Sphagnum compactum DC. in Lam. & DC.
Cushion Peat Moss
Sphagnum compactum var. imbricatum, Sphagnum rigidum
Basis for Listing
Sphagnum compactum (cushion peat moss) has a nearly continuous world distribution, without zonal affinity. It has been recorded regionally from Ontario, Manitoba, Iowa, Wisconsin, and the Upper Peninsula of Michigan (McQueen and Andrus 2007). In Minnesota, S. compactum was first found in St. Louis County in 1996 (Border Lakes Subsection; Janssens 2010). An intensive local search at and near the original locality has been unable to relocate the species, and it is thought that the original small population was extirpated by voucher collection during the original ecological survey. Years of additional surveys throughout the state in suitable mesohabitats have resulted in the discovery of only two additional records, both in Koochiching County (Border Lakes Subsection). Given the species’ extreme rarity and limited geographic range in the state, its restrictive habitat requirements and the vulnerability of populations to degradation or destruction, Sphagnum compactum was listed as a threatened species in 2013.
Sphagnum compactum belongs to the section Rigida, which includes only one other North American species (S. strictum), which is restricted to the Atlantic seaboard and the southeastern states. In the field, S. compactum plants are pale green to golden brown and form dense compact cushions, often in shallow areas on bedrock. The broadly ovate branch leaves are the most diagnostic, and on first sight they are similar to those of the species in the section Sphagnum (represented in Minnesota by S. magellanicum [Magellan's sphagnum)], S. centrale, and S. papillosum [papillose shpagnum]). However, S. compactum leaves are broadly truncate at the apex and abruptly involute (with 5-6 teeth but without squamae) rather than cucullate (though they appear cucullate). The leaves are also somewhat subsecund and are strongly inrolled. In addition, the branches of S. compactum grow upward, concealing the capitulum. Other somewhat similar species in field aspect are S. squarrosum and S. subsecundum. The stem leaves of S. compactum are very small (0.3-0.7 mm [0.01.0.03 in.]) compared with those of S. squarrosum. Sphagnum subsecundum (also with small stem leaves) and relatives (usually with larger stem leaves) have obvious capitula, grow in less dense and compact clones, and have a very different branch-leaf pore structure.
Sphagnum compactum is not a significant peat former but grows in small turfs on wet and sandy soil, siliceous rocks, or bare peat, often in seepage, in late snow melt areas, and on low banks of roadside ditches (Crum 1984; Cleavitt et al. 2001; McQueen and Andrus 2007). The St. Louis County population was found within a carpet covering a paludifying forest-floor hollow of a young jack-pine stand on the Vermilion Granite (Ojakangas and Matsch 1982; Reich et al. 2001), with thin local till. The other species within the carpet were, in order of decreasing abundance, Sphagnum russowii (Russow's sphagnum), S. angustifolium, Polytrichum commune (polytrichum moss), and S. girgensohnii (Gergensohn's sphagnum). The two Koochiching County populations both occurred in very shallow depressions on bedrock in northern dry bedrock pine (oak) woodlands. Associated species included Vaccinium angustifolium (lowbush blueberry), Maianthemum canadense (Canada mayflower), and Sphagnum russowii, among others.
Biology / Life History
Evidently a pioneer species among sphagna, with a globally wide but locally scattered distribution (Janssens 2006).
Conservation / Management
The apparent rarity of S. compactum in Minnesota is somewhat of a puzzle. It is easily recognized and differentiated from look-alikes, so it probably has not been overlooked. Its mesohabitat preferences make it stand out among other bryophytes (except in the northern Sphagnum-dominated muskegs; Janssens 2006). Its North American and global distribution suggest no major disjunctions (McQueen and Andrus 2007) nor should there be any immediate threat from climate change.
Best Time to Search
The best time to search for S. compactum is from May through September or essentially anytime the ground is not covered by snow.