Scleria verticillata Muhl. ex Willd.
Basis for Listing
Scleria verticillata (whorled nutrush) is a slender sedge that occurs in the central and eastern U. S. and adjacent portions of southern Canada. It is generally rare or local throughout the northern part of its range. In Minnesota, it has very unusual and restrictive habitat requirements that limit the species to a rare native plant community called a calcareous fen. Calcareous fens are localized, fragile wetlands that depend on specific hydrologic and geologic conditions and are themselves unique and quite rare in Minnesota. They develop where cold calcium-rich (or calcium/magnesium-rich) groundwater seeps to the surface. In places where the seepage has remained uninterrupted for perhaps thousands of years, partially decomposed plant remains accumulate and become peat.
Before the 1850s, there may have been as many as 2,200 ha (5,436 ac.) of calcareous fen habitat just in the Lower Minnesota River Valley between Mendota and Shakopee (Big Woods and St. Paul-Baldwin Plains and Moraines subsections). Human activities have reduced the total to fewer than 40 ha (99 ac.). The other major concentration of calcareous fen habitat is along the ancient beach ridges of Glacial Lake Agassiz in northwestern Minnesota (Red River Prairie and Aspen Parklands subsections). Most of the calcareous fens in that region have now been destroyed by agriculture, gravel quarrying, and water management projects (draining or impoundment).
The decline in acreage of calcareous fen habitat has been closely paralleled by a decline in S. verticillata, which was listed as a threatened species in Minnesota in 1984.
Scleria verticillata is a slender annual sedge that can be very difficult to detect when it is growing among other sedges and grasses. It has fine linear leaves no more than 2 mm (0.08 in.) wide. It has 2-6 flower clusters spaced apart from one another on the main stem. Each tiny flower has a long-pointed scale, and each cluster of flowers has an inconspicuous bract. An obvious field character is the white stone-like achene (a kind of one-seeded fruit). The achene is rough with transverse ridges and a blunt disk area where the achene was attached to the plant.
The only other member of the genus Scleria in Minnesota is S. triglomerata (tall nutrush). It is also a rare species, but it is much larger, has a strongly triangular stem, and occurs in dry prairies.
Technical Description: Plants annual. Culms slender, erect, arising singly or few together, 10--40 cm (4-16 in.) long, smooth, or minutely scabrous, triangular in cross-section. Rhizomes absent. Leaves: sheaths pubescent, at least near the summit; blades glabrous, ascending or nearly erect, 0.5--2 mm (0.02-0.08 in.) wide, not surpassing the inflorescence, flat or keeled. Inflorescence terminal, unbranched, 1--10 cm (0.4-4 in.) long, consisting of 2--8 compact, sessile, widely spaced glomerules; bracts setaceous, scabrous, about equaling or slightly exceeding the subtended glomerule. Spikelets 3--10 per glomerule; each with 2--10 unisexual flowers and consisting of both male and female flowers. Floral scales with narrow scabrous tips. Achenes white or whitish, with irregular transverse ridges, 1--2 mm (0.04-0.08 in.) long, ± spherical or obscurely 3-angled, apiculate; bases trigonous, stipe-like, not papillose. Maturing mid-July through September (Smith 2018).
Scleria verticillata is entirely restricted to calcareous fens in the prairie regions of Minnesota and seems to occur only in the better quality, least disturbed examples of this specialized plant community. It has never been found in the spring fens or ribbed fens that sometimes occur in the large peatland complexes of the forested regions of the state. Within calcareous fens, S. verticillata acts as a pioneer species on exposed marl and along the margins of shallow pools. Conditions there are quite severe, with a high pH, high mineral content, and low dissolved oxygen. Scleria verticillata is usually found in association with other strict fen species such as Rhynchospora capillacea (hair-like beak rush) and Carex sterilis (sterile sedge).
Biology / Life History
Because S. verticillata is an annual, its reproduction depends entirely on seed production. It is not known how long seeds remain viable in the peaty substrate of a calcareous fen. The seeds are not adapted for wind dispersal but may be aided in transport by small animals. Ridges on the achene could help them adhere to mud or marl and thus to birds’ feet. Or possibly, the plants are browsed or grazed, and ingested achenes are deposited elsewhere.
Conservation / Management
The conservation of S. verticillata in Minnesota is tied to conservation of the specialized calcareous fen habitat upon which it relies. 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 hydrologic conditions, calcareous fens can be indirectly affected by remote activities such as gravel quarrying or groundwater appropriation that occur several kilometers away. Without safeguarding the hydrologic regime of the entire system supporting calcareous fens, the security of this habitat, and therefore, S. verticillata is uncertain.
Best Time to Search
It can be very difficult to see Scleria verticillata when mature fruits are not present, so searches are most effective if limited to the period between the middle of July and the middle of September.
Conservation Efforts in Minnesota
As part of a directed search effort for calcareous fens by the Minnesota County Biological Survey, several previously unknown fens have been discovered, some of which harbor viable populations of S. verticillata. This major contribution to conservation offers a sound basis for establishing protection plans. Still remaining to be resolved, however, is the broader issue of how climate and land use changes may affect critically important hydrologic conditions occurring outside the boundaries of protected fens.
Welby R. Smith (MNDNR), 2023
(Note: all content ©MNDNR)
References and Additional Information
Core, E. L. 1936. The American species of Scleria. Brittonia 2:1-105. + illustrations.
Reznicek, A. A., J. E. Fairey III, and A. T. Whittemore. 2002. Scleria. Pages 242-251 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 23. Oxford University Press, New York, New York.
Smith, W. R. 1983. A range extension of Scleria verticillata in Minnesota. The Michigan Botanist 22:27-30.
Smith, W. R. 2018. Sedges and rushes of Minnesota: the complete guide to species identification. University of Minnesota Press, Minneapolis. 696 pp.