Rare Species Guide

  Synonyms

Hypnum dimorphum

  Basis for Listing

Heterocladium dimorphum (spaced-out tangle moss) has a disjunct distribution in the northern hemisphere in the boreal bioclimatic zone ranging across parts of North American and northern and central Europe. It has been recorded regionally from Ontario, South Dakota, and the Upper Peninsula of Michigan. In Minnesota, only a single population has been recorded from Cook County (North Shore Highlands Subsection; Janssens 2005). With only one population documented in Minnesota, there is too little information available to detect a statewide population trend at this time. Given its apparent rarity, and because the only known population is located in an area that is highly vulnerable to destruction, Heterocladium dimorphum was designated a species of special concern in 2013.

  Description

Heterocladium dimorphum is a small moss, growing in open to crowded mats that are rigid and dark-green to yellow-brown in color. Stems are loosely pinnate, with stem leaves approximately 0.7-1.0 mm (0.03-0.04 in.) long; branch leaves are smaller yet. Stem leaves are cordate-ovate at the base, ending in a slender, abruptly narrowed and somewhat recurved apex. Stem leaves have a double costa (midrib), which is often indistinct, ending usually 1/3-1/2 the leaf length (Crum and Anderson 1981). Branch leaves are wide-spreading when moist, rounded or oblong-ovate, and acute (especially towards the branch tips) or rounded at the apex, with serrulate margins (Crum and Anderson 1981). The double costa on these branch leaves is sometimes short and indistinct and sometimes appears as only a single costa. In general, the stem leaves are paler than the branch leaves and abruptly acuminate. In contrast, the branch leaves are bluntly short-pointed, crowded and overlapping when dry, but widely spreading when moist, and darker and firmer in texture than the stem leaves. The overall appearance is something like that of Thuidium; however, the structure of the gametophytes is different.

  Habitat

Heterocladium dimorphum grows on boulders or in the crevices of cliffs, such as shaded northwest- to east-facing shaded areas of northern mesic cliffs, northern wet cliffs, or bedrock lakeshore and occasionally on the soil or humus of shaded banks of streams or on the bark of trees near the base (Crum and Anderson 1981).

  Biology / Life History

The genus Heterocladium, as well as the specific epithet "dimorphum", refers to the differentiated branch and the two forms of leaves (stem and branch). Plants are dioicous, meaning that male and female reproductive organs are on separate plants.

  Conservation / Management

Presently, the known population is located in an area that is highly vulnerable to destruction by trampling from rock scrambling and climbing. Care should be taken to avoid areas where moss is observed growing on rock when participating in these activities. Additional survey work is needed to clarify the species’ abundance and distribution at the site.

  Best Time to Search

The best time to search for Heterocladium dimorphum is from May through September or essentially anytime its cliff habitat is not covered by snow.

  Authors/Revisions

Erika Rowe (MNDNR), 2018

(Note: all content ©MNDNR)

  References and Additional Information

Crum, H. A., and L. E. Anderson. 1981. Mosses of eastern North America. In two volumes. Columbia University Press, New York, Yew York. 1330 pp.

Janssens, J. A. 1997. Bryophyte floristics at Sugarloaf Cove, Cook County, Minnesota. Report submitted to the Sugarloaf Interpretive Center Association, Duluth, Minnesota. 20 pp.

Janssens, J. A. 2005. Proposed candidates of endangered, threatened, and special concern species of bryophytes for Minnesota: update June 2005. Report to the Minnesota Department of Natural Resources, County Biological Survey, St. Paul. 18 pp.

Janssens, J. A. 2009. MS Access database on Minnesota bryophytes. Lambda-Max Ecological Research, Minneapolis, Minnesota.

Magill, R. E. 2007. Pterigynandraceae in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico [web application]. <http//www.efloras.org/florataxon.apx?>. Accessed 14 October 2016.

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. <http.//www.natureserve.org/explorer>. Accessed 10 June 2009.