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For information on Wisconsin's rare plants, contact:
Kevin Doyle
608-416-3377

Limestone Oak Fern (Gymnocarpium robertianum)

Life history

Species overview

Limestone Oak Fern (Gymnocarpium robertianum), a Wisconsin Special Concern plant, is found on moist shaded dolomite or, less commonly, sandstone cliffs. Elsewhere in the region, it is found in dense, mature northern white-cedar swamps. The optimal identification period for this species is late May through late September.

Synonyms: Gymnocarpium dryopteris var. pumilum, Dryopteris robertiana, Phegopteris robertiana, Polypodium robertianum, Thelypteris robertiana

Identification

  • Distinguishing characteristics: Leaves and rachis densely and finely glandular; stipe, rachis, and axes with many very small, stalked, capitate, white glands; suprabasal pinna pair usually divided more like the basal pinnae than the distal ones; stalks of the basal pinnae 0.5 to 1 cm long. Distinguished from G. dryopteris by pubescence and the leaves with the terminal third longer the than the two triangular basal pinnae, so the fronds appear less like an equilateral triangle.
  • Flower characteristics:
  • Fruit characteristics: Sori on the anterior vein-branches, round, submarginal; no indusium.
  • Leaf characteristics: Blade green, densely and finely glandular, deltoid, to 14 cm long by 16 cm wide, terminal third typically longer the than the two triangular basal pinnae; basal pinnae segments ovate-oblong, obtuse, sessile, the lowest basal one mostly less than a fourth as long as the main rachis; ultimate segments to 9 mm, with entire to crenate, mostly recurved margins.

Phenology

  • Blooming phenology:
  • Fruiting phenology: June through August
  • Optimum time to identify: The optimal identification period for this species is late May through late September

Other

  • Growth form: Fern
  • Vegetative reproduction: Rhizomatous
  • Life cycle: Perennial
  • Comments: Associated Species: Thuja occidentalis, Acer spicatum, Draba arabisans, Zigadenus glaucus, Cystopteris bulbifera, Adiantum pedatum, Athyrium felix-femina, Cornus canadensis.

State status

Status and Natural Heritage Inventory documented occurrences in Wisconsin

The table below provides information about the protected status - state and federal - and the rank (S and G Ranks) for Limestone Oak Fern (Gymnocarpium robertianum). See the Working List Key for more information about abbreviations. Counties shaded blue have documented occurrences for this species in the Wisconsin Natural Heritage Inventory database. The map is provided as a general reference of where this species has been found to date and is not meant as a range map.

Documented locations of Gymnocarpium robertianum in the Natural Heritage Inventory Database as of July 2015.


Summary Information
State StatusSC
Federal Status in Wisconsinnone
State RankS1S2
Global RankG5
Tracked by NHIY

Habitats and landscapes

The Natural Heritage Inventory has developed scores indicating the degree to which each of Wisconsin's rare plant species is associated with a particular natural community or ecological landscape. This information is similar to that found in the Wildlife Action Plan for animals. As this is a work in progress, we welcome your suggestions and feedback.

General habitat information

  • Habitat description: Found on moist shaded dolomite or, less commonly, sandstone cliffs. Elsewhere in the region, it is found in dense, mature northern white-cedar swamps.
  • Soils: Moist, calcareous, rocky soils.

Natural communities

This table lists the natural communities that are associated with Limestone Oak Fern. Scores for natural community associations are: "significant" association (score=3), "moderate association" (score=2) or the species can be present but is only weakly associated with the community (score=1).

Natural communities score
Northern Wet-mesic Forest 3
Moist Cliff 3

Ecological landscapes

This table lists the ecological landscape association scores for Limestone Oak Fern. The scores (3=High, 2=Moderate, 1=Low, 0=None) also correspond to the map.

Map of the Ecological Landscapes of Wisconsin.

Species guidance

The Endangered Resources Program has developed avoidance measures and management guidelines for plants on the Natural Heritage Working List. These are a work in progress, and we welcome your suggestions and feedback. Sources used in developing this information can be found here.

Avoidance measures

These are specific actions designed to avoid "take" (mortality) of this species.

  • Avoid broadcast spraying of herbicides; use care with spot spraying.
  • Avoid known individual plant locations and conduct operations elsewhere when they are least likely to cause damage. Ideally, this would involve frozen, snow-covered ground. However, in areas of the state where frozen conditions are unreliable, very dry soils late in the growing season might be the best available alternative. Consult with a biologist, if needed.
  • Avoid direct disturbance to sensitive microsites such as seeps, cliffs, and moss-covered boulders.

Management guidance

Management guidelines are additional considerations that may help maintain or enhance habitat for this species

  • Minimize disturbance to hydrology, including soil disturbance from rutting.
  • Avoid rapid and dramatic reductions in canopy cover or basal area in wet areas to reduce risk of swamping.
  • Maintain high forest canopy cover; this species requires shaded habitat conditions.
  • Follow BMPs, especially around streams and use care near ravines, steep slopes, cliffs, rock outcrops, etc.
  • Buffer management around unique microhabitats such as ephemeral ponds, seeps, etc.

Photos


Limestone Oak Fern  Photo.

The lowest pair of pinnae on limestone oak fern are not as large as those on common oak fern (G. dryopteris), giving it less of a three-parted appearance.

Photo © Mary Ann Feist.

Limestone Oak Fern  Photo.

Limestone oak fern has hairs along the rachis, which are visible to the naked eye.

Photo © Mary Ann Feist.

Limestone Oak Fern  Photo.

Photo © Chris Noll.

Limestone Oak Fern  Photo.

Photo by Thomas Meyer, Wisconsin DNR.

Limestone Oak Fern  Photo.

Photo © Mary Ann Feist.


Support for Wisconsin's rare plant information has been provided by the Division of Forestry, the Endangered Resources Fund and the Wisconsin Rare Plant Preservation Fund. To donate, visit the Natural Resources Foundation of Wisconsin [exit DNR].

Last revised: Friday, August 10, 2018
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