Asplenium platyneuron (L.) B.S.P.
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Basis for Listing
Asplenium platyneuron (ebony spleenwort) is a small, evergreen fern of hardwood forests. As of 2017, there were only about a dozen documented occurrences in Minnesota, all were from the southeastern counties. The counties in question have been the subject of intensive botanical surveys beginning in 1990, so the relatively few discoveries of A. platyneuron is probably a true reflection of its rarity in Minnesota. Concern for this species however, goes beyond just its rarity. For example, forest management issues are complex and they can involve competing resource interests, some of which may not be compatible with the habitat needs of A. platyneuron. Given its rarity and the small size of known populations, A. platyneuron was listed as a special concern species in Minnesota in 1996.
Asplenium platyneuron is a small, woodland fern with relatively narrow, once-pinnate, evergreen leaves that arise from a short rhizome. The leaves grow erect to a maximum height of about 50 cm (1.6 ft.) and a width of about 5 cm (2.0 in.); the sterile leaves are somewhat smaller than, and not as erect, as the fertile leaves. The individual segments of the leaf are 1-2.5 cm (0.4-1.0 in.) long, 15-40 in number, and attached to the rachis in an alternating pattern. The base of each leaflet has an ear-shaped lobe (auricle) on the upper side that overlaps the rachis. The petiole of each leaf is reddish brown, shiny and smooth, and it comprises 1/4 to 1/3 of the length of the entire leaf. Asplenium platyneuron superficially resembles a small-sized Polystichum acrostichoides (Christmas fern), but the rachis and petiole of the latter fern are light-colored and very scaly.
In Minnesota, A. platyneuron is found in dry and mesic hardwood forests, predominantly in the rugged stream-dissected terrain of the Paleozoic Plateau. Common tree species in these habitats include Quercus spp. (oaks), Tilia americana (basswood), and Ostrya virginiana (ironwood). The best places to look seem to be rocky slopes and mossy banks, often on sand. Asplenium platyneuron has been described as unusual among the spleenworts in that it grows on both rocks and in soil (Wagner and Johnson 1981). It has also been described as a habitat generalist and, in Ontario, is drought-tolerant and grows in both sun and shade (Wagner et al. 1993). Often, the forests where A. platyneuron is found are in a dynamic stage of succession where competitive relationships among the various plant species are shifting and evolving.
Biology / Life History
Like all ferns, A. platyneuron reproduces by spores rather than seeds, and it has a two-stage life cycle. There is an underground gametophyte stage and an aboveground sporophyte stage. This complicates matters when trying to get a census of populations, especially when trying to determine reproductive success and recruitment into the population. It might also be useful to think of A. platyneuron as a rather mobile species, perhaps able to take advantage of shifting habitat opportunities in dynamic landscapes.
Conservation / Management
Wagner and Johnson (1981) suggest that A. platyneuron was once quite rare in the Great Lakes region but has become more common and widespread since the 1960s. That idea was challenged by Peck (1982) who has direct experience with this species in Minnesota. He suggests that in the Driftless Area, which includes Minnesota, the populations of A. platyneuron were always small and transient and it was consequently overlooked by earlier botanists. Whichever the case, very few populations are currently known to occur in Minnesota, and it seems prudent that the presence of the species should be taken into account when making land use or management decisions.
Conservation Efforts in Minnesota
No known conservation efforts have been undertaken specifically on behalf of A. platyneuron.
Peck, J. H. 1982. Ferns and fern allies of the driftless area of Illinois, Iowa, Minnesota, and Wisconsin. Milwaukee Public Museum Press, Contributions in Biology and Geology Book 53, Milwaukee, Wisconsin. 140 pp.
Wagner, W. H., Jr., and D. M. Johnson. 1981. Natural history of the ebony spleenwort, Asplenium platyneuron (Aspleniaceae), in the Great Lakes area. The Canadian Field-Naturalist 95:156-166.
Wagner, W. H., Jr., R. C. Moran, and C. R. Werth. 1993. Asplenium. Pages 229-245 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 2. Oxford University Press, New York, New York.