Marsilea vestita Hook. & Grev.
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Basis for Listing
Marsilea vestita (hairy waterclover) was proposed as a special concern species in Minnesota in 1984 because it was known from only three historical records, all from shallow ponds on Sioux quartzite near Luverne in Rock County and near Pipestone and Jasper in Pipestone County. It was not listed at that time however, because the survival of extant populations could not be proven. In 2006-2008, the Minnesota Biological Survey documented approximately 15 new locations for this species in ephemeral rainwater pools on Sioux quartzite outcrops in Pipestone and Rock counties. The largest population is in Pipestone National Monument, where it occurs in numerous shallow depressions on a bedrock ridge. Several populations are on outcrops in heavily grazed pastures. Johnson (1993) also reported specimens from near Gary in Norman County, Crookston in Polk County, and Roscoe in Stearns County. These records have not been verified however, and are discounted pending authenticated documentation. Marsilea vestita was ultimately listed as an endangered species in Minnesota in 1996.
Marsilea vestita is a close ally of the ferns, although the shape of the leaf outwardly resembles that of a four-leaf clover. It has roots only at the nodes of the underground runners. The sporocarps (structures that contain the spore cases) have rather long, coarse, reddish hairs and are solitary (Lellinger 1985; Johnson 1993).
In Minnesota, M. vestita seems to prefer moist soil at the margins of shallow prairie pools, and ephemeral rainwater pools on rock outcrops. These areas are sparsely vegetated and receive direct sunlight. The seasonal fluctuation of water levels in the shallow ponds appears to be an essential part of the habitat requirement.
Biology / Life History
Marsilea vestita has sterile leaves and a fertile stalk. The fertile stalk bears the spore-containing capsules, which contain male and female spores. When the spores are ripe, the capsule splits open allowing the entry of water. As the spores leave the capsule, the male spore fertilizes the female spore. This species also reproduces asexually by means of long creeping underground runners. It can develop both aquatic and terrestrial forms depending upon the habitat (Cobb 1984; Lellinger 1985). Known populations appear to occur only in seasonal habitats that develop in spring and early summer, and may go dormant by late summer. If the required habitat conditions do not return the following year, the population can survive as durable sporocarps that remain viable in the soil for up to 100 years, raising the possibility that they will reappear when conditions become more favorable.
Conservation / Management
The greatest threat to known M. vestita populations in southwestern Minnesota include any land uses that alter or destroy ephemeral pools on bedrock outcrops, including: mining, severe overgrazing, and broadleaf herbicide applications for weed control in pastures. Elsewhere, the conversion of native prairie to row crops has resulted in the extensive destruction of prairie wetlands that also formed habitats for this species.
Best Time to Search
The best time to find M. vestita on outcrops in southwestern Minnesota is in mid-May through June, but plants may also be seen in pools as late as September in years with adequate rainfall to maintain rainwater pool habitats.
Conservation Efforts in Minnesota
Several of the known M. vestita populations are on public lands including Pipestone National Monument, Touch the Sky Unit of the Tallgrass National Wildlife Refuge, and Split Rock Creek State Park. Pipestone National Monument staff have made an extensive effort to mark plant locations, prevent damage by visitors, and consult experts on management strategies.
Cobb, B. 1984. A field guide to ferns and their related families, northeastern and central North America. Houghton Mifflin Company, Boston, Massachusetts. xv + 281 pp.
Great Plains Flora Association. 1986. Flora of the Great Plains. University Press of Kansas, Lawrence, Kansas. 1,402 pp.
Johnson, D. M. 1993. Marsilea. Pages 332-335 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 2. Oxford University Press, New York, New York.
Larson, G. E. and J. R. Johnson. 1999. Plants of the Black Hills and Bear Lodge Mountains. South Dakota State University, Brookings, South Dakota. 608 pp.
Lellinger, D. B. 1985. A field manual of the ferns and fern-allies of the U.S. & Canada. Smithsonian Institution Press, Washington, D.C. 389 pp.