Sphagnum acutifolium var. robustum Russow
Plants ± moderate-sized, stiff and open, compact on exposed sites, capitulum flat-topped and often stellate; green or variegated red and green, lacking metallic sheen when dry. Stems typically mixed green and red; superficial cortical cells mostly rectangular and uniporose with a single round to ovate pore in distal portion of cell usually free from cell wall, some cells and occasionally whole stems may be aporose. Stem leaves lingulate, 1.3--1.6 mm, apex broadly rounded or pointed and notched (sometimes denticulate), border strong and broadened at base (more than 0.25 width); hyaline cells short sinuoid-rhombic, mostly efibrillose, 0--1(--2) septate. Branches long and slender, never 5-ranked. Branch fascicles with 2 spreading and 1--2 pendent branches. Branch leaves ovate-lanceolate, 1.3--1.6 mm, concave, straight, apex strongly involute; hyaline cells on convex surface with numerous round to elliptic pores along the commissures, grading from small round pores near the apex to large elliptic pores at the base, concave surface usually with large round pores throughout, but sometimes restricted to proximal portions of leaf. Sexual condition dioicous, but some specimens apparently monoicous. Spores 18--33 µm, coarsely papillose on both surfaces; proximal laesura more than 0.5 spore radius.
Capsules mature late spring to early summer. Minerotrophic and shade-tolerant, common on the margins of mires, open portions of poor to rich fens, up through timberline in montane regions in wet coniferous forests; Alta., B.C., Man., N.B., Nfld., N.W.T., N.S., Ont., P.E.I., Que., Sask., Yukon; Alaska, Calif., Colo., Conn., Del., Idaho, Ill., Ind., Iowa, Maine, Md., Mass., Mich., Minn., Mont., Mo., N.H., N.J., N.Y., N.C., Ohio, Oreg., Pa., R.I., Tenn., Utah, Vt., Va., Wash., W.Va., Wis., Wyo.; Eurasia.
Sporophytes are uncommon. This species is associated with Sphagnum centrale, S. fallax, S. fimbriatum, S. girgensohnii, and S. squarrosum. Because of its not particularly distinct phenotype as well its strong tendency to produce hemiisophyllous stem leaves, Sphagnum russowii is probably the most frequently misidentified Sphagnum species. The combination of the flat, stellate capitulum, unranked branch leaves, and lingulate stem leaf will usually suffice to identify this species. Sphagnum capillifolium has a rounded capitulum and a pointed stem leaf while S. subtile halso has a rounded capitulum$ but a shorted and more triangular stem leaf. In montane and Arctic mires it can be confused with S. warnstorfii$ but the latter species usually has conspicuously 5-ranked branch leaves. As one might expect in such a widespread and common species, the characters can vary considerably. For example, one regularly finds plants that are consistent in every respect with the description of this species except that they lack stem cortical pores. Some stem leaves have almost no septations in the stem leaf hyaline cells while other forms have most of the cells septate. As with similar variation in the likewise common S. fuscum, there is no consistent pattern and so taxonomic recognition of the variants is unwarranted.