Draba arabisans Michx.
Arabian Whitlow Grass
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Draba arabisans var. superiorensis
Basis for Listing
In Minnesota, Draba arabisans relies on a rather specific type of cliff habitat that occurs in only limited amounts in two areas of the state. Rare plant surveys of potential habitats have been completed in all but a portion of the northeast, where searches are now in progress. Results to date reveal that even in what appears to be suitable habitat, D. arabisans is rarely present. The small cluster of sites in the southeast corner of the state are isolated from the main range of the species by approximately 483 km (300 mi.) and are believed to have special phytogeographical significance. On their own, the populations in Fillmore and Olmsted counties could warrant threatened or endangered status, but given the two ranges of D. arabisans in Minnesota, it was listed as a special concern species in 1984.
Draba arabisans is a perennial herbaceous plant and a member of the mustard family. The stems are 10-40 cm (3.9-15.7 in.) tall and may be branched or unbranched. There are both basal leaves and stem leaves, which are narrow and grow to about 6 cm (2.4 in.) in length. The margins of the leaves are often sharply toothed and the surfaces are uniformly stellate-hairy. The flowers are white and 4-6 mm (0.16-0.24 in.) long, and arranged in a loose, glabrous raceme that can reach 10 cm (3.9 in.) in length. The fruit is a somewhat flattened capsule 7-12 mm (0.28-0.47 in.) long and 1/4 or 1/5 as wide. The capsules are glabrous and lanceolate to narrowly oblong in shape, and they quickly develop a very noticeable and characteristic spiral twist. The twisted capsule is perhaps the best single field character, although D. cana, another rare cliff species, also has flattened and spirally twisted capsules. The fruits often change in color from green to yellow/tan as the season progresses.
In Minnesota, D. arabisans is found exclusively in association with cliffs. These are sheer bedrock exposures with substantial vertical faces. Of the numerous cliffs in the southeastern part of the state, the only ones that support populations of D. arabisans are the rare maderate cliffs (Minnesota Department of Natural Resources 2005). These limestone or dolomite cliffs face northeast to northwest with cool, moist microclimates maintained by a system of karst sinkholes and subterranean ice. The cliff habitats where D. arabisans is found in the northeast portion of the state are structurally similar to those in the southeast, although they are made up of quite different bedrock, and many of these tend to be drier and sunnier. The northern cliffs that support populations of D. arabisans occur primarily along the shore of Lake Superior on south to east-facing diabase, basalt, gabbro, and rhyolite bedrock. Several additional northern populations occur on inland cliffs near the Canadian border on north to west-facing shale, argillite, and grawacke of the Rove Formation.
Biology / Life History
Draba arabisans is a perennial herbaceous plant that produces insect-pollinated flowers. However, it has been reported that the flowers are often self-pollinated without the aid of insects (Mulligan and Findlay 1970). The seeds develop in a dehiscent capsule and are released when the capsule dries and twists open. The seeds do not posses any obvious structure that would indicate a specialized form of dispersal. It is likely that birds or other small animals play a role in the movement of seeds, although gravity and the movement of water and soil may also play a role. Plant species that specialize in cliff habitats are probably better adapted to environmental extremes than species in surrounding terrestrial communities. These extremes include rapid fluctuations in temperature and moisture, absence or paucity of soil in the rooting zone, and periodic dislocation of the actual substrate in which they root (Minnesota Department of Natural Resources 2003). By all indications, D. arabisans handles these conditions very well although specific adaptive strategies are not known.
Conservation / Management
It would seem that rare plant populations occurring on cliffs would need very little, if any, special conservation or management considerations. To a large extent that is true; however some potential threats have been identified by ecologists and land managers. Recreational rock climbing is gaining in popularity and although rock climbing is benign in many respects, it can conflict with conservation goals for the rare plant species that live in the crevices and small ledges of certain cliffs.
Conservation Efforts in Minnesota
Draba arabisans is known to occur on publicly owned lands at several locations in Minnesota. Management responsibilities for these sites are spread over several agencies, including the Minnesota Department of Natural Resources, Minnesota Department of Transportation, the U.S. Forest Service, and at least two municipalities. At a few of these sites management issues relating to biological conservation have been addressed, but outcomes have not been assessed.
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.
Mulligan, G. A., and J. N. Findlay. 1970. Sexual reproduction and agamospermy in the genus Draba. Canadian Journal of Botany 48:269-270.