Basis for Listing

At this time (2018), there is only one record of Juncus subtilis (slender rush) from Minnesota. It is a herbarium specimen collected on a sandy lake shore in Cook County (Border Lakes Subsection) in 1998 by a botanist working for the Minnesota Biological Survey. For years the Biological Survey has targeted lake shores for rare plant searches, yet J. subtilis has been found only once. All searches before and since then have been fruitless. It is certainly possible that J. subtilis could have been missed on occasion, and it may exist in habitats not yet searched; but there can be little doubt this is a very rare species in Minnesota. It is also rare in the region, being absent from neighboring states and rare in adjacent portions of Canada (Brooks and Clements 2000).

The ecological integrity of the site where J. subtilis has been found is not known to be threatened. However, the singular nature of the occurrence raises concerns that some unforeseen event, human-created or otherwise, could wipe out the only known population in the state.  Hence, Juncus subtilis was listed as an endangered species in 2013.

  Description

Plants of the genus Juncus, commonly known as rushes, are grass-like plants with small inconspicuous flowers. The fruits of a Juncus are small dry capsules, and the seeds are nearly microscopic in size.

Juncus subtilis is a perennial, 3-20 cm (1-8 in.) tall, typically submerged or emergent from shallow water. The stems are thin and weak, barely, if at all, rigid. The rhizomes are slender and form mats just beneath the substrate. The leaves arise from the base of the stems and from along the length of the stems. The leaf blades are capillary and 0.2-0.3 mm (0.008-0.012 in.) wide; the auricle of the leaf is membranous and 0.1-0.8 mm (0.004-0.031 in.) long. The inflorescence is 1-7 cm (0.4-3.0 in.) long and consists of 1-12 obconic-shaped glomerules, each with 1-3 flowers. The involucral bract at the base of the inflorescence is no more than 1 cm (0.4 in.) long and does not extend past the inflorescence. Each flower has 6 tepals, 2.0-4.4 mm (0.8-1.7 in.) long, the inner 3 are somewhat longer than the outer 3; there are 6 stamens in each flower, 1.0-1.5 mm (0.04-0.06 in.) long; the anthers are shorter or equal in length to the filaments. The capsules are narrowly ovoid in shape, 2.5-5.0 mm (0.1-0.2 in.) long, and surpass the tepals; the apex of each capsule is acutely tapered to a beak; the seeds are numerous, yellowish, 0.3-0.5 mm (0.01-0.02 in.) long, and lack tail-like appendages (Brooks and Clements 2000).

Unlike other Minnesota rushes, the stems of J. subtilis are usually too weak to support the plant; so it lies on the ground or floats in shallow water.

  Habitat

The lone Minnesota population occurs in shallow water on a gently sloping and sandy-bottomed bay (or littoral zone) of a remote lake. A portion of the population also occurs on the beach, just above water level (Gerdes 2001). This appears to be a typical habitat for this species throughout its range.

  Biology / Life History

Juncus subtilis is a perennial plant that overwinters as seeds that have been released from the capsules and as rhizomes rooted in the substrate. In addition, the flowers of J. subtilis are sometimes modified into tiny structures, called “bulbils”. A bulbil is essentially a small plantlet and serves as an alternative means of reproduction.

The littoral zone of a lake can be a turbulent habitat, where large waves might dislodge plants and wash them ashore. The action of lake ice in the spring may also have an impact on the habitat of this species. Although the effect of all this on populations of J. subtilis can only be speculated, it is reasonable to assume the species has evolved strategies to cope with these conditions. In fact, these extreme conditions may serve to spread plants to potentially suitable sites elsewhere within a watershed.

  Conservation / Management

Although J. subtilis is very rare in Minnesota and is currently known to occur at only one site, no immediate conservation action is recommended. At this time, the suitability of the habitat and the viability of the population cannot be improved by management action. Low-impact recreational use of the area such as canoeing, fishing, and swimming should have no lasting impact on the plants. However, anything that goes beyond simple recreational use of the habitat, or use of areas immediately adjacent to the habitat, must be evaluated before being initiated.

  Best Time to Search

The phenology of J. subtilis in Minnesota is poorly known. It is likely dependent on lake levels, which can vary from month to month and year to year. Taking this uncertainty into account, the best time to search for J. subtilis is likely July through early September. Years with below normal lake levels may expose more shoreline habitat, making access easier and improving chances of finding plants.

  Authors/Revisions

Welby Smith (MNDNR), 2018

(Note: all content ©MNDNR)

  References and Additional Information

Brooks, R. E., and S. E. Clemants. 2000. Juncus. Pages 211-255 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 22. Oxford University Press, New York.

Gerdes, L. B. 2001. A contribution to the flora of the Rove Slate Bedrock Complex Landtype Association, northern Cook County, Minnesota, U.S.A. Thesis, Michigan Technological University, Houghton. 79 pp.

Minnesota Department of Natural Resources. 2003. Field guide to the native plant communities of Minnesota: the Laurentian mixed forest province. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 352 pp.

NatureServe. 2009. NatureServe Explorer: An online encyclopedia of life [web application]. Version 7.1. NatureServe, Arlington, Virginia.

Smith, W. R. 2018. Sedges and rushes of Minnesota: the complete guide to species identification. University of Minnesota Press, Minneapolis. 696 pp.