Juncus articulatus L.
Basis for Listing
For many years, Juncus articulatus (jointed rush) was known in Minnesota only by a herbarium specimen collected on the shore of White Bear Lake in Ramsey County in 1920 (Eastern Broadleaf Forest Province). More than 80 years passed before it was found again (Laurentian Mixed Forest Province) and then only after intensive searching. It is still known from only a small handful of sites, nearly all are lakeshores and are considered potentially vulnerable. Threats include lakeshore development, artificial manipulation of lake levels, and degradation of water quality.
Juncus articulatus is not rare everywhere. In fact, it has a rather broad distribution across North America and Eurasia. Why it is so rare in Minnesota is unknown. What appears to be suitable habitat is not in short supply in the state, yet repeated searches of potential habitat almost always turn up nothing, revealing this to be one of the rarest rushes in Minnesota. Hence, J. articulatus was listed as an endangered species in 2013.
Juncus articulatus is a rush that grows to a height of 15-55 cm (6-22 in.). Rushes are grass-like plants, and there are about 20 species occurring in Minnesota. Distinguishing the species of Juncus can be a challenge and usually requires careful measurements of small structures. Juncus articulatus is a perennial; the stems are produced singly or few together by a rhizome that may reach 5 cm (2 in.) in length. The leaves occur at the base of the stem and along the length of the stem; the leaf blades are circular in cross-section and 0.5-1.5 mm (0.02-0.06 in.) wide; the auricles are yellowish or whitish and 0.5-1.5 mm (0.02-0.06 in.) long. The inflorescence is an open panicle, 3-12 cm (1-5 in.) long, with 7-30 glomerules borne on ascending or stiffly spreading branches; each glomerule is hemispheric or obconic in shape, 4-8 mm (0.16-0.32 in.) across, and contains 5-12 sessile flowers. The tepals of the flower are six in number and 1.8-2.6 mm (0.07-0.10 in.) long; the inner 3 and the outer 3 are about equal in length; they are erect or ascending. Each flower has 6 stamens, which are 1.0-1.5 mm (0.04-0.06 in.) long; the anthers are shorter or equal in length to the filaments. The seed capsules are ellipsoid or ovoid in shape, 2.7-3.6 mm (0.11-0.14 in.) long; they exceed the tepals in length and have a beaked apex; the seeds are yellowish, 0.4-0.5 mm (0.016-0.020 in.) long, and lack tail-like appendages (Clements 1985; Brooks and Clements 2000).
To date, most records of J. articulatus in Minnesota are from sandy lakeshores. These habitats typically flood in the spring, after the snow and ice melt, then are exposed later in the summer, when water levels drop. Similar habitats can be anticipated around the margins of marshes or other non-permanent wetlands, especially where cyclical or periodic perturbations create openings for opportunistic species like J. articulatus.
Biology / Life History
Although the habitats of J. articulatus typically experience seasonal fluctuations in water levels along with corresponding scouring or deposition of soil, this does not seem to be a problem for J. articulatus. In fact, the pattern of rising and falling water levels may be necessary for J. articulatus to complete its life cycle.
While this species is a perennial, individuals are probably not long-lived. The short-lived nature of J. articulatus is probably offset by a high reproductive potential, via numerous seeds. The seeds are released in the fall, when water levels are usually low, allowing them to settle into the substrate. The seeds overwinter easily and some, or most, will germinate the following year. Whether or not J. articulatus maintains a seed bank in the substrate is unknown. The rhizomes also overwinter, buried in the substrate, and can send up new stems the following year, even if they are flooded for some period of time.
Conservation / Management
Any population trend of J. articulatus in Minnesota will likely parallel the trend of wetlands and lakeshore habitat. Negative trends seem to follow lakeshore development. Rip-rap placed on a lakeshore will have unpredictable consequences for native lakeshore plants, though it will likely reduce the size and extent of the habitat. Similar problems could arise from removal of emergent vegetation and floating-leaved vegetation from the littoral zone, which buffer the shore from strong waves. Sand blankets, weed-rollers, and other habitat-modifying activities should also be avoided.
Best Time to Search
The best time to search for J. articulatus is when it has reached full development and has mature seeds. The timing in any given year will depend on local factors and weather conditions; however, prime dates are likely to be during August and early September.
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.
Clements, S. E. 1985. A key to the rushes (Juncus spp.) of Minnesota. The Michigan Botanist 24:33-37.
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.
Minnesota Department of Natural Resources. 2005. Field guide to the native plant communities of Minnesota: the eastern broadleaf 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. 394 pp.
Minnesota Department of Natural Resources. 2006. Tomorrow's habitat for the wild and rare: An action plan for Minnesota wildlife, comprehensive wildlife conservation strategy. Division of Ecological Services, Minnesota Department of Natural Resources. 297 pp. + appendices.