Basis for Listing

In Minnesota, Gymnocarpium robertianum (northern oak fern) occurs at a handful of locations in the northern counties (Laurentian Mixed Forest Province) and at two locations in the southeastern counties (Paleozoic Plateau Section). The records from northern Minnesota are from conifer swamps dominated by Picea mariana (black spruce) or Thuja occidentalis (northern white cedar). These tend to be swamps enriched by subsurface water, the supply of which is dependent on an intricate and somewhat delicate hydrological regime. Habitats that develop under this particular regime are much rarer than the more common bog forests, which are maintained primarily by surface waters.

The occurrences in southeastern Minnesota are on algific talus slopes. These habitats are small (less than one acre [< 1 ha] each) and quite rare. They are dependent on extensive karst formations and are believed to have very little resiliency in the face of habitat disturbance.

Both of the species’ habitat types are considered stable old-growth or climax communities that develop in the absence of disturbance. Given the small number of known populations, the species’ restricted habitat requirements, its absence in apparently suitable habitat, and the vulnerability of the known populations to degradation or destruction, Gymnocarpium robertianum was designated special concern in 2013.

  Description

Gymnocarpium robertianum is a small fern, 5-20 cm (2-8 in.) long, with a long thin black rhizome. The leaf blade (frond) is triangular in shape and 3-parted. The rachis (axis of the leaf) and stipe (stalk of the leaf) are minutely glandular. The sori are round and located near the margin of the frond; indusia are absent.

There are two similar species in Minnesota: G. continentale (Nahanni oak fern) and G. dryopteris (western oak fern). The easiest way to know you have G. robertianum is to look for glands on the surface of the leaf blade. Glands are found on both surfaces of the leaf of G. robertianum, only the lower surface of G. continentale, and nowhere on the leaf of G. dryopteris (Pryer 1993).

  Habitat

Populations in the southeastern portion of the state are on algific talus slopes. These are small unique habitats, where environmental conditions simulate those normally found in boreal habitats. These conditions develop when cold air drains from subterranean limestone karst formations and emerges through forested and north-facing talus slopes and associated rock outcrops. Northern populations of G. robertianum occur in conifer swamps of a type known to ecologists as forested rich peatlands. These are usually non-acidic forested wetlands, dominated by Thuja occidentalis (northern white cedar) and/or Picea mariana (black spruce), often within a hydrological setting where iron-rich springs or seeps are observed or groundwater is evidently moving just below the peat surface.

  Biology / Life History

Like all ferns, healthy populations of G. robertianum are comprised of two “types” of plants: sporophytes and gametophytes. The sporophytes are the familiar leafy stage, which can be easily counted and mapped. The gametophytes live entirely underground and cannot be easily detected. The gametophyte population might be analogous to a seed bank of angiosperms. It is important to consider this when trying to evaluate the extent or viability of a population because gametophytes may be just as important to population health as sporophytes.

The biology and life history of G. robertianum does not seem to differ greatly from that of other small ferns that occur in the same habitat. Yet clearly there is something that makes this species rare, while so many others are common. It may be a narrower tolerance of water chemistry parameters or perhaps a lesser ability to compete under prevailing conditions.

  Conservation / Management

Most habitats of G. robertianum are stable old-growth or climax forested wetland communities that develop in the absence of major disturbance and where the hydrology has not been altered. In other words, these habitats are not adapted to fire, large-mammal grazing, flooding, or other perturbations that are sometimes used to manage disturbance-dependent habitats. In essentially all cases, the appropriate management activity would be to leave the habitat alone. Small habitats, especially the algific talus slopes, need to have a significant buffer zone established around the core habitat. The size of the buffer would vary, depending on the position of the habitat within the landscape. Any sinkholes that might be feeding the algific system need to be included in the buffer. Allowable activities within such a buffer zone would need to be determined with great care.

  Best Time to Search

The presence of mature fully developed leaves are needed for positive identification of G. robertianum. This usually occurs between about June 1 and September 20.

  Conservation Efforts in Minnesota

Two of the three algific talus slopes where G. robertianum is known to occur are in public ownership (Wykoff Balsam Fir Scientific and Natural Area and Forestville/Mystery Cave State Park) and are being managed for purposes of conservation.

  Authors/Revisions

Welby Smith (MNDNR), 2018

(Note: all content ©MNDNR)

  References and Additional Information

Choberka, E. G., M. R. Penskar, and P. J. Higman. 2001. Special plant abstract for Gymnocarpium robertianum (limestone oak fern). Michigan Natural Features Inventory, Lansing, Michigan. 2 pp.

Minnesota Department of Natural Resources. 2003. Field guide to the native plant communities of Minnesota: the Laurentian mixed 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. 352 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.

NatureServe. 2009. NatureServe Explorer: An online encyclopedia of life [web application]. Version 7.1. NatureServe, Arlington, Virginia.

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.

Pryer, K. M. 1993. Gymnocarpium. Pages 258-262 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 2. Oxford University Press, New York, New York.

Schultz, J. 2002. Conservation assessment for limestone oak fern (Gymnocarpium robertianum). United States Forest Service, Eastern Region, Milwaukee, Wisconsin. 31 pp.

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