Rare Species Guide

  Synonyms

Arabis holboellii var. consanguinea, Boechera retrofracta, Arabis holboellii var. tenuis, Arabis retrofracta, Arabis holboellii var. retrofracta

  Basis for Listing

There are two small population clusters of Boechera collinsii (Collins’ rock cress) in Minnesota. They appear to be at the eastern edge of the range of the species which is centered in the northern Great Plains. One is in the northeast corner of the state (Border Lakes and North Shore Highlands subsections), where it occurs on cliff habitats; the other is in the northwest corner of the state (Aspen Parklands Subsection), where it occurs on sand dunes. The habitats are so dissimilar and the populations so far apart that it has been thought they may actually be two different species, which is not a simple question to answer. There are more than 100 species of Boechera in North America (Al-Shehbaz, and Windham 2010), and the taxonomy is very complex, so it is difficult to be certain. Even taken as a single species, it is very rare and deserving of threatened status. The northwestern population may even be endangered. It is localized in a small sand dune system, which is a very rare and vulnerable habitat in Minnesota. The plants in the northeast are perhaps more secure since their habitat is associated with bedrock cliffs, which are not intensively exploited. However, even in the northeast it is very rare with only scattered individuals. For these reasons, B. collinsii was listed as a threatened species in Minnesota in 1984.

  Description

Boechera collinsii is reported to be a perennial, but specimens from Minnesota appear to be biennial. The first year, it produces only a basal rosette of simple leaves. Each leaf is oblanceolate in shape with entire or dentate margins and a ciliate petiole. The second year, it produces a single unbranched flowering stem about 50 cm (20 in.) tall. The stem is covered with simple and 2-rayed hairs and small clasping leaves. Small white flowers are produced on the upper portion of the stem and tend to hang downward. When the flowers are pollinated, they develop into long slender smooth seed pods (siliques) that hang straight down along the stem (Al-Shehbaz and Windham 2010).

  Habitat

The populations in northeast Minnesota occur in crevices on dry north- and east-facing diabase cliffs. In these northeastern counties, B. collinsii may be associated with the rare Woodsia scopulina ssp. laurentiana (Rocky Mountain woodsia) and Asplenium trichomanes ssp. trichomanes (maidenhair spleenwort). The Kittson County populations occur primarily in dry prairie and sand dune habitat where they are associated with typical xerophytes such as Juniperus horizontalis (creeping juniper), Orthocarpus luteus (owl clover), and Hedyotis longifolia (bluets). Boechera collinsii has also been found in a bur oak savanna.

  Biology / Life History

Very little is known about the specific biology of Boechera collinsii. It is a biennial or short-lived perennial. The flowers are presumably pollinated by small flying insects, though the species involved are largely unknown. The seeds are small and appear to be dispersed by gravity and possibly small animals, including insects and mammals, which may cache seeds, and potentially by ground foraging birds.

  Conservation / Management

At least one known B. collinsii population may be in danger from rock-climbing activities, but other threats to cliff-dwelling populations may be minimal. Two occurrences in Kittson County grow where there is heavy foot traffic, which could injure plants unless they can be avoided. Potential threats to non-cliff populations include land clearing for agriculture, vegetation succession that leads to a more wooded habitat, livestock grazing, and contact with herbicides.

  Best Time to Search

Flowering occurs in May and June, but it is perhaps easier to identify Boechera collinsii when fruits are mature, from July to September.

  Conservation Efforts in Minnesota

No specific conservation actions have been undertaken on behalf of B. collinsii. However, because most populations occur on publicly owned land, future management actions to protect this species and its habitat may be possible.

  Authors/Revisions

Welby R. Smith (MNDNR), 2023

(Note: all content ©MNDNR)

  References and Additional Information

Al-Shehbaz, I. A., and M. D. Windham. 2010. Boechera. Pages 348-412 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexcio. Volume 7. Oxford University Press, New York, New York.

Windham, M. D., and I. A. Al-Shehbaz. 2006. New and noteworthy species of Boechera (Brassicaceae) I: Sexual diploids. Harvard papers in Botany 11(1):61-88.