McDonald's rock-cress was federally listed as endangered without critical habitat in 1978. A recovery plan was published for the California populations in 1990.

Historical Status and Current Trends

McDonald's rock-cress is one of several closely related endemic species (species restricted to a well-defined geographic area) which have evolved in the Siskiyou Mountains region of southwest Oregon and northwest California. This species was not discovered in Oregon until 1980. It is an attractive plant, as are many of the endemic rock-cress species of the Siskiyou Mountains. Taxonomic studies are currently under way to investigate the relationship of the Oregon population to those in California.

Description and Life History

McDonald's rock-cress is s perennial species in the mustard family (Brassicaceae). This species has a branched caudex (short, vertical, often woody stem at or just beneath the ground surface) and several simple stems that measure 5 to 20 centimeters (2 to 8 inches) in height. The lower leaves are in rosettes (a cluster of leaves in a circle), are spatulate (rounded above and narrowed to the base), measure 1 to 2 centimeters (0.4 to 0.8 inches) long and 4 to 7 millimeters (0.2 to 0.3 inches) wide, are toothed, and are essentially smooth. The petals are rose or purple in color and measure 9 to 11 millimeters (0.35 to 0.43 inches) long. The fruits are siliques (elongate, dry, and open at maturity) that measure 3 to 4 centimeters (1.2 to 1.6 inches) long. Flowering typically occurs from late April through June.

This species is distinguished from other rock-cress species by being almost glabrous (without hairs or glands) and by possessing spatulate basal leaves 1 to 2 centimeters (0.4 to 0.8 inches) long.

Habitat

McDonald's rock-cress occurs on serpentine soils (high in magnesium, iron, and certain toxic metals). This species is found below 1500 meters (4920 feet) elevation in dry, open woods or brushy slopes, with sanicles (Sanicula spp.), violets (Viola spp.), and onions (Alium spp.).

Reasons for Decline

Mining activities and collection of specimens has contributed to the decline of this species.

Range

This species is currently cnsidered to be restricted to Curry and Josephine Counties in southwest Oregon on USDA Forest Service land and private land, and in adjacent Del Norte County, California. It has also been reported from Mendocino County, California.

References

U.S. Fish and Wildlife Service. 1978. Determination of Five Plants as Endangered species. Federal Register 43:44810-44811. https://ecos.fws.gov/docs/frdocs/1978/78-27314.pdf

U.S. Fish and Wildlife Service. 1990. McDonald's Rock-cress, (Arabis mcdonaldiana Eastwood), Recovery Plan. U.S. Fish and Wildlife Service, Portland, Oregon. 40 pp.