Hill’s thistle (Cirsium hillii) COSEWIC assessment and status report: chapter 5

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Habitat

Habitat requirements

Throughout its range Cirsium hillii is found in a variety of open, dry, sandy, fire-prone habitats, including such communities as gravel hill or bluff prairies, dry mesic to mesic sand prairies, pine barrens, oak barrens, sand dunes, oak savannah, and open woods (Higman & Penskar 1999, Penskar 2001). In Michigan, Wisconsin, and Ontario, it is also known from alvar grasslands.

In Minnesota, the species occurs in relatively disturbed sites, in association with many non-native, invasive species, such as Kentucky bluegrass (Poa compressa and P. pratensis) and Sweet white clover (Melilotus alba) (Penskar 2001). E.J. Hill, who discovered the species, noted this tendency as well in 1910, observing that, in the prairie areas south and west of Chicago, it grew in, "railway enclosures fenced off from the surrounding prairie before the land had been touched by the plow. Here it is one of the few native denizens of the prairie that seems able to compete with an introduced vegetation. It may return again to ground from which it had been excluded by cultivation, when this is seeded down for a time for pasture or meadow, and left a few years untilled. It thus takes on the character of a pasture or meadow thistle, and grows successfully beside the white and the red clover, timothy, and the most common meadow and pasture grasses of drier grounds, Poa pratensis and P. compressa." Cusick (1995) actually noted that, "The name "pasture thistle" is appropriate, since the plants are avoided by grazing animals and therefore stand out in grazed pastures." In Ontario C. hillii has been noted in association with Poa compressa, Melilotus alba, Ambrosia artemisiifolia, Centaurea maculosa, Daucus carota, Hypericum perforatum, and Chrysanthemum leucanthemum.

One essential aspect of the habitat of Cirsium hillii is its need for open, or relatively open conditions. For example, in Michigan, Penskar (2001) notes that, "Within the extensive jack pine barrens of the Lower Peninsula, C. hillii thrives best in the most open fire-prone areas, where it occurs in prairie-like communities." He also refers to its prime habitat within the oak-pine barrens in the Upper Peninsula as being, "openings in relatively small colonies within a relatively large savanna remnant." All of the Ontario stations have an open aspect to them, alvars, savannahs, or woodland, although these communities may be ‘degraded’ with lack of disturbance. Jones (1996) has noted from the population on Great La Cloche Island that, “It does not grow in amongst broad-leaved plants or anything directly overtop of it.” Schaefer (1995) noted C. hillii at Barney Lake, Bruce County, in open alvar grassland, but that a majority of the plants observed were actually in an 80% shaded woodland of cedar, poplar & spruce, with Pteridium aquilinum dominating the understory (C. hillii was 2^nd dominant) and occurring along an overgrown path.

It has been suggested that historically, at least in the eastern portion of its range, C. hillii, “may have survived along the buffalo traces where trampling and grazing provided adequate habitat disturbance.” (The Nature Conservancy 1990). There are several references in the Ontario records to disturbance-related habitats. For example Jones (1995) refers to the population at Evansville High Pavement on Manitoulin as, “Also grows in spots of exposed dirt. In this area it seems to be growing in and along a 4-wheeler trail, so perhaps it likes some disturbance.” Another essential aspect relates to the fact that the species does not germinate well in a thick duff layer (see below). The Nature Conservancy (1990) notes that, “Thin-soil prairie ridges and sand prairie/savanna sites (which provide habitat for the bulk of extant occurrences [in the US]) appear to provide appropriate germination and development sites without significant animal-induced disturbance regimes.”

Of the 41 stations in Ontario for which habitat information has been noted, it is found in the following habitats: limestone pavement-alvar meadow; Jack Pine-Juniperus communis treed alvar shrubland or savannah; small patches in white spruce dominated forest; jack pine–trembling aspen alvar; open jack pine–white cedar woods; jack pine–trembling aspen–Juniperus communis alvar; white cedar–white spruce–Juniperus horizontalis open alvar grassland; white cedar-dominated upland coniferous and mixed forest; jack pine–white spruce–balsam fir woodland; openings in white cedar–white spruce forest; white cedar-tamarack-white spruce-black spruce open woods; white cedar-tamarack woodland; white cedar-white spruce-trembling aspen alvar; red oak-white pine savannah on dunes; and sand dunes at edge of woods. It has also been noted on the Bruce Peninsula in coniferous woods over old dolostone shingle or cobblestone beaches (Owen Sound Field Naturalists, 2001; see Figures 3 and figure4. At the other end of the spectrum, it also occurs in open alvars, including ones just above the Lake Huron high water line on the Bruce Peninsula (Owen Sound Field Naturalists, 2001), or at Fisher Harbour on Little La Cloche Island (see Figure 5). 

The above communities are always supported by a substrate of limestone bedrock or sand. Usually the sites are relatively dry, although the author observed a small population at Site 55 in 2002 that was in a moist, sandy alvar nested within a white cedar-tamarack woodland, in association with such wet meadow species as Tofieldia glutinosa, Aster umbellatus, Solidago ohioensis, and Potentilla fruticosa (see Figure 6).

