Round-leaved greenbrier (Smilax rotundifolia) COSEWIC assessment and status report: chapter 4

Distribution

Global range

Round-leaved greenbrier occurs naturally across much of eastern and central North America, from southern Nova Scotia to northern Florida, westward through southern Ontario to Michigan and southwest to eastern Texas (Figure 2). It is globally secure (G5), and of the states where it has a conservation status rank it is S5 (secure) in all but Illinois, where it has a rank of “S3?” (NatureServe 2006).

Figure 2. Global range of round-leaved greenbrier, based on Holmes (2002).

Figure 2. Global range of round-leaved greenbrier, based on Holmes (2002).

Canadian range

Round-leaved greenbrier occurs in two widely separated regions in Canada, around the north shore of Lake Erie in the Carolinian Forest of the Great Lakes Plains (e.g. Soper and Heimburger 1982), and in the Atlantic Coast Plain of southern Nova Scotia (e.g. Roland 1998). Various Ontario-focused literature has failed to note the Nova Scotia population or has been inaccurate in describing it. White et al. (1982) and Soper and Heimburger (1982) called round-leaved greenbrier "rare in Canada", while Argus and White (1977) considered it "rare and possibly extirpated in Canada", despite the fact that it had been documented as a locally common species in southern Nova Scotia since Fernald’s expedition in 1920 (Fernald 1921).

The status of the Ontario population was unclear until 1982 with Argus and White (1977) and White et al. (1982) indicating that it was possibly or probably extirpated in Ontario. It was also not recorded in detailed surveys of the Essex County flora (Botham 1981) and natural areas (Oldham 1983).

Although Ontario populations are nearly contiguous with the core distribution of round-leaved greenbrier further south, plants in Nova Scotia are disjunct from the nearest populations in southern Maine by approximately 200 km across the Gulf of Maine (Holmes 2002; Figure 3). In Nova Scotia, round-leaved greenbrier is a fairly common species over the region within about 115 km of the southernmost end of Nova Scotia (Roland and Smith 1969, Roland 1998). Its northern limit appears to be quite abrupt beyond that zone; it is a strong indicator species for the region supporting the greatest diversity of Atlantic Coastal Plain flora in Nova Scotia (S. Blaney, M. Elderkin, pers. comm.). Figure 3 maps the known distribution of the species from herbarium and sight records compiled by the Atlantic Canada Conservation Data Centre up to 2006. Within its limited region of occurrence, it is present in shrub thicket and forest near the shores of most river systems and larger lakes, although it tends to be less common toward headwater regions (Roland and Smith 1969, S. Blaney, pers. comm.).

Figure 3. Distributional area of round-leaved greenbrier in Nova Scotia.

Figure 3.  Distributional area of round-leaved greenbrier in Nova Scotia.

The number of solid circles do not reflect the total number of populations (>50) documented for the province. (Map by Pamela Mills, Nova Scotia Department of Natural Resources, Kentville, Nova Scotia. Reprinted from COSEWIC 2001.)

The 13 extant populations in Ontario cover an Extent of Occurrence of about 2500 km² based on two convex polygons including Essex and Kent counties in the westernmost polygon and Niagara/Norfolk counties in the eastern portion of the species range in the province; its Area of Occupancy is 13 km² if a 1x1 km grid is applied and about 40 km² if a 2x2 km grid is used. The >50 Nova Scotia populations have an Extent of Occurrence <5000 km² and an Area of Occupancy > 50 km².

Biogeographical and phytogeographical history in Ontario

In Ontario, the distribution of this species appears to be concentrated at the two extremities of the Carolinian Zone, possibly reflecting the likely paths of post-glacial migration from the south, across the two points of land between adjoining lakes (Figure 4). Why there are so few collections in between is difficult to explain, considering that many birds eat the fruit and may be important agents of dispersal and the relatively similar habitats and moderated climates in the region between the two centres of distribution. It is conceivable that perhaps the forested habitats between these centres were cleared before comprehensive botanical surveys were conducted in this region. The region is also dominated by clay soils rather than sandy soils that may not have served as optimum substrate for this species.

The open squares in Figure 4, representing unsubstantiated literature reports from Soper and Heimburger (1982) appear to have no documented basis. These authors specifically stated that this species occurred only in Essex and Kent counties, thereby excluding the Middlesex County records (upper two open squares in Figure 4). No information is available to substantiate the two Kent County records (lower two open squares in Figure 4) mapped by Soper and Heimburger (1982) since the catalogue of records compiled by Soper and used by him for mapping is no longer available.

Figure 4. Known and reported occurrences of round-leaved greenbrier in Ontario.

Figure 4. Known and reported occurrences of round-leaved greenbrier in Ontario.

Filled circles represent sites confirmed within the last 20 years. The open star at Point Pelee is an extirpated population; open squares are unsubstantiated literature reports from Soper and Heimberger (1982).

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