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The History of Invasion and Current Status of Glossy Buckthorn,
Rhamnus frangula, in Southern Ontario
Paul M. Catling and Z. Sue Porebski
Biological Resources Division, Agriculture Canada Centre for Land and Biological Resources Research, Wm. Saunders
Building, Central Experimental Farm, Ottawa, Ontario K1A 0C6
Catling, Paul M., and Z. Sue Porebski. 1994. The history of invasion and current status of Glossy Buckthorn, Rhamnus frangula, in southern Ontario. Canadian Field-Naturalist 108(3): 305-310.
In southern Ontario Glossy Buckthorn, Rhamnus frangula, became established in three urban centres, London, Ottawa and Guelph, at the turn of the century. By 1930, it had still only been found in these three urban centres, and by 1950 it had not extended beyond 40 km from any of them. By 1970, it had spread to sites up to 150 km distant but still appeared to have a sparse and largely urban distribution in the southern part of the province. Currently Rhamnus frangula occurs throughout much of southern Ontario and is locally common. In some sites it comprises more than 90% of the green
biomass over areas of several acres and it has become a major component of regionally and provincially significant plant communities. Berries of Rhamnus frangula are eaten by American Robins, Bohemian Waxwings, Cedar Waxwings, Rose- breasted Grosbeaks and Starlings. The shrub probably has a number of different avian and mammalian dispersal agents.
Over the next 10-20 years, it is expected that R. frangula will become abundant and dominant in open and semi- open wet- lands over most of the southern part of the province. While small scale management technology is available, the problem is one that requires a consideration of management on a large scale.
Key Words: Rhamnus frangula, Glossy Buckthorn, alien, weed, distribution, dispersal, ecology, invasion, Ontario.
Glossy Buckthorn, Rhamnus frangula L. is one of three species of Buckthorns occurring without culti- vation in eastern Canada. The group is characterized by leaves with veins converging toward the tips and fruits that are fleshy and black when ripe (e.g., Soper and Heimburger 1982). The native Alder-leaved
Buckthorn, Rhamnus alnifolia L'Her and the intro- duced Common Buckthorn, Rhamnus cathartica L., both have serrated leaves, the former being a thorn- less shrub less than 1 m high, the latter, a large shrub or small tree to 6 m high with some branches ending in a sharp thorn. The Glossy Buckthorn is a shrub or small tree to 6 m tall, like Common Buckthorn, and is also introduced from Europe. It differs in having smooth-margined, mostly alternate leaves and in lacking thorns. In winter, it can be distinguished by its greyish, and pubescent twigs, alternate buds, and ascending branches. While the berries of Common
Buckthorn are retained on the shrub until the follow- ing spring (unless eaten), those of Glossy Buckthorn have mostly fallen by early November.
Several authors have alluded to invasion and replacement of natural communities by Glossy
Buckthorn (e.g., Voss 1985; Taft and Solecki 1990) and it is evidently a rapidly increasing problem in southern Ontario (e.g., Dugal 1990, 1992; White et al. 1993). Native to Europe, it was recently rated as one of the six principal invasive aliens of wetlands in Canada, and one of the four principal invasive aliens of Canadian uplands (White et al. 1993). In a national survey it was rated as second (to Purple
Loosestrife, Lythrum salicaria) in both the extent to
which it is spreading in natural habitats and its severity of impact (White et al. 1993). In addition to the threat it poses to natural plant communities, R. frangula is an alternative host to crown rust fungi which attack oats (e.g., Ginns 1986). Here we pre- sent information relating to the history of its inva- sion and current status in southern Ontario.
Methods
The identifications of herbarium material of
Rhamnus frangula from southern Ontario herbaria
(including DAO, CAN, OAC, QK, TRT, TRTE and
WAT, acronyms from Holmgren et al. 1990) were verified. Collection dates and locations of confirmed specimens were recorded in dBase IV database files
(1992. Ashton-Tate, Scotts Valley, California) which were later interfaced with Quikmap version 2.51 mapping software (1990. ESL Environmental
Sciences Ltd., Sidney, British Columbia) so as to produce maps of collections up to various dates.
Habitats were surveyed in the Ottawa area to obtain a subjective impression of abundance and threat to native vegetation. In addition the recent literature relating to R. frangula was reviewed.
Early Establishment
The earliest collections in Ontario were from the period around the turn of the century. It was collect- ed at London in 1898 by J. Dearness, at Ottawa in
1899 by J. Macoun and at Guelph in 1906 by W.
Scott. It was "well naturalized" in London, Ontario in 1898 (Macoun 1898). Although it is sometimes
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cultivated (Bailey 1949), it appears that all of these early collections were from non-cultivated plants.
