Polystichum acrostichoides (Michx.) Schott
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Basis for Listing
Polystichum acrostichoides (Christmas fern) is a wintergreen fern that is common in deciduous forests over most of its range. It is of concern in Minnesota, because it reaches the very northwestern limit of its range in the state and is known from only two localities; one in Houston County and one in Winona County (Blufflands Subsection). When P. acrostichoides was first located in the state in 1979, it was presumed this species had just been overlooked in the past. For that reason, it was listed as special concern in 1984, a relatively low status. Subsequent field searches failed to find any more locations, indicating it was not simply overlooked, it was likely quite rare, so its status was raised to threatened in 1996. Since then, the Minnesota Biological Survey continued both extensive and intensive rare plant searches of potential habitats, and yet found no additional populations of P. acrostichoides; so in 2013, its status was elevated to endangered.
Polystichum acrostichoides has a relatively large, compact rhizome and dimorphic leaves. Sterile fronds are once-pinnate, arching, and up to 76 cm (30 in.) long. Stipes are 1/2 to 1/3 as long as the blades. Leaf pinnae are alternate, spreading at right angles, sharply auricled at the upper base; paler beneath, with hair-like scales; darker green and scaleless above. Fertile, upper pinnae are abruptly smaller than sterile pinnae. Sori are in 2 rows on either side of the midrib. Indusia are thick, entire, and blackish when dry (Wagner 1993).
In Minnesota, P. acrostichoides has been found in mesic hardwood forests on north-facing or east-facing river bluffs (Peck 1982). Soils of moderate moisture and neutral pH are preferred.
Biology / Life History
Polystichum acrostichoides is a long-lived, forest fern, adapted to moist, low-light conditions. The leaves (fronds) are often reported as evergreen, though a more appropriate term might be wintergreen. New leaves are produced in early spring and survive for one year. They senesce and die as new leaves are produced the following spring. The leaves have been found to perform net photosynthesis during the winter in the temperate climate of Ohio (Minoletti and Boerner 1993); however, it is unknown whether they perform the same function in Minnesota, where the winter sun is weaker and the temperatures are colder. To aid survival, the leaves have been reported to reorientate from upright to prostrate in the fall, thereby reducing their exposure to cold, drying winds, yet still allowing photosynthesis to proceed (Noodén and Wagner 1997).
Conservation / Management
There are fragments of high quality mesic forests scattered throughout the southeastern counties of Minnesota. Only two of them are currently known to support populations of P. acrostichoides, though others may be colonized by wind-borne spores in the future, and some may already have spore banks in the soil which could develop into sporophyte colonies. Potential conflicts might arise between preserving colonies and habitat of P. acrostichoides and exploitative uses of these forests. Dramatic and long-term deleterious changes would likely result from livestock grazing, logging, and certain aspects of tree farming. Invasive species could present a more insidious threat, especially Rhamnus cathartica (common buckthorn), Alliaria petiolata (garlic mustard), and various species of non-native earthworm. It is no longer reasonable to assume that a forest fragment will be self-perpetuating, if simply left alone. Regular monitoring is necessary.
Best Time to Search
Because P. acrostichoides has wintergreen leaves, searches can be conducted any time of the year, except when snow is covering the ground.
Conservation Efforts in Minnesota
The Minnesota Biological Survey has been completed across the likely range of this species in the state.
Welby Smith, MN DNR, 1988, 2008, and 2017
Coffin, B., and L. Pfannmuller, editors. 1988. Minnesota's endangered flora and fauna. University of Minnesota Press, Minneapolis. 473 pp.
Cranfill, R. 1980. Ferns and fern allies of Kentucky. Kentucky Nature Preserves Commission, Frankfort, Kentucky. 284 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.
Minnesota County Biological Survey. 1994. Natural communities and rare species of Houston County. Biological Report No. 50. Minnesota Department of Natural Resources, St. Paul, Minnesota.
Minnesota County Biological Survey. 1994. Natural communities and rare species of Winona County. Biological Report No. 49. Minnesota Department of Natural Resources, St. Paul, Minnesota.
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.
Minnesota Department of Natural Resources, Division of Fish and Wildlife. 1995. Statement of need and reasonableness in the matter of proposed amendment of Minnesota Rules, Chapter 6134: endangered and threatened species. Minnesota Department of Natural Resources, St. Paul, Minnesota. 336 pp.
Minoletti, M. L., and R. E. J. Boerner. 1993. Seasonal photosynthesis, nitrogen and phosphorus dynamics, and resorption in the wintergreen fern Polystichum acrostichoides (Michx.) Schott. Bulletin of the Torrey Botanical Club 120(4):397-404.
Nood?n, L. D., and W. H. Wahner, Jr. 1997. Photosynthetic capacity and leaf reorientation in two wintergreen ferns, Polystichum acrostichoides and Dryopteris intermedia. American Fern Journal 87(4):143-149.
Ownbey, G. B., and T. Morley. 1991. Vascular plants of Minnesota: a checklist and atlas. University of Minnesota Press, Minneapolis, Minnesota. 320 pp.
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, D. H. 1993. Polystichum. Pages 290-299 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 2. Oxford University Press, New York, New York.