterns of buckthorn density, dominance, and size variations within our study area
note associations of habitat factors (e.g., soil moistness, ground cover, competitor presence, microclimate, and disturbance recency) with presence (or absence) of buckthorn