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McDonald's rockcress
Arabis macdonaldiana

General Information

Official Status: Endangered, the McDonald’s rockcress is federally listed under the Endangered Species Act as Endangered, and is listed Endangered by the State of California.

Date Listed: September 28, 1978; Federal Register 43 FR 44810.

Critical Habitat: Critical habitat has not been designated for McDonald’s rockcress.

Recovery Plan: The Recovery Plan for McDonald’s rockcress (PDF, 2.8 MB) was approved in February 1984.

5-Year Review: A 5-Year Review (PDF, 3.1 MB) was finalized in 2013.

  • McDonald’s rockcress was the second plant listed as Endangered under the Endangered Species Act. 

McDonald's rockcress, Photo Credit: Dave Imper, USFWS

McDonald's rockcress

Photo Credit: Dave Imper, USFWS

arrow button Photo Gallery for McDonald's rockcress

Identifying Characteristics:

McDonald’s rockcress was discovered in northern Mendocino County, California in 1902, and described as a distinct species the following year. Its discoverer was Alice Eastwood, one of the earliest and most well known female botanists.  McDonald’s rockcress is a perennial herbaceous member of the mustard family (Brassicaceae), characterized by relatively large, very stiking lavender or crimson-purple flowers and a deep green rosette, usually flattened, composed of broadly oblanceoate or spatulate leaves from which the flowering stems arise.  The species is distinguished from other rock-cress species by being almost glabrous (without hairs or glands) with the spatulate basal leaves toothed, 0.4-0.8 inches long.

Current Geographic Range:

Until 1979, it was believed that McDonald’s rockcress was restricted to Red Mountain, in Mendocino County.  In 1980 the taxonomy defining the species was revised to include populations of purple flowered rockcress located in Del Norte County, California and Josephine and Curry Counties, Oregon.

McDonald’s rockcress is currently considered to be restricted to Mendocino and Del Norte Counties, the very west portion of Siskiyou County in California, and the southern extent of Curry and Josephine Counties in southwest Oregon.  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.
Life History:

Individuals of Mcdonald’s rockcress may reach 25-50 years old, based on an average 0.5-1 centimeter growth per year.  Plants often do not flower and fruit every year.  McDonald’s rockcress typically flowers from late April through June.  Up to 12 elongate, dry fruit called siliques, may be produced, from which very small, slightly winged seed are discharged.  Some long term monitoring data indicate a relative stable population density and cover, while data collected for at least one stand at Red Mountain, Mendocino County, document a decline in response to encroachment by knobcobe pine in absence of fire.  It is not known if McDonald’s rockcress thrives on the shallow, serpentine soils because of the limited competition that those harsh soils support, or if due to an adaptation to the soil chemistry of the freshly exposed serpentine soil.

General Habitat Characteristics:

McDonald’s rockcress is restricted to soils derived from ultramafic rocks, chiefly peridotite.  Soils may range from recently exposed serpentine to very old weathered lateritic soils.  A pronounced red color is often evident in the lateritic soils because of the abundance of iron.  These soils are also high in heavy metals such as copper, chromium and nickel.  The habitat is often very steep and unstable, with an open tree canopy of generally less than 5 percent cover.  Elevation ranges up to about 4,900 feet on the slopes of Preston Peak and Sanger Peak in the Siskiyou Mountains.  Vegetation association ranges from dry Jeffrey Pine, knobcone pine, or incense cedar woodlands to brushy or very open, rocky scree slopes.  In addition to scattered trees, associated vegetation includes a diverse array of herbs and shrubs, such as montane penny-cress, Bolander’s lily, and multiple species of buckbrush, fescue grass, iris, snakeroot, lomatium, stonecrop, violet, phlox, onion, and others.  Serpentine barren habitats in general often support a great variety of endemic plants, many of which are sensitive or rare. 

Population and Habitat Status:

No population estimate has been made for the Red Mountain portion of the distribution.  The Del Norte populations were estimated to include in excess of twenty thousand individuals in the late 1980s.  While habitat for McDonald’s rockcress is scattered across 5 counties and more than 100 miles from one end to the other, the actual occupied habitat is relatively small.  Based on mapping conducted in 2003 at Red Mountain, in Mendocino County, the amount of occupied habitat there was estimated as less than 30 acres.  Based on habitat mapping conducted in the 1980s, occupied habitat in Del Norte County, California and Curry and Josephine Counties, Oregon likely exceeds 1,000 acres.  However, no estimate of plant density was made within the mapped areas.  Much, if not the majority, of the habitat occupied by McDonald’s rockcress remains covered under existing mining claims, although most are not proven or active.  

Threats:

At the time of listing, the only known population of McDonald’s rockcress at Red Mountain, in Mendocino County, was threatened by proposed nickel mining.  While the revised taxonomy has increased the amount of occupied habitat and overall population of McDonald’s rockcress, the habitat occupied by the species across its range will always remain vulnerable to mining due to the abundance of heavy metals in the soils.  Another potential threat that has not been well characterized is the progressive encroachment into its habitat by knobcone pine and other serpentine-tolerant species, in the absence of fire.  Off-road vehicle recreation and overcollection has impacted McDonald’s rockcress in some areas.

Conservation Needs:

Only a small portion of the distribution of McDonald’s rockcress is located within wilderness, thus currently off-limits from mining.  The long-term conservation of this species will require that a portion of the range, representative of the species with respect to genetic and habitat diversity, be set aside in permanently protected areas, or subject to permanent mining withdrawals.  In addition, further research is needed on the rate at which habitat for McDonald’s rockcress is becoming unsuitable as a result of encroachment by competing shrubs and trees, as a result of fire suppression and forest succession.  Prescribed fire may be an integral element in the long-term management regime for this species.

Related Documents:

No other documents other than those listed in the general information section above.

Other Informational Weblinks:

Last updated: August 8, 2013

Arcata Fish and Wildlife Office, 1655 Heindon Road, Arcata, California 95521, USA
Tel: (707) 822.7201 Fax: (707) 822.8411