Marsilea vestita Hook. & Grev.
Basis for Listing
Although Marsilea vestita (hairy waterclover) is widespread in the western half of the U.S. it is extremely rare in Minnesota where it reaches the eastern edge of its range. It was proposed as a special concern species in Minnesota in 1984, because it was known from only three historical records. The records were from shallow pools on Sioux quartzite near Luverne in Rock County and near Pipestone and Jasper in Pipestone County (Inner Coteau Subsection). In a monograph on the genus, Johnson (1993) also reported specimens from near Gary in Norman County, Crookston in Polk County, and Roscoe in Stearns County. It has been revealed that those reports were in error and have been discounted. Marsilea vestita was not listed in 1984 because the survival of the species in Minnesota could not be confirmed at that time.
Field surveys have since confirmed the continued existence of two of the historically known populations and, in the process, discovered a small number of previously unknown populations (Minnesota River Prairie Subsection). All Minnesota populations are quite small and occur in small microhabitats that are very limited in extent. It was also believed that nearly all the suitable habitat had been searched and few, if any, additional populations would be discovered. For these reasons, M. vestita was listed as an endangered species in Minnesota in 1996.
Marsilea vestita is considered a true fern, though the shape of the leaf outwardly resembles that of a four-leaf clover. That is the source of the common name ‘waterclover’. It is a semiaquatic, deciduous perennial forming diffuse or dense colonies (clones) through the production of slender stolons that grow over the surface of the ground. The roots occur only at nodes along the stolons. The leaves arise singly from the nodes at intervals of 1.5—5 cm (0.6-2.0 in.) They may be floating on the surface of the water or rising above the surface on slender erect petioles that can be up to 20 cm (8 in.) long. The leaf blade is divided into 4 equal segments very similar to the leaf of a clover, though the surface of M. vestita leaves usually has hairs, those of a true clover do not. The reproductive structure is a sporocarp that is about the size of a pea and is initially covered with coarse, appressed hairs. It grows on a short stalk from the base of the petiole.
In Minnesota, M. vestita seems to occur only in direct association with ephemeral rainwater pools on outcrops of Sioux quartzite that occur in a prairie setting. The plants root in thin layers of sediment (organic and otherwise) that accumulate over time. This microhabitat is sparsely vegetated and receives direct sunlight. The seasonal fluctuation of water levels in the shallow pools appears to be an essential part of the habitat. However, the exact nature or timing of seasonal changes that favor M. vestita are not completely understood.
In states west of Minnesota, such as North and South Dakota, the habitat requirements of M. vestita are not nearly so restrictive. It is reported to occur in wet ditches, abandoned agricultural fields, and similar seasonally wet habitats that may have a history of human activity.
Biology / Life History
Marsilea vestita has sterile, photosynthetic leaves with a short fertile stalk attached near the base of the petiole. The fertile stalk bears the male and female reproductive structures that develop within a unique organ called a sporocarp. This is a small firm pea-size structure that can remain dormant in the soil for many years, even decades. Germination of the spores occurs within the sporocarp when it breaks open and water enters.
This species also reproduces asexually by means of long rhizomes, or stolons, that grow over the surface of the ground and root at the nodes. The stolons can grow more than a meter (a yard) in length and produce several branches. The stolon dies during the winter, but at least some of the rooted nodes survive and resume growth the following spring.
Known populations of M. vestita appear to occur only in small seasonal wetlands that develop in spring and early summer. It may go dormant by late summer if the wetlands become dry. If the required habitat conditions do not return the following year, the population can survive as durable sporocarps that remain viable until conditions become more favorable. Because of this ability, M. vestita may seem ephemeral, easy to find some years and hard to find other years. However, established populations are relatively long-lived. Populations are known to have persisted at Pipestone National Monument (Pipestone County) for at least 80 years (and counting) and at Blue Mounds State Park (Rock County) for at least 73 years (and counting). Both of these populations are probably centuries old.
Conservation / Management
The greatest threat to known M. vestita populations in southwestern Minnesota are activities that degrade or destroy ephemeral pools on bedrock outcrops, such as mining, overgrazing by livestock, and herbicide applications for weed control in surrounding grasslands. Management of surrounding grasslands by fire should have no lasting impact on populations of M. vestita since rooted nodes and sporocarp reserves of M. vestita likely occur where the vegetation is too sparse to carry a fire.
Best Time to Search
The best time to find Marsilea vestita on outcrops in southwestern Minnesota is from 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.
Welby R. Smith (MNDNR), 2020
(Note: all content ©MNDNR)
References and Additional Information
Great Plains Flora Association. 1986. Flora of the Great Plains. University Press of Kansas, Lawrence, Kansas. 1,402 pp.
Johnson, D. M. 1986. Systematics of the new world species of Marsilea (Marsileaceae). Systematic Botany Monographs 11:1-87.
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.
Lellinger, D. B. 1985. A field manual of the ferns and fern-allies of the United States & Canada. Smithsonian Institution Press, Washington, D.C. 389 pp.
Tryon, R. 1980. Ferns of Minnesota. University of Minnesota Press, Minneapolis. 176 pp.