Basis for Listing

Isoetes melanopoda (prairie quillwort) occurs in Minnesota at the northwestern edge of its range, which is discontinuous and disjunct. In fact, the Minnesota populations of I. melanopoda appear to be disjunct 250 km (155 mi.) or more from the nearest extant population. The potential for its occurrence in Minnesota is further limited by its unusual habitat requirements. It appears to be restricted to a semiaquatic microhabitat associated with bedrock outcrops of Sioux quartzite. Such outcrops are limited to four or five counties in southwestern Minnesota (North-Central Glaciated Plains Section), and the specific microhabitat itself is even more limited. Because the habitat occurs among rock outcrops, it is largely immune to land conversion practices such as plowing and ditching. However, these sites are frequently grazed by cattle, which may cause permanent trampling damage to the habitat. For these reasons, I. melanopoda was listed as an endangered species in Minnesota in 1984.

  Description

Isoetes is an ancient genus that is morphologically distinct from anything else that might be encountered in Minnesota. And yet, this species can be very difficult to spot among the grasses and rushes with which it grows. Isoetes melanopoda is functionally amphibious, though generally classified as terrestrial. It is a perennial, though perhaps seasonally ephemeral. The leaves are linear (quill-like), bright green becoming pale to lustrous black toward the base, and deciduous. They are tufted, erect to spreading, and grow to 40 cm (16 in.) in length (but usually half that length). The leaves are stiff but pliant, dilated at the base, tapering gradually to the tip (Taylor et al. 1975).

Each leaf of I. melanopoda bears a single sporangium (a structure containing spores) embedded in a cavity at its base (inward side) that is covered by a thin membrane called a velum. This feature can be easily seen in the field and is adequate to verify the plant as an Isoetes.  It is usually necessary to conduct a microscopic examination of the megaspores to differentiate species of Isoetes, but I. melanopoda is the only member of the genus Isoetes found in southwestern Minnesota.

  Habitat

Isoetes melanopoda is restricted to the prairie region of the state, specifically where Sioux quartzite bedrock is exposed at the surface. Where these outcrops are exposed as flat terraces on gentle slopes, shallow pools may form in depressions in the rock itself. These pools are sustained by seepage from between the strata or directly from rain. Isoetes melanopoda occurs in soil pockets in the pools or on the margins, which typically persist for at least six weeks in the spring. The plants are usually rooted in shallow fine-textured sediments 3-5 cm (1-2 in.) deep. During the early stages of development, the plants may be submerged or emergent in 5- 10 cm (2-4 in.) of water. But by the time they mature in midsummer, they are usually stranded in mud. This is a very discrete and uncommon habitat type in Minnesota. It is also the preferred habitat of other rare species such as Heteranthera limosa (mud plantain), Plantago elongata (slender plantain), Callitriche heterophylla (larger water starwort), Eleocharis wolfii (Wolf’s spikerush), and Marsilea vestita (hairy waterclover).

  Biology / Life History

Mature I. melanopoda plants have fertile leaves that contain male spores (microspores) and female spores (megaspores). Each individual plant will normally produce both types of spores, which are released when the leaf senesces at the end of the growing season. Upon germination, the male spores produce sperm, and the female spores produce eggs. Fertilization occurs only in a moist or wet habitat when male and female spores are in close proximity. The fertilized egg develops into a young I. melanopoda plant. In turn this plant produces microspores and megaspores, completing a two-part life cycle. This is the only means of reproduction known in I. melanopoda.

The individual plants are perennial but appear to be short-lived. However, populations appear to be relatively stable over long periods of time. The location where this species was first discovered in Minnesota in 1945 (Moore and Tryon 1946) was known to still support this species in 2018.

Long-range dispersal of spores resulting in colonization by I. melanopoda of unoccupied habitats has undoubtedly occurred historically, but considering the habitat specificity of the species and the necessity of both male and female spores to land in close proximity, it must be a very rare event.

  Conservation / Management

Habitat degradation may be the greatest threat to I. melanopoda in Minnesota. While its habitats are not prime targets for agriculture, they are frequently grazed by livestock and are increasingly being mined, with the rock being crushed for gravel. Evidence has shown that cattle will trample the fragile habitats, which damages or destroys the native vegetation and allows non-native species to invade. The broadcast application of herbicides applied for pasture "improvement" is a potential threat, though the specific susceptibility of Isoetes to chemical herbicides is not known.

  Best Time to Search

Immature plants can be easily seen in May, but the best time to search for Isoetes melanopoda is June and early July when plants have reached maturity. Plants may persist through September in years with adequate rainfall.

  Conservation Efforts in Minnesota

Most known populations of I. melanopoda in Minnesota are on state and federal lands, including a National Wildlife Refuge, a National Monument, State Parks, and Wildlife Management Areas. A few populations persist in privately owned pastures.

  Authors/Revisions

Welby R. Smith (MNDNR), 2020

(Note: all content ©MNDNR)

  References and Additional Information

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.

Moore, J. W., and R. M. Tryon, Jr. 1946. A new record for Isoetes melanopoda. American Fern Journal 36(3):89-91.

Smith, W. R. 2023. Ferns and lycophytes of Minnesota: the complete guide to species identification. University of Minnesota Press, Minneapolis. 368 pp.

Taylor, W. C., N. T. Luebki, D. M. Britton, R. J. Hickey, and D. F. Brunton. 1993. Isoetaceae. Pages 64-75 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 2. Oxford University Press, New York, New York.

Taylor, W. C., R. H. Mohlenbrock, and J. A. Murphy. 1975. The spores and taxonomy of Isoetes butleri and I. melanopoda. American Fern Journal 65(2):33-38.

Tryon, R. 1980. Ferns of Minnesota. University of Minnesota Press, Minneapolis. 176 pp.