Rare Species Guide

  Synonyms

Carex muricata var. sterilis

  Basis for Listing

Carex sterilis (sterile sedge) 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.

Because of concern about the loss of calcareous fens, intensive surveys for C. sterilis and calcareous fen habitats were conducted beginning in the 1980s and continue today. Many new occurrences were discovered, but observations at the collection sites indicate that several populations are in jeopardy. Approximately half of the occurrences are on unprotected private land. In Stearns County, C. sterilis occurs in habitats that have been degraded by grazing. In Dakota County, artificial lowering of the water table (drawdown) lead to the reported destruction of at least two fens that had previously supported C. sterilis. One was the result of a quarry several miles away and the other a result of a sewage treatment plant. Just the reverse happened to a fen in Norman County, which was severely damaged by flooding caused by a water impoundment project. All these sites were on publicly owned land, which seems to have provided inadequate protection. Remote activities that disrupt the natural hydrological system supporting calcareous fens are occurring at several other sites, elevating the threat to this species' survival in Minnesota. Carex sterilis was listed as a threatened species in Minnesota in 1984.

  Description

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.

Technical Description: Culms densely cespitose, to 75 cm (30 in.) long. Rhizomes less than 1.0 cm (0.4 in.) long or not discernable. Leaves flat or plicate, to 2.4 mm (0.09 in.) wide; tips narrowly pointed; basal sheaths brown, weakly fibrous. Spikes 3--6 per culm (usually 4--5), with 8--25 perigynia each, typically pistillate with one or a few staminate flowers at the base, sometimes unisexual, 4--7 mm (0.2—0.3 in.) long. Pistillate scales acute, 1.7--2.5 mm (0.07—0.10 in.) long; the tip reaching or slightly exceeding the base of the perigynial beak. Perigynia glabrous, ovate, spreading or reflexed, 2.1--3.5 mm (0.08—0.14 in.) long, 1.2--2.3 mm (0.05—0.09 in.) wide; dorsal surface with fine continuous veins; ventral surface veinless or with veins on the lower half of the perigynia; tapered or contracted to a beak 0.8--1.5 mm (0.03—0.06 in.) long. Achenes biconvex, 1.3--1.8 mm (0.05—0.07 in.) long; style deciduous. Maturing mid-May to early August (Smith 2018).

  Habitat

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 spikerush), Rhynchospora capillacea (hair-like beak rush), and Scleria verticillata (whorled nutrush).

  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 acres) 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 led to encroachment of shrubs, trees, and invasive species, such Phalaris arundinacea (reed canary grass) and the European subspecies of Phragmites australis ssp. australis (common reedgrass).

  Best Time to Search

The best time to search for Carex sterilis is when mature perigynia are present from early June to late July.

  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.

  Authors/Revisions

Welby R. Smith (MNDNR), 2023

(Note: all content ©MNDNR)

  References and Additional Information

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, 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.

Smith, W. R. 2018. Sedges and rushes of Minnesota: the complete guide to species identification. University of Minnesota Press, Minneapolis. 696 pp.