Polemonium occidentale ssp. lacustre Wherry
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Polemonium occidentale var. lacustre
Basis for Listing
Polemonium occidentale ssp. lacustre was first documented in St. Louis County, Minnesota in 1944. Prior to that discovery, this subspecies was unknown to science. A total of only six sites in Minnesota and Wisconsin have been located since 1944, even after extensive survey efforts. The closely related subspecies occidentale occurs in mountainous habitats in the western United States. The most likely threats to P. occidentale ssp. lacustre are changes in the hydrology of its wetland habitat caused by disruption of groundwater and surface water drainage, either by natural processes or by human intervention. Given its extreme rarity and the vulnerability of its habitat, P. occidentale ssp. lacustre was listed as an endangered species in Minnesota in 1996.
Polemonium occidentale ssp. lacustre is a singled-stemmed, perennial plant up to 10 dm (40 in.) tall, growing from a horizontal rhizome. Leaves are divided, with a variable number of narrow leaflets. The blue, bell shaped flowers are arranged in a compact cluster (Lakela 1965).
Polemonium occidentale ssp. lacustre occurs in forested swamps with Picea mariana (black spruce), Larix laricina (tamarack), and Thuja occidentalis (northern white cedar). Common associated shrub species include Alnus incana ssp. rugosa (speckled alder) and Betula pumila (bog birch). Within suitable forests, P. occidentale ssp. lacustre is found in open and sparsely forested portions of the swamps, as well as in moderately deep shade, usually in hummocks of Sphagnum magellanicum (sphagnum moss). The saturated conditions usually originate from groundwater seepage. Although populations of P. occidentale ssp. lacustre are extremely rare in Minnesota, one or two of the populations are relatively large, consisting of numerous clones.
Biology / Life History
Polemonium occidentale ssp. lacustre is a perennial herbaceous species that reproduces sexually by seeds and asexually by subterranean rhizomes. The species appears to flower where light is plentiful and to persist primarily by asexual means in shadier conditions. In some Minnesota populations, flowering appears to be most prolific in open areas and in areas where Arceuthobium pusillum (dwarf mistletoe) infestations have killed the canopy trees. At a site in Wisconsin where strip-harvesting treatments had occurred, flowering was most prolific in the open strips.
Conservation / Management
All of the recorded P. occidentale ssp. lacustre sites in Minnesota are in conifer swamps that have experienced episodes of selective logging. Because little is known about the requirements of this species, it is difficult to assess what effect this past land use has had on populations. It is possible that minimal selective logging may not adversely affect populations because P. occidentale ssp. lacustre seems to prefer small forest openings. However, any logging operation should occur in winter to prevent damage to the soil and to minimize disruptions in water flow. Slash should be removed from the site to maintain potential microhabitat requirements of this endangered species. Perhaps a greater threat to habitat integrity is from road construction, beaver activity, or other activities that could disrupt the hydrological system that sustains this habitat.
Best Time to Search
Leaves of P. occidentale ssp. lacustre are visible and identifiable throughout the summer, but the best time to search for this species is during flowering, from late June through July.
Conservation Efforts in Minnesota
The Minnesota DNR participated in a collaborative, federally funded survey project with the state of Wisconsin that targeted this species. The Minnesota Biological Survey has also conducted rare species surveys in significant portions of the anticipated range of P. occidentale ssp. lacustre, and survey work continues.
Anderson, C., R. Lake, J. Dobberpuhl, and N. Sather. 1994. Status of Polemonium occidentale ssp. lacustre. Interim report to United States Fish and Wildlife Service, Office of Endangered Species, Twin Cities, Minnesota. 13 pp.
Gleason, H. A., and A. Cronquist. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. Second Edition. New York Botanical Garden, Bronx, New York. 910 pp.
Lakela, O. 1965. A flora of northeastern Minnesota. University of Minnesota Press, Minneapolis, Minnesota. 541 pp.
NatureServe. 2006. NatureServe Explorer: An online encyclopedia of life [web application]. Version 5.0. NatureServe, Arlington, Virginia.
Ownbey, G. B., and T. Morley. 1991. Vascular plants of Minnesota: a checklist and atlas. University of Minnesota Press, Minneapolis, Minnesota. 320 pp.
U.S. Forest Service. 2000. Population viability assessment in forest plan revision. Questions for plant population viability assessment panel: Polemonium occidentale. United States Forest Service, Region 9, Duluth, Minnesota.