We base this assumption on the fact that there is no reference to cultivation on specimen labels. By 1930
Glossy Buckthorn was still known from only these localities (Figure 1A). By 1950, 50 years after initial establishment, it was still associated with these three urban areas (Figure IB).
The early association with urban areas is not sur- prising for an introduced species, but what is a little surprising for a plant that aggressively invades natu-
ral habitats is the fact that it remained associated with these urban areas for so long. Even now it is primarily a dominant plant in natural communities near urban areas, although its rate of moving out into natural and agricultural landscapes has increased.
One might argue that the few early botanists restricted themselves to collecting in urban areas and that is why the plant appears to have been confined for so long to the three above mentioned centres.
This notion is readily dispelled by examining a map of the distribution of the related native species
Figure 1. Collections of Rhamnus frangula in southern Ontario based on herbarium speci- mens examined (see methods). A, up to 1930. B, up to 1950.
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Catling and Porebski: Glossy Buckthorn in Southern Ontario
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Figure 2. Collections of Rhamnus frangula in southern Ontario based on herbarium speci- mens examined (see methods). A, up to 1970. B, up to 1994.
Rhamnus alnifolia by 1930 (Figure 3). The edaphic tolerance of R. frangula appears to overlap complete- ly with that of R. alnifolia, so that a habitat difference cannot be advanced as an explanation for the differ- ent distributions of these two species. By 1930, the much shorter and less conspicuous R. alnifolia had been collected over a broad area of southern Ontario and was not associated with urban centres, suggesting the extensive collecting of the early botanists. Thus the early confinement of R. frangula to the urban centres is not an artefact of collecting locations, but is an accurate picture of the first stages of its invasion.
Spread from urban centres
By 1950, R. frangula appears to have spread up to
40 km from initial sites of establishment in Ottawa and 20 to 30 km from initial sites of establishment in
London and Guelph (Figure IB). By 1970, it had spread to sites up to 150 km distant from the three urban centres but still appeared to have a sparse dis- tribution in the southern part of the province (Figure
2A). In 1982, Soper and Heimburger noted that it was rather local and chiefly near cities in southern
Ontario. Their 1982 map is basically the same as the
"up to 1970" map (Figure 2A).
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Present Distribution and Status
Currently Rhamnus frangula occurs throughout much of southern Ontario and is very common in some regions of the province (Figure 2B). It has clearly "escaped" major urban centres, where its early dominance in urban natural areas acted as an advance warning. Not only are more locations shown than on the 1970 map, but new regions of southern Ontario have been colonized including the
St. Lawrence River region of eastern Ontario and the
Lake St. Clair region of extreme southwestern
Ontario. The map suggests that R. frangula is absent from most of the Canadian Shield region and from the Bruce Peninsula and Manitoulin Island. Its absence from the latter area is supported by the com- prehensive survey of Morton and Venn (1984).
Dispersal
Howell and Blackwell (1977) implicated the intro- duced European Starling (Sturnus vulgaris) with the spread of Rhamnus frangula in Ohio. The basis for this was largely Ridley's (1930) report of starlings dispersing the fruit and on the fact that the starling and Glossy Buckthorn invaded Ohio at about the same time. Although Glossy Buckthorn was estab- lished at three widely separate locations in Ontario prior to 1900, starlings were not observed in the province until 1914 (Snyder 1951). They had been seen at a number of places in southern Ontario by
1920, but were not reported as breeding in Ontario until 1922 (Bowman 1987). By 1935, they were abundant in southern Ontario (Snyder 1951). Since field-naturalists with an interest in compiling region-
al lists of birds were numerous in the province at the time, it seems very unlikely that starlings arrived much earlier than the first report in 1914, i.e., after
R. frangula.
With respect to Ridley's (1930) report, it was based on observations in Sweden, and while it is likely that starlings have played a role in the spread of R. frangula in Ontario and generally in North
America, there is little evidence. Lindsey (1939) reported that the related alien, Rhamnus cathartica formed 8.3% of the food of starlings in New York
State in November. Cramp (1988) reported
Bohemian Waxwings (Bombycilla garrulus) feeding on R. frangula in Europe. In Huntley Township near
Ottawa, American Robins {Turdus migratorius),
Bohemian Waxwings, and Cedar Waxwings
{Bombycilla cedrorutn) have been observed eating the fruit of Glossy Buckthorn (S. J. Darbyshire and
M. Runtz, personal communication). In Ottawa,
American Robins, Rose-breasted Grosbeaks
(Pheucticus ludovicianus) and starlings have been observed feeding on berries (M. Goselin, personal communication). It seems most likely that in North
America, R. frangula has a number of different avian and mammalian dispersal agents.
