Best Survey Period State Distribution

citation: 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, MI. 2 pp. fern and lacks the pronounced, extended tips of the main leaf segments so evident in well-developed fronds of G. robertianum. Best survey time/phenology: Gymnocarpium robertianum is recognizable whenever its foliage is present and well-developed, conservatively from about mid-June through leaf senescence. July and August are likely the best periods to seek and reliably identify this species. Habitat: Limestone oak fern inhabits cool, rocky woods and swamps. In northern white cedar swamps, the principal habitat for this species in Michigan, it grows in cool, wet substrates, particularly in organic soils and often in areas of thick moss cover, including Sphagnum mosses. As indicated by the common name, this fern prefers alkaline or calcium-rich substrates, and it can also be found colonizing limestone or dolomite ledges and cliffs. Biology: Spores (sori) are produced from June through August. The frond of this species is deciduous. Conservation/management: To conserve this species, it is necessary to maintain its habitat. This includes maintenance of the hydrologic cycle of northern white cedar swamps as well as providing an intact canopy. The effects of timber harvesting in cedar swamps is poorly known, but it is expected that this species could experience habitat degradation through excessive clearcutting. The cycle of beaver activity within swamps is a natural disturbance feature that undoubtedly affects populations as well. Ledges and cliffs that contain limestone oak fern should not be disturbed, and are likely vulnerable to recreational activities such as rock climbing. Comments: Hybrids between Gymnocarpium robertianum and other oak fern species are very rare, according to Pryer (1993). W.H. Wagner (1966) described G. X heterosporum as a hybrid between G. appalachianum and G. robertianum. In 1981, Sarvela described a hybrid known only from Sweden, G. X achriosporum. This is a hybrid between G. dryopteris Funding for abstracts provided by Michigan Department of Natural Resources-Forest Management Division and Wildlife Division. Copyright 2004 Michigan State University Board of Trustees. Michigan State University Extension is an affirmative-action, equalopportunity organization.