Rare Species Guide

  Synonyms

Polemonium occidentale var. lacustre, Polemonium occidentale ssp. lacustre

  Basis for Listing

Polemonium lacustre was first documented in St. Louis County, Minnesota in 1944. Prior to that discovery, this species was unknown to science. A total of only six sites in Minnesota and Wisconsin have been located since 1944, even after extensive survey efforts. Prior to 2023, this species was considered a subspecies of the more western Polemonium occidentale; however, recent genetic work by Rose and Systema (2022, 2023) elevated P. occidentale ssp. lacustre to the species level as P. lacustre.  The most likely threats to P. 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. lacustre was listed as an endangered species in Minnesota in 1996.

  Description

Polemonium 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).

  Habitat

Polemonium 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. 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. lacustre are extremely rare in Minnesota, one or two of the populations are relatively large, consisting of numerous clones.

  Biology / Life History

Polemonium 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. 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. 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 Polemonium 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. lacustre, and survey work continues.

  References and Additional Information

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. . Accessed 19 July 2006.

Ownbey, G. B., and T. Morley. 1991. Vascular plants of Minnesota: a checklist and atlas. University of Minnesota Press, Minneapolis. 320 pp.

Rose, J. P. and K. J. Sytsma. 2022. Phylogeography and genetic variation in Western Jacob's ladder (Polemonium occidentale) provide insights into the origin and conservation of rare species in the Great Lakes region. Molecular Ecology 32: 79-94.

Rose, J. P. and K. J. Sytsma. 2023. A new combination for a narrowly endemic Polemonium (Polemoniaceae). Novon 31: 33-35.

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.