Ridley (1930) noted that the fresh fruits of R. frangula were reported to be able to float for about three weeks and the dry seed floated for one week, but he noted that it was of little importance because
R. frangula does not grow near water. Quite to the contrary in Ontario, R. frangula grows in and near water and in many situations where it is periodically flooded. However, ripe berries that we placed in
Figure 3. Collections of Rhamnus alnifolia in southern Ontario up to 1930 based on herbarium specimens examined (see methods).
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Catling and Porebski: Glossy Buckthorn in Southern Ontario
309
water sank immediately. Since they do not dry out on the shrub over winter as in Common Buckthorn, they cannot take advantage of spring flooding. Thus, there seems to be little reason to attach much signifi- cance to dispersal by water.
Future Prospects
Based on climatic zones (Brown et al. 1980) cur- rently occupied by Glossy Buckthorn in Ontario and observed range of habitats in the province, it may be expected to spread into the Bruce Peninsula, along the Lake Huron shore and Georgian Bay shorelines and probably to Manitoulin Island and much of the
Canadian Shield. It is also expected to become abun- dant and dominant in open and semi-open wetlands over most of the southern part of the province.
Although R. frdngula is a source of medicinal drugs and possibly a useful source of nectar for honey production (Risnes 1980), the problems facing natural communities as a result of its abundance seem to far outweigh the benefits. In the Ottawa area, it has become a dominant plant in moist to mesic organic soils of acid, neutral or alkaline reac- tion. It occurs also on sand, clay, limestone rock and pure peat (e.g., Dugal 1989). Habitats range from fields to dense woodlands but light is nevertheless a limiting factor and in dense woodland the shrubs, or small trees up to 10 cm dbh, are only dominant along the more open edges. Wetland habitats are character- istic and include open and treed fen, sedge marsh, swamps of Red Maple, ash, cedar, alder, etc., as well as shorelines. Its invasion should be of as much con- cern as that of Purple Loosestrife, Lythrum salicaria, since it appears to have a much broader habitat range than the latter species and appears to be much less dependent on human disturbance to estab- lish and become dominant. Furthermore, it has the capacity to invade certain low nutrient wetland habi- tats such as fens and shores where rare and endan- gered native species are frequently concentrated. In the Ottawa area, it has invaded, then dominated and replaced native vegetation, especially in sedge marshes, fens, maple swamps and mesic woodland edges (Dugal 1992, personal observation). In many places, it represents more than 90% of the green biomass over areas of several acres. In some places, it has become a major component of regionally and provincially significant plant communities such as the Leitrim fen near Ottawa (e.g., Dugal 1990,
1992), the Sifton Bog near London (D. Sutherland, personal communication), and old growth pine stands in the lower Ottawa valley. Rhamnus frangula may have a devastating effect on many southern
Ontario bogs and fens which are already a high con- servation priority (e.g., Riley 1989).
A number of articles have been published on spa- tially limited methods of control of R. frangula (e.g.,
Heidorn 1991; Post et al. 1989) and these methods
will continue to be useful, but more extensive man- agement plans may be appropriate. With the problem being in natural communities, R. frangula appears to be an ideal candidate for biocontrol. Some investiga- tion has already been done on the potential for bio- control of the related alien, Rhamnus cathartica, in
Canada (Maw 1981; Malicky et al. 1970). Although both species are a menace to natural communities, R. frangula is generally considered to be a more serious problem now (e.g. White et al. 1993), even though it is apparently still less widespread in Ontario than R. cathartica (cf. Soper and Heimburger 1982). To some extent this could be a consequence of the recent preoccupation of environmentalists with wet- lands, but regardless, both R. frangula and R. cathar- tica have become very serious problems which are rapidly increasing in magnitude.
If the alien domination of the natural areas of major urban centres is indicative of what can occur within extensive natural landscapes, as seems to be the case with Glossy Buckthorn, then domination by alien vegetation certainly appears likely to be a major threat to Canadian biodiversity.
Acknowledgments
W. J. Cody of Agriculture Canada and two anonymous reviewers provided useful comments on the manuscript. S. J. Darbyshire of Agriculture
Canada, M. Runtz of Arnprior, and M. Goselin of the Canadian Museum of Nature provided observa- tions of birds feeding on Glossy Buckthorn. M.
Goselin assisted with literature search relating to bird feeding habits.
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Received 30 March 1994
Accepted 3 November 1994