Rare Species Guide

  Synonyms

Empetrum rubrum var. atropurpureum

  Basis for Listing

Like the closely related Empetrum atropurpureum (purple crowberry), E. nigrum is a plant of the far north that occurs at the comparatively southern latitude of Minnesota. It exists only in a cold microclimate found on a small island in Lake Superior in Cook County. It is apparently absent from the other Great Lakes. In Minnesota, E. nigrum is known by a single herbarium specimen collected in 1948 by Gerald B. Ownbey (Butters and Abbe 1953). There have been repeated attempts to relocate this species at its original collection site as well as in other suitable habitats, but it has not been found. Empetrum nigrum was listed as an endangered species in Minnesota in 1996.

  Description

Empetrum is a genus of low, creeping evergreen shrubs with inconspicuous flowers and a berry-like fruit (drupe). The two species in Minnesota, E. nigrum and E. atropurpureum, are closely related and difficult to distinguish. In E. nigrum, the branchlets and margins of expanding leaves (new leaves) are not tomentose (they lack woolly hairs) but are minutely glandular. The mature leaves are divergent at first, but soon become reflexed, and the fruit is black. In E. atropurpureum, the branchlets and margins of expanding leaves are white-tomentose but not glandular and the leaves are divergent or ascending (angled in the forward direction along the stem) but rarely reflexed, and the fruit is purple.

  Habitat

Empetrum nigrum was found on a small island composed of Precambrian rock. The island is in Lake Superior, less than 3 km (1.9 mi.) from the mainland. The interior of the island is forested with Picea spp. (spruce) and Abies spp. (fir) trees, and the margins consist of massive bedrock exposures. Empetrum nigrum occurred on moist ledges of exposed bedrock just far enough from the lake to escape wave action and ice scouring.

  Biology / Life History

Much like E. atropurpureum, E. nigrum is a small, low-growing, woody shrub that forms a mat. Stems are prostrate and repeatedly branched. The mat may be compact, especially if growing in a narrow crevice, or it may sprawl across the rocky surface with the younger branchlets creeping outward from the main mat. Branches and branchlets are quite uniformly and densely covered with small, evergreen leaves. Leaves are only 4-8 mm (0.2-0.3 in.) long and leaf margins roll inward toward the underside of the leaf. Details of the species' life history in Minnesota are not known. According to Gleason and Cronquist (1991), North American plants belong to the circumboreal tetraploid group sometimes identified as E. nigrum var. hermaphroditum (Hagerup) T. Soerensen, which have perfect flowers (flowers having both male and female parts) but sometimes partly unisexual flowers. The European plants are diploid and the flowers are unisexual (plants are dioecious, separate male and female plants). Voss (1985), however, emphasizes that even specialists have difficulty placing the circumpolar members of Empetrum into well-defined species or subspecies and that one approach is to consider E. nigrum as a single, highly variable species. That interpretation would include Minnesota's other rare Empetrum, E. atropurpureum, even though E. atropurpureum appears distinct in Minnesota.

Because E. nigrum is an evergreen species, it can be seen and identified any time of the year that it is not covered by snow.

  Conservation / Management

Empetrum nigrum is at risk because it is extremely rare and suitable habitat is very limited. The places where this plant occurs or could possibly occur are remote and not readily accessible, so it is not greatly threatened by human activities. The species faces a greater threat from nesting gulls that utilize the same rocky coast. Ring-billed gull (Larus delawarensis) populations in particular have increased dramatically in the Lake Superior basin in the past 20 or so years. Significant disturbance of shore edge vegetation by gull activity has been well documented (Bernard et al. 1971; Cuthbert and McKearnan 1998). Global climate change is also a likely threat to this species in Minnesota.

  Conservation Efforts in Minnesota

The Minnesota Biological Survey for rare plants has been nearly completed in northeastern Minnesota, although some potential habitat for E. nigrum still remains to be searched. Botanists generally agree that significant new populations are unlikely to be found.

  References and Additional Information

Bernard, J. M., D. W. Davidson, and R. G. Koch. 1971. Ecology and floristics of Knife Island, a gull rookery on Lake Superior. Journal of the Minnesota Academy of Science 37:101-103.

Butters, F. K., and E. C. Abbe. 1953. A floristic study of Cook County, northeastern Minnesota. Rhodora 55:21-55.

Cuthbert, F. J., and J. McKearnan. 1999. U.S. Great Lakes gull survey: 1998 Progress Report. Report submitted to the U.S. Fish and Wildlife Service, Fort Snelling, Minnesota.

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.

Love, D. 1960. The red-fruited crowberries in North America. Rhodora 62:265-292.

Shugart, G. W. 1976. Effects of Ring-billed Gull nesting on vegetation. The Jack-Pine Warbler 54(2):50-53.

Smith, W. R. 2008. Trees and shrubs of Minnesota. University of Minnesota Press, Minneapolis. 703 pp.

Sobey, D. G. and J. B. Kenworthy. 1979. The relationship between Herring Gulls and the vegetation of their breeding colonies. Journal of Ecology 67 (2):469-496.

Soper, J. H., and E. G. Voss. 1964. Black Crowberry in the Lake Superior region. The Michigan Botanist 3:35-38.

Southern, W. E., W. L. Jarvis, and L. Brewick. 1976. Food habits and foraging ecology of Great Lakes region Ring-billed Gulls. Pages 100-144 in Proceedings of the fish-eating birds of the Great Lakes and environmental contaminants symposium. Toxic Chemical Division and Ontario Region Canadian Wildlife Service, 2-3 December 1976, Toronto, Ontario, Canada.

Voss, E. G. 1985. Michigan Flora. Part II: Dicots (Saururaceae-Cornaceae). Cranbrook Institute of Science Bulletin 59 and the University of Michigan Herbarium. University of Michigan, Ann Arbor, Michigan. 727 pp.