Atrichum tenellum (Rohl.) Bruch & Schimp. in B.S.G.
Little Saw Moss
Basis for Listing
Atrichum tenellum (little saw moss) has a patchy distribution in the temperate regions of the northern hemisphere. In Minnesota, only four populations have been confirmed from three counties: Lake, Pine, and Fillmore (Laurentian Mixed Forest Province and Paleozoic Plateau Section). This species is reportedly rare throughout its entire range, and further inventory work is needed to clarify the species’ abundance and distribution in the state as well as the potential threats to its survival (Janssens 2005).
This green to brownish-green moss is small and delicate in appearance, with erect and unbranched stems that are less than 2.0 cm (0.8 in.) tall. Leaves are somewhat crowded near the top of the stem and are 3.0-6.0 mm (0.12-0.24 in.) long and up to 1.5 mm (0.06 in.) wide, ovate-lanceolate, tapering to an acute apex. They are scarcely undulate (or wavy) when moist but are strongly crisped or contorted when dry. Costa (distinct midrib of leaf) extends to the leaf apex or nearly so, with 2-5 visible lamellae (green ridges or projections on the costa made up of cells) that have variable height along the length of the costa. Leaf margins consist of small widely spaced teeth that are well-developed at the apex, becoming smaller and disappearing near the middle of the leaf (Crum and Anderson 1981).
This species has been previously confused with the rare A. crispum (wave-leaved crane’s-bill moss); however, A. tenellum is easily distinguished by the absence of papillae (small projections) on marginal teeth and border, which are visible under a microscope (Ireland 1991). In addition, the lamellae of A. crispum are always low and discontinuous (0-3 and 1-4 cells high), unlike A. tenellum which has 2-5 lamellae, 4-7 cells high (FNA 2007). Capsules are oval to cylindrical, upright to inclined, rarely exceeding 2.5 mm (0.10 in.) in length. Setae (stalk supporting the capsule) are up to 2 mm (0.08 in.) long (Smith Merrill and Ireland 2007).
Atrichum tenellum may also be confused with other species of Atrichum due to the distinctively ridged nerves, called ‘lamellae’, along the length of the leaf. However, both A. crispum and A. tenellum have lamellae that are smaller in stature and less well-developed marginal teeth (Janssens 2014) and neither have leaves that are undulate when moist. Additionally, compared to other species in the Polytrichaceae family (e.g. Polytrichum spp.) which also have lamellae, the leaves of Atrichum species lack differentiated sheaths that wrap along the stem (Janssens 2014).
As a colonist, A. tenellum inhabits wet soils in open wet hardwood forests. It has also been reported in other parts of its range to occur in a range of disturbed open acidic habitats on moist and sandy humus and loams. These include stream sides, lake margins, ditch banks, and open rich graminoid-dominated peatlands.
Biology / Life History
The uncommon shortly cylindrical inclined capsule of this dioicous (separate male and female plants) species develops on a yellowish seta that matures in late summer.
Conservation / Management
This species has few distinguishing characteristics and may be easily overlooked and, as such, is seldom collected. Until more populations are confirmed and a clear trend in habitat is defined for this species, it will be difficult to say with certainty whether it is only limited to wet forests. Consequently, although conservation and management of this species should be based on its habitat as currently understood (wet forests), this may change as more is learned about this species. Climate change, coupled with the effects of insects and disease, such as emerald ash borer, could also threaten the species' viability in Minnesota, since the loss of moist habitat conditions would likely be detrimental to the species. Similarly a loss of canopy cover, either through harvest removal or emerald ash borer mortality, could also result in a loss of habitat, with a temporary rise in the water table leading to a conversion of wet forest to wet meadow or shrub swamp.
Best Time to Search
The best time to search for A. tenellum is from May through September or essentially anytime the ground is not covered by snow.
Erika R. Rowe (MNDNR), 2018
(Note: all content ©MNDNR)
References and Additional Information
Crum, H. A., and L. E. Anderson. 1981. Mosses of eastern North America. 2 volumes. Columbia University Press, New York, Yew York. 1330 pp.
Flora of North America Editorial Committee, editors. 2007. Flora of North America north of Mexico. Volume 27, Bryophytes: Mosses, Part 1. Oxford University Press, New York, New York. 734 pp.
Ireland, R. R. 1991. Cuticular papillae of Atrichum crispum. The Bryologist 94(1):73-76.
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 Resoucres, County Biological Survey, St. Paul. 18 pp.
Janssens, J. A. 2009. MS Access database on Minnesota bryophytes. Lambda-Max Ecological Research, Minneapolis, Minnesota.
Janssens, J. A. 2014. Noteworthy mosses and liverworts of Minnesota. Part II: Species fact sheets. Minnesota Department of Natural Resources. 208 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.
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.
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.
Smith Merrill, G. L., and R. R. Ireland. 2007. Polytirchaceae. Pages 121-161 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 27, Bryophytes: Mosses, Part 1. Oxford University Press, New York, New York.