Jones (2000) characterizes typical C. hillii habitat on Manitoulin Island, based on her observations at numerous stations, as, “Openings within jack pine or spruce-dominated savanna-woodland with Juniperus communis, ground cover of Cladina rangiferina, Carex richardsonii or Danthonia spicata, Arctostaphylos uva-ursi. Other associates are Geum triflorum, Schizachyrium scoparium, Linnaea borealis, Pteridium aquilinum, Vaccinium angustifolium, Oryzopsis asperifolia, Senecio obovatus, and mosses Dicranum sp., Hypnum sp. Substrate is coarse loamy sand to sandy loam or sand on bedrock. Can be adjacent to pavement alvar. Burn evidence can be present to old or very old, to no visible evidence at all of recent fire.” On the Bruce Peninsula, Johnson (pers. comm. 2002) considers typical C. hillii habitat as, “Open, scrubby, conifer woods on bedrock.” He also provides an excellent, detailed characterization of typical habitat along the shores of the Bruce peninsula from his work at Site 52. He notes C. hillii habitat as, “In very open woods (usually coniferous, never deciduous) (including near-alvars), less often in true open alvars, thus often in alvars and near-alvars, and always where canopy very open to absent. White cedar (most often dominant), tamarack, white spruce, and black spruce are the main trees present. The most frequent ground cover associates include: Carex eburnea, Juniperus horizontalis, Iris lacustris, Arctostaphylos uva-ursi, and Schizachyrium scoparium. Apparently always where the forest burned in the early 1900s. Always dry. Usually on dolostone bedrock, although by far the most populous site (2001) was on small dolostone stones. Always where undisturbed (except that 2 plants were along trails), but populations mainly small.” (Johnson 2002). On the Bruce Peninsula it has been noted as occurring up to six or seven km inland (Owen Sound Field Naturalists, 2001).

Frequently associated species in the US sites include typical prairie/savanna grasses such as big bluestem (Andropogon gerardii), little bluestem (Schizachyrium scoparius), Indian grass (Sorghastrum nutans), poverty grass (Danthonia spicata), hair grass (Deschampsia flexuosa), June grass (Koeleria macrantha), and a variety of goldenrods, asters, and other prairie forbs (Higman & Penskar 1999). In Ontario, again from the 41 stations with habitat information cited, a total of 26 herbaceous associates have been noted, 1 lichen, 2 mosses, 7 shrubs, and 11 tree species. Danthonia spicata is most commonly noted as the dominant species in the ground layer with C. hillii, with Arctostaphylos uva-ursi the most commonly noted associate, followed by the lichen Cladina rangiferina, Pteridium aquilinum, and Schizachyrium scoparium. jack pine, white spruce, and white cedar are the trees most often noted as dominant in the habitat, and Juniperus communis is the most noted dominant shrub, followed by Juniperus horizontalis.

Shoreline areas of woodland alvar on Manitoulin Island and the associated islands, and the west shore of the Bruce Peninsula, are critical to the long-term maintenance of Cirsium hillii in Canada. These woodland alvars have existed historically in a landscape shaped by natural disturbances such as drought and fire. The landscape is also shifting rapidly, on Manitoulin Island, and to a greater degree on the Bruce Peninsula, as estate homes and condominiums are developed in the prime shoreline sites. With this development comes a cessation of natural fires and the usual habitat degradation associated with subdivisions. Only six stations of C. hillii are believed to be extirpated, with the reasons for their demise less than conclusive. Although the species appears to persist in some remnant sites, the pressure to control fire near human habitation will increase the likelihood of succession to woodland and therefore of loss of the population.

Trends

Alvar and savannah habitats have been lost on Manitoulin Island and the Bruce Peninsula over the past 100 years, the result of forest succession and concurrent suppression of natural periodic fire, clearing of land for agriculture, increasing use of open shoreline areas (old & new) for housing and recreation (including ATVs), and extraction of sand for commercial uses. The south shore of Manitoulin Island and the west shore of the Bruce Peninsula are highly desirable locations to live and this fact will almost certainly have implications for the privately held populations of C. hillii. The projected population increases for southern Ontario would indicate that this trend of growth in north Bruce County and Manitoulin Island is likely to increase (Western Ontario Smart Growth Panel, 2003). Of the 64 extant stations for C. hillii, about 20 are either fully or partially protected in public or private parks or reserves.

Protection/ownership

Of the 64 extant stations for Cirsium hillii, the ownership breakdown is as follows:

• 36 are privately owned by single or multiple landowners;
• 11 are publicly owned by government conservation organizations, e.g., either as provincial or national park (Sites 9, 15, 39, 44, 48, 49, 60, 61, 62, 63 and 64)
• 4 are privately owned by conservation NGOs (Site 50 by the Federation of Ontario Naturalists and Sites 42 and 54 by the Nature Conservancy of Canada);
• 5 are owned by First Nations
• 8 are split between public and private ownership (Sites 4, 13, 14, 16, 22, 33, 47 and 53).

Of the six extirpated sites, all are privately owned. Ownership of each site is noted within the “Extant Populations” section.

Of the 11 stations either within full public ownership or owned by conservation NGOs, management or active protection which would benefit Cirsium hillii is known to be occurring only at Site 60, a Provincial Park site.

The majority of the extant stations (36) are in private hands. Another seven of the stations are partly held in private. Some of these 43 sites are recognized as provincially significant Areas of Natural and Scientific Interest (ANSI), for example that at Site 52. ANSIs are currently Category 2 within the Provincial Policy Statement for Ontario, this category permitting development and site alteration, “if it can be demonstrated that there will be no negative impacts on the natural features or on the ecological functions for which the area is identified.” (Ontario Government, 1997). As required by the Planning Act, local planning authorities “shall have regard to” policy statements issued under the Act. Fortunately, some of the highest quality and most populous stations of C. hillii are within public ownership, for example Sites 9, 15, 44, and 60.