Pellaea atropurpurea (L.) Link
Purple Cliff Brake
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Basis for Listing
As of 2008, approximately 17 populations of Pellaea atropurpurea had been documented in four southeastern Minnesota counties. In that region, the species is confined to a very narrow and specific range of prairie habitats that occur on steep, south-facing, sandstone exposures. These habitats are now isolated from important ecosystem processes, particularly fire, that are needed to sustain them. Without these processes, the habitats need intensive management or they can essentially vanish in only a few decades. Given the rarity of P. atropurpurea and the vulnerability of its habitat, it was listed as a special concern species in Minnesota in 1984.
Pellaea atropurpurea is a small, distinctive-looking rock plant. The leaves are clustered on a short rhizome, 5-50 cm (2.0-19.7 in.) long, and 2-18 cm (0.8-7.1 in.) wide. The sterile leaves are somewhat shorter and less divided than the fertile leaves. The petiole and rachis are reddish purple to nearly black, lustrous, and densely hairy, especially on the upper surface with short, curly, appressed hairs. The leaf blade is elongate-deltate in outline, and usually 2-pinnate near the base. The smallest segments of the leaf (the pinnae) are arranged perpendicular to the rachis or ascending, and usually number 3-15. The segments are linear-oblong in shape, 1-7.5 cm (0.4-3.0 in.) long and no more than half as wide, leathery in texture, and sparsely hairy near the mid-rib on the lower surface. The margins of the segments are weakly recurved or flat on fertile segments, the borders are whitish and wavy, and the tip is obtuse to slightly mucronate. The more common P. glabella also occurs in Minnesota, and may occur in the same habitat as the rare P. atropurpurea. It looks like a somewhat smaller version of P. atropurpurea with a smooth or only slightly hairy petiole and rachis.
Pellaea atropurpurea is found in steep, south facing, bluff prairies in the southeastern corner of the state. These are small, short grass-dominated habitats within a matrix of deciduous forest. Within these habitats, P. atropurpurea occurs in crevices of sandstone and dolomite outcrops or in thin, sandy/rocky soil over bedrock. The growing conditions are characteristically hot, dry, and droughty.
Biology / Life History
Pellaea atropurpurea is a perennial fern of dry, sunny, rocky habitats. The leaves are produced from a short rhizome that is usually wedged into a small crevice in a sandstone or dolomite outcrop. The species can also be found growing on large, bouldery rocks that have come loose or fallen from cliffs, and on actual cliff faces. The leathery leaves and the wiry stem resist desiccation.
Conservation / Management
Management of the habitats where P. atropurpurea occurs requires immediate attention. The sites are in small prairie openings (often only 0.8-1.2 ha (2-3 ac.) in size) in oak forests that are kept open by the combination of shallow soil, south-facing exposure, and wildfire. The wildfire part of the equation is now missing (a casualty of habitat fragmentation and deliberate fire suppression policies), and the habitats are suffering as a consequence. Woody plants, especially aggressive colonizers such as Juniperus virginiana (eastern red cedar) and Rhus glabra (smooth sumac), can quickly take over the habitat. Controlled burns can, in some cases, restore open conditions to habitats that have become overgrown, but manual cutting of brush and tree saplings may need to precede a burn. Once open conditions have been restored, fire alone may be enough to control brush. It must be stated that P. atropurpurea itself is very vulnerable to fire, and it will not resprout. Fire serves only to maintain the open nature of the habitat, and it will normally burn through the grass and shrubs but go around the exposed rocks where P. atropurpurea occurs. This is labor-intensive management but it is necessary, and if timed and executed carefully, it will benefit a number of prairie species, both plant and animal.
Conservation Efforts in Minnesota
No known conservation efforts have been directed towards this species in Minnesota.