Carex sterilis Willd.
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Carex muricata var. sterilis
Basis for Listing
Carex sterilis is generally limited to temperate and boreal habitats in the glaciated regions of North America, but it is common only in the center of its range in portions of Michigan and southern Ontario. Elsewhere it appears to be rare or local because of the highly localized nature of its special habitat (Reznicek and Ball 1980). This is certainly true in Minnesota where C. sterilis ranges over a relatively large portion of the state but occurs almost exclusively in small, fragile, calcareous fen habitats. Calcareous fens are highly specialized wetlands maintained by surface discharge of cold, calcium rich groundwater. In Minnesota, these fens occur in the prairie region where agricultural land use competes with native habitats and, in this case, the groundwater upon which the fen habitat depends. As a result, the acreage of calcareous fens in Minnesota has decreased with an attendant decline in the overall extent of C. sterilis.
Carex sterilis is a conspicuous member of the sedge family, but it can be easily confused with C. interior (interior sedge), which often occurs in the same habitat. Both species have fairly slender leaves a few millimeters wide arising from a dense clump. On flowering culms, C. sterilis typically has 4 spikes (flower clusters or heads), the terminal one either all pistillate (female) or all staminate (male), or nearly so. Carex interior usually has 3 spikes, the terminal one largely pistillate with staminate scales at the base that give the spike a tapered (clavate) base. Some plants in a population of C. sterilis produce all-male spikes that appear quite different from the fertile plants, but they still exhibit the densely clumped growth form and other characteristics of this species.
Carex sterilis appears to be an obligate fen species in Minnesota. Most populations are found in the mineral-rich calcareous fens of the prairie region. A calcareous fen is a special kind of peatland that is maintained by the surface discharge of cold, oxygen-poor, calcium-rich groundwater. Occurrences of C. sterilis in the spring fens of the transition and northern forested regions are much rarer. These fen habitats that are associated with large peatlands seem to lack the mineral richness of calcareous fens, but in a few cases do support small populations of C. sterilis. Calcareous fen communities are typically wet, open, sunny, and dominated by sedges; C. sterilis itself sometimes being a dominant species. It frequently occurs with other rare fen species such as Eleocharis rostellata (beaked spike-rush), Rhynchospora capillacea (hair-like beak-rush), and Scleria verticillata (whorled nut-rush).
Biology / Life History
Carex sterilis has a densely clumped (cespitose) growth form. These clumps appear to be able to persist for many years. This species is unusual among the sedges in that some plants may produce only male flowers and the spikes (flower heads) appear quite different than those on plants with female flowers. The male plants still have the same densely cespitose growth form. Carex sterilis is well adapted to the cold, nutrient-rich, and oxygen-poor groundwater that characterizes its calcareous fen habitat.
Conservation / Management
The conservation of C. sterilis in Minnesota is tied directly to conservation of its specialized calcareous fen habitat. A typical calcareous fen may be less than 2 ha (5 ac.) in size, but the recharge area that maintains its groundwater supply may cover thousands of hectares. Because of their dependence on delicate groundwater hydrology, fens can be indirectly affected by activities several kilometers away. Without safeguarding the entire hydrologic system maintaining these fens, the security of C. sterilis habitat is uncertain. Livestock grazing, herbicide application, sedimentation, and the construction of wildlife and stock ponds have also damaged calcareous fens. Artificial lowering of the water table, fire suppression, or a combination of these factors has lead to encroachment of shrubs, trees, and invasive species, such as Phalaris arundinacea (reed canary grass) and Phragmites australis (common reed grass).
Conservation Efforts in Minnesota
Several of the calcareous fen populations of C. sterilis are on state-owned land. This, unfortunately, has not afforded adequate protection for this species to date. A remaining challenge is the broader issue of how land use changes may affect critically important hydrologic conditions occurring outside the boundaries of protected fens.
Ball, P. W., and A. A. Reznicek. 2002. Carex. Pages 254-572 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 23. Oxford University Press, New York.
Reznicek, A. A., and P. W. Ball. 1980. The taxonomy of Carex section Stellulatae in North America north of Mexico. Contributions from the University of Michigan Herbarium 14:153-203.