Cirsium pumilum var. hillii (Canby) Boivin
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Cirsium pumilum ssp. hillii, Cirsium hillii
Basis for Listing
Cirsium pumilum var. hillii is a native Midwestern thistle that is experiencing a decline as a result of the loss of its prairie and sandy woodland habitat. Thistles in general have gained a decidedly negative reputation because of the unintentional introduction of two non-native thistles that have caused great damage to agricultural and natural ecosystems in Minnesota and across North America. Those species, C. arvense (Canada thistle - a regrettable misnomer as it is a native to Asia, not Canada) and C. vulgare (bull thistle), should not be thought of in the same context as the five non-aggressive, native thistles that inhabit plant communities throughout Minnesota. Of the native thistles, C. pumilum var. hillii is the rarest, not only in Minnesota, but across its range. Cirsium pumilum var. hillii was listed as a special concern species in Minnesota in 1984.
The stems of C. pumilum var. hillii are solitary, erect, 25-60 cm (9.8-24 in.) tall, with few if any short branches. The leaves are usually shallowly lobed (especially the leaves near the top of the stem) and the margins are covered with fine spines. The pink to purplish flowering head is relatively large at 5-7 cm (2.0-2.8 in.) tall, and it is surrounded by 8-10 series of spine-tipped phyllaries (bracts). There is usually only 1 flowering head per plant, or 1 flowering head per branch if branches are present, and it is borne on a peduncle that is elevated above the uppermost leaves (Frankton and Moore 1966). The general impression is of a short, stocky thistle with a single large flower head.
Cirsium pumilum var. hillii is primarily a species of southern dry prairies and southern dry savannas, and to a lesser extent drier examples of southern mesic prairies and woodlands (central dry pine woodland and central poor dry pine woodland) with scattered Quercus spp. (oaks) or Pinus banksiana (jack pine). These habitats are typically on level outwash plains, river terraces, and rolling terrain formed of glacial till. In some cases, bedrock bluff prairies that develop on thin soil over sandstone bedrock also provide suitable habitat. In all cases, the soils are well-drained and produce drought-like conditions during periods of low rainfall.
Biology / Life History
Cirsium pumilum var. hillii is considered a biennial or a monocarpic perennial (Keil 2006), and sometimes a perennial (Johnson and Iltis 1963). As a biennial, it would produce only a rosette of basal leaves the first year, a flowering stem the second year, and then die before the third year. As a monocarpic perennial it might remain in the sterile rosette stage for more than one year, but still flower only once then die. The flowers are insect-pollinated and the seeds are wind-dispersed.
Conservation / Management
The native plant communities that C. pumilum var. hillii is associated with in Minnesota are considered fire-dependent (Minnesota Department of Natural Resources 2005). This means that periodic fire is needed to maintain habitat conditions suitable for the perpetuation of the species. Prior to the fragmentation of prairie and savanna habitats, the frequency of wildfire in C. pumilum var. hillii habitat was probably every 5-10 years. After a period of 15 -20 years without fire, accumulated plant materials will create a dense thatch, which may inhibit the establishment of C. pumilum var. hillii seedlings. After a period of perhaps 20-30 years without fire, the growth of shrubs and trees may create too much shade for the species to grow to maturity, produce flowers, and set seeds. This gradual degradation of habitat can be reversed by establishing a regime of prescribed, controlled burns. However, C. pumilum var. hillii is not known to maintain a seed bank in the soil, so when the last plant is gone there may be no hope of recovery.
Conservation Efforts in Minnesota
Cirsium pumilum var. hillii occurs in several habitats that are owned by public and private conservation agencies. A few of these habitats are effectively managed by prescribed burns. However, the effects of management efforts (or lack of management) on populations of C. pumilum var. hillii have not been monitored.
Frankton, C., and R. J. Moore. 1966. An evaluation of the status of Cirsium pumilum and Cirsium hillii. Canadian Journal of botany 44:581-595.
Johnson, M. F., and H. Iltis. 1963. Preliminary reports on the flora of Wisconsin. No. 48 Compositae I - Composite Family. Transactions of the Wisconsin Academy of Science Arts and Letters 52:255-342.
Keil, D. J. 2006. Cirsium. Pages 95-164 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 19. Oxford University Press, New York, New York.
Minnesota Department of Natural Resources. 2005. Field guide to the native plant communities of Minnesota: the eastern broadleaf forest province. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 394 pp.