Rare Species Guide

  Synonyms

Athyrium pycnocarpon, Diplazium pycnocarpon

  Basis for Listing

This woodland fern reaches the northwestern limit of its range in Minnesota, where it is confined to wooded bluffs in deep stream valleys in the southeastern corner of the state. Field surveys have found Homalosorus pycnocarpos (narrow-leaved spleenwort) absent from large areas of apparently suitable habitat, especially habitats that show a history of human exploitation. The habitats tend to be very fragile and are vulnerable to damage from a variety of human activities, especially activities that lead to erosion, soil compaction, canopy removal, or consequences of livestock grazing. Homalosorus pycnocarpos was originally listed as a special concern species in Minnesota in 1984, but given its rarity and threats to its habitat, it was reclassified as threatened in 1996.

  Description

Homalosorus pycnocarpos has long-lived subterranean stems (rhizomes) that are clothed in the persistent bases of petioles from previous years. The rhizome will produce several leaves each year, some reaching one meter in length. The blade of each leaf is oblong-lanceolate in outline and gradually tapered to an acutely pointed apex. The structure of the blade is 1-pinnate, meaning it has one series of divisions. Each leaf has 20-30 pair of pinnae (leaflets). The pinnae are linear in shape with pointed tips. The margins of the pinnae are nearly entire to shallowly scalloped. The veins are pinnate and fork once or twice. Sori occur on the undersides of the pinnae. They are elongate, straight, and attached to a vein.

  Habitat

In Minnesota, Homalosorus pycnocarpos is a forest species found on north- and northeast-facing bluffs in old-growth maple-basswood forests. It is most often found near the base of the slope in rich organic loam near a creek, a stream, or a groundwater seep. Homalosorus pycnocarpos is commonly associated with such notable ferns as Deparia acrostichoides (silvery spleenwort), Dryopteris goldieana (Goldie's fern), and other plants considered rare in Minnesota. Although H. pycnocarpos is a very rare species, and most populations consist of few individuals, some populations may comprise 100 or more individual plants.

  Biology / Life History

Each plant sends up several leaves from the rhizome each year. The sterile leaves emerge first, in mid- to late May in Minnesota. This is later than many of the herbaceous species in its habitat. The purpose of the fertile leaves is to conduct photosynthesis, they have no role in reproduction. The fertile fronds are produced from the same rhizome in mid-summer. These are often somewhat longer than the sterile fronds, standing up to 1 m (39 in.) or more in height. The fertile leaves produce spores in sporangia that are found on the underside of the leaf. The spores are very small and light and easily carried on wind currents. All the leaves are deciduous and die back to the ground before winter.

  Conservation / Management

Homalosorus pycnocarpos seems to be absent from large areas of apparently suitable habitat. Because it is distinctive and conspicuous, it is unlikely that surveys in occupied habitat would have failed to find it. Known occurrences are limited to fragile habitats in deep stream valleys in the Paleozoic Plateau or "Driftless Area".  Homalosorus pycnocarpos was probably not common even at the time of human settlement, and habitat changes since human settlement have likely reduced its populations. At the present time, the best way to preserve this rare species is to maintain and protect the forest habitat where it occurs. Special consideration should be given in forest management, as even careful, selective logging could potentially alter the habitat if canopy openings increased light and temperature conditions on the forest floor. This would tend to dry and warm the soil, which could disrupt competitive relationships of plants and shift species composition. Any land uses employing heavy machinery may potentially damage the soil, especially on slopes. Livestock grazing could also degrade H. pycnocarpos habitats and directly injure plants.

  Best Time to Search

The best time to search for Homalosorus pycnocarpos is from late May through August, when the leaves are fully developed.

  Conservation Efforts in Minnesota

Several H. pycnocarpos populations occur in State Parks, where they are protected from most threats. Other populations are in State Forests and Wildlife Management Areas, where resource management plans should incorporate habitat protection needs for this species.

  Authors/Revisions

Welby R. Smith (MNDNR), 2021

(Note: all content ©MNDNR)

  References and Additional Information

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.

Kato, M. 1993. Diplazium. Pages 252-253 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 2. Oxford University Press, New York, New York.

Ownbey, G. B., and T. Morley. 1991. Vascular plants of Minnesota: a checklist and atlas. University of Minnesota Press, Minneapolis. 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 No. 53, Milwaukee, Wisconsin. 140 pp.

PPG I. 2016. A community-derived classification for extant lycophytes and ferns. Journal of Systematics and Evolution 54(6): 563-603.

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

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