Dryopteris marginalis (L.) Gray
Marginal Shield Fern
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Basis for Listing
Although Dryopteris marginalis (marginal shield fern) is widespread in the northeastern United States, it was not discovered in Minnesota until 1981, when a single population was found in Houston County (Paleozoic Plateau Section). It has likely been a part of Minnesota’s flora since before human settlement but has been overlooked because of its extreme rarity as well as its superficial resemblance to common members of the genus Dryopteris (wood ferns). This perceived rarity was the reason it was listed as threatened in 1984. The higher rank of endangered was rejected because it was thought that additional searches by experienced botanists might discover other sites in Minnesota. Those searches have now been completed, yet only one additional population was discovered (Fillmore County; Paleozoic Plateau Section), and it consists of perilously few individuals. For these reasons, the status of D. marginalis was elevated from threatened to endangered in 2013.
Dryopteris marginalis is a fairly large green leafy fern. The leaf blades are rather leathery and evergreen and are divided in a bipinnate pattern. The petioles are stout and beset with chaffy scales. The defining feature of this species is the marginal placement of the sori (hence the name, marginal shield fern). This means the sori are arranged along the very edge of the leaves, specifically at the base of the sinuses between the teeth of the pinnules. The other species in the genus have sori located inwards from the margin.
Contrary to the popular impression of ferns inhabiting moist and dark habitats, D. marginalis prefers rather dry woodland or overgrown savanna habitats. Both Minnesota sites have a patchy canopy that includes Juniperus virginiana (eastern red cedar) and Quercus macrocarpa (bur oak). The soils are sandy at both sites, one a large sand terrace and the other a sandstone bluff. The possibility that D. marginalis might also be found in more heavily shaded forest habitats cannot be discounted.
Biology / Life History
Like most ferns, D. marginalis has both fertile and sterile fronds. Sori first appear on the undersides of fertile fronds in early summer as tiny pale greenish specks. As they develop and ripen in July and August, the marginal sori enlarge and darken, becoming quite prominent. Each sorus is actually a mass of spore cases, or sporangia, that contain 64 spores each (Cobb 1984). A single mature D. marginalis can produce hundreds of thousands, or even millions, of spores each year (Cobb 1984). When spores are ripe and sporangia are dry, the spore cases burst open, and the tiny spores fall to the ground or are dispersed by the wind. Spores that end up in suitable habitat develop into prothalli, or gametophytes.
The prothallus starts as a one-celled body and promptly puts down a tiny root-like hair to anchor itself to the soil. It grows, cell by cell, becoming a small membranous green body, about 0.6 cm (0.25 in.) in size, eventually developing microscopic sexual organs on the underside (Cobb 1984). Moisture is required for fertilization. The egg begins its initial development anchored to the female sex organ under the gametophyte. It grows a root downward into the soil and sprouts a stem upward, which curves through the notch of the heart-shaped gametophyte. Nutrition stored in the gametophyte lasts only long enough for the first root and leaf to develop, at which point the seedling is an independent and self-sufficient individual. Spores of D. marginalis develop into prothalli after about two weeks, and the gametophytes develop into young ferns after about 3½ months (Cobb 1984).
Conservation / Management
Little is known about the habitat needs of D. marginalis in Minnesota. Quarrying or some extreme form of recreational use may potentially threaten its habitat, but that has not been determined. Vegetation succession that follows fire suppression is likely a more immediate threat; however, that has not been assessed either.
Best Time to Search
Searching for this species can be conducted at any time because the firm and leathery leaves are typically evergreen. However, it is easiest to identify D. marginalis in July and August, when the submarginal sori (spore clusters) ripen to a prominent dark brown (Cobb 1984).
Welby Smith, MN DNR, 1988 and 2008; Welby Smith and Mike Lee, MN DNR, 2018
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.
Coffin, B., and L. Pfannmuller, editors. 1988. Minnesota's endangered flora and fauna. University of Minnesota Press, Minneapolis. 473 pp.
Gleason, H. A., and A. Cronquist. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. Second Edition. New York Botanical Garden, Bronx, New York. 910 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.
Minnesota Department of Natural Resources. 2005. Field guide to the native plant communities of Minnesota: the eastern broadleaf forest province. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 394 pp.
Minnesota Department of Natural Resources, Division of Ecological Resources. 2008. Rare species guide: an online encyclopedia of Minnesota's rare native plants and animals [Web Application]. Minnesota Department of Natural Resources, St. Paul, Minnesota. Accessed 1 July 2009.