Braun's Rock-cress

Arabis perstellata

Description

Braun's rock-cress, Arabis perstellata, a perennial member of the mustard family (Brassicaceae or Cruciferae), is comprised of small rock cress and large rock cress subspecies that occupy distinct geographic areas in Kentucky and Tennessee, respectively. E. L. Braun described this taxon in 1940 from plants collected on wooded hillsides along Elkhorn Creek in Franklin County, Kentucky, distinguishing it from similar species in the area. Current taxonomy recognizes his distinction between A. shortii and A. perstellata. A subspecies of A. perstellata was found in 1959 by R. B. Channel on steep limestone cliffs above the Stones River in Davidson County, Tennessee. He distinguished this subspecies from the already known variety— A. perstellata var. perstellata, commonly called the small rock cress—by its generally larger size, thinner and more entire leaves, and lesser pubescence. The Tennessee sub-species, classified as A. perstellata var. ampla, is commonly called large rock cress.

Both varieties have round stems and alternate leaves. Their stems and foliage have a grayish coloration due to the large quantity of hairs. Their stems arise from horizontal bases and grow up to 31.5 in (80 cm) long, often drooping from rock ledges. Each year a basal rosette of leaves is produced, and new flowering branches emerge from the old rosette of the previous season. Their lower leaves are 1.6-6.0 in (4-15 cm) in length and are obovate to oblanceolate with slightly toothed and pinnatifid margins. Their upper leaves are smaller—up to 1.4 in (3.6 cm) long—and are elliptic to oblanceolate, with coarse teeth along the margin. Both surfaces of their leaves are stellate-pubescent. The inflorescence is an elongated raceme with numerous flowers. Their flowers have four petals that are 0.12-0.16 in (3-4 mm) long, are white to lavender, and have four pale green sepals that are 0.08-0.12 in (2-3 mm) long. There are six stamens, with two shorter than the other four. The ovary is elongate, two-chambered, and develops into a silique. Fruiting stalks are about 0.4 in (1 mm) long at maturity; siliques are up to 1.6 in (4 cm) long and are covered with both simple and stellate hairs. Flowering is from late March to early May. Fruits mature from mid-May to early June. Their oblong seeds are reddish-brown, somewhat flattened, about 0.04 in (1 mm) long, and, in places, minutely hairy.

Habitat

A. perstellata is typically found on wooded steep slopes with limestone outcrops. The outcrops tend to be moist but not wet; rarely, plants can be found on seepy outcrops. They also may be found in protected areas, such as around the bases of larger trees, or in areas where there is little competition, such as around areas regularly scoured by talus movement or erosion. The plants have a well-developed system of rootstocks that allow them to persist in these inhospitable sites. Sometimes plants display a weedy tendency, colonizing recent road cuts or animal paths through the woodlands. The plants survive in full shade or filtered light, but are not found in full sunlight.

Distribution

In the late twentieth century, the small rock cress was known from 27 populations in Kentucky—24 in Franklin County, two in Owen County, and one in Henry County—and its distribution showed a strong correlation with the Kentucky River and its tributaries, primarily Elkhorn Creek. No sites have been found south of Frankfort along the Kentucky River, although appropriate habitat appears to be present. The 1991 status survey revealed that eight historical sites had been extirpated, but it also found eight new populations. Of the 27 populations, 10 had less than 100 individual plants and 12 had 20 or less.

The large rock cress was known in the late twentieth century from only two populations in Rutherford County, Tennessee. This subspecies was historically associated with calcareous bluff habitat overlooking the Stones River. The known populations occurred on rocky knobs about 15 mi (24 km) from that river, a somewhat anomalous site compared to historic ones. Prior to the 1991 status survey, there were three records of large rock cress in Davidson County and two in Rutherford County. All three of the sites in Davidson County have been extirpated, and one of the sites in Rutherford County could not be relocated, though one additional population was discovered there during the status survey. The smaller of the two remaining populations had only 25 plants in an area covering about 0.06 acre (0.02 hectare). The other population contained several hundred plants scattered over about 2.2 acres (0.9 hectare). Both sites are on private land and are threatened from competition by weedy invaders.

Threats

Both rock-cress subspecies are endangered throughout their range due to habitat alteration; residential, commercial, or industrial development; timber harvesting; livestock grazing and trampling; and competition with native and exotic weedy species, especially the European garlic mustard (Alliaria petiolata ). The immediate threats to the 27 remaining populations of the small rock cress can be identified as follows: eight are threatened by weedy competitors, four by weedy competitors and trampling, two by trampling, two by road work, and one by logging. A. perstellata does not survive in full sunlight, and the removal of trees through timber harvesting in areas where this species occurs would likely have severe consequences. Plants are also at risk from destruction by heavy logging equipment. One of the largest populations was severely damaged by roadwork while the species was proposed for listing. The remaining 10 populations do not appear to have any immediate threats, but they are vulnerable to the threats listed above, as well as other habitat alterations and potential inbreeding problems as neighboring populations decline. All of the Kentucky populations are on privately owned land. Three receive limited protection through their inclusion in state-designated natural areas.

Both of the remaining large rock cress populations in Tennessee are threatened from competition by weedy invaders and potentially by livestock grazing and trampling. The smaller site also appears to be made up of older individuals, and there is little evidence of reproduction.

Most populations of this species are very small and cannot support the collection of plants for scientific or other purposes. Inappropriate collecting for scientific purposes or as a novelty could be a threat to the species.

Conservation and Recovery

Active management is required to ensure that a species so few in numbers and so narrowly distributed continues to survive at all sites; more positive measures are needed than benign neglect.

Kentucky state law provides no protection for plants. In Tennessee, A. perstellata is protected under the Rare Plant Protection and Conservation Act of 1985, which controls the removal of plants from state properties for scientific, educational, or propagative purposes, and the disturbance of the species on private lands without the landowner's consent.

Contacts

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1875 Century Blvd., Suite 200
Atlanta, Georgia 30345
http://southeast.fws.gov/

U. S. Fish and Wildlife Service
Asheville Ecological Services Field Office
160 Zillicoa St.
Asheville, North Carolina 28801-1082
Telephone: (828) 258-3939
Fax: (828) 258-5330

References

Braun, E. L. 1940. "New Plants from Kentucky." Rhodora 42: 47-49.

Collins, J. L., et al. 1978. "The Rare Vascular Plants of Tennessee." Journal of the Tennessee Academy of Science 53: 128-133.

Jones, R. L. 1991. "Status Survey Report on Arabis perstellata var. perstellata. " Unpublished report to the Asheville Field Office, U. S. Fish and Wildlife Service, Asheville, N.C. 94 pp.

Medley, M. E., and J. M. Baskin. 1986. "Endangered, Threatened, and Rare Plants and Animals of Kentucky." Transactions of the Kentucky Academy of Science 47: 84-97.

U. S. Fish and Wildlife Service. 1997. "Recovery Plan for Arabis perstellata Braun (Braun's Rockcress)." U. S. Fish and Wildlife Service, Atlanta. 21 pp.