|Listed||September 28, 1978|
|Description||Perennial with purple to pink flowers and a basal rosette of spatula-shaped leaves.|
|Habitat||Serpentine soils in yellow pine forests.|
The perennial McDonald's rock-cress, Arabis mcdonaldiana, grows from a woody base and forms several smooth, unbranched stems. Size depends on variations of moisture in the soil. Individuals range in size from small, stunted plants about 3 in (7.6 cm) high to plants in deeper soil that can grow to a height of 30 in (76.2 cm). Spatula-shaped evergreen leaves are grouped in a rosette at the base of the plant. Leaves along the stems are oblong. Between two and 20 purple to pink flowers grow along the stems, depending on plant size. Blooming is from late March to late June.
McDonald's rock-cress grows in full sun in serpentine soils 3,000-4,000 ft (914.4-1,219.2 m) in elevation in yellow pine forests on Red Mountain, California. Most of the populations occupy open habitats comprised of tree slopes, rocky ridges, and barren rock outcrops. They usually grow on west-facing slopes that are cleared of snow sooner than other parts of the mountain, although some populations, which are the densest, occur in habitats with north and east exposures or in sheltered saddles. This rock-cress does not like competing vegetation and is highly adapted to fire. Areas burned over during a 1985 fire have produced the most numerous and robust populations. Plants grown in greenhouses and introduced to the site are often eaten by the same rodents that shun wild plants, suggesting that wild plants may take in minerals from the soil that make them less palatable to herbivores. The red soils of the region are rich in nickel and chromium.
Red Mountain (Mendocino County), California, is the only known location of this species, which is estimated to cover an area of 3.5 sq mi (9.1 sq km) of highlands. About 21 sites are known, consisting of populations as small as 10 to as large as 1,000 individuals. A 1988 survey estimated the total number at roughly 10,000 plants. A taxonomically similar species is found to the north in nearby Del Norte and Curry counties, Oregon. This near-relative has not been fully described.
This species is found largely on land administered by the Bureau of Land Management (BLM), but several sites are on private property. As the soil is rich in nickel and chromium, the region has been heavily mined. When the species was listed as endangered, population sites were threatened by a mining company that operated claims over much of the area. The company has since suspended active mining and sold its holdings on Red Mountain to a silvi-culture (forest development) firm. While tree planting in the rock-cress colonies is not expected, plants could be adversely affected by windblown herbicides, and usage will need to be carefully monitored.
The most vigorous populations of McDonald's rock-cress occur in relatively deep soil in a sheltered saddle between two ridges. After the fire in 1985, the knob cone pine became a dominant plant in the habitat and is slowly shading the area, which the rock-cress will eventually find intolerable.
Conservation and Recovery
The cessation of active mining removed the most immediate threat to the survival of McDonald's rock-cress. The population is stable, well-adapted, and not immediately in danger of extinction. In 1984 the BLM, in conjunction with the U. S. Fish and Wildlife Service, began a 10-year project to survey population sites and to study the biology of the rock-cress. BLM land on Red Mountain is under consideration for designation as a wilderness area.
In 1985 M. F. Baad conducted a three-year study of the 19 Red Mountain populations. Each was unique with respect to density, size, and reproductive classes. Some apparently reproductive populations did not set seed before predation by rodents; others did not flower during the course of the study; others were reproductively successful. In spite of the fact that seed and seedling survival appear to be the weakest link in the life cycle, the overall population increased slightly during the course of the study and seems to be maintaining stable densities. Although the species is slow growing, it is also long lived, with large individuals in excess of 25 years old.
U.S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N.E. 11th Ave.
Portland, Oregon 97232-4181
Baad, M. 1987. "Geographic Distribution of Rare Plants on Public Lands within the Red Mountain Area, and a Study of the Population Dynamics and the Reproductive Biology of Arabis mcdonaldiana. " Bureau of Land Management and the U. S. Fish and Wildlife Service, Sacramento.
Knight, W., and I. Knight. 1971. "A Botanical Glimpse of Red Mountain." Four Seasons—Journal of the East Bay Regional Parks District 4 (1).
Raven, P. H. 1977. "The California Flora." In Terrestrial Vegetation of California. Edited by M. Barbour and J. Major. Wiley-Interscience.
Rollins, R. 1973. "Purple Flowered Arabis of the Pacific Coast of North America." Contributions of the Gray Herbarium 204: 149-154.
U. S. Fish and Wildlife Service. 1985. "Recovery Plan for McDonald's Rock-cress, Arabis mcdonaldiana, an Endangered Plant." U. S. Fish and Wildlife Service, Portland.