Rhytidiadelphus squarrosus var. calvescens (Lindberg) Warnstorf
Plants large, soft, 2--4 mm wide measured across leafy stem, to 15 cm long. Stems irregularly branched to pinnate; branches commonly to 20 mm. Stem leaves not crowded (stem visible between leaves) except at tip, erect-spreading to squarrose, broadly ovate-triangular, not plicate or rugose, 2.3--4.2 × 1.1--2 mm; base not or scarcely sheathing, broadly rounded to the insertion; apex abruptly narrowed to a long, channeled acumen; costa extending to 1/3 leaf; median cells 40--75 × 5--7 µm, smooth; alar cells distinct, shorter and wider than basal cells with thin, nonporose walls. Branch leaves ovate to lanceolate, 1.3--2.8 × 0.4--1.3 mm. Capsule ovoid, 1--2.2 mm.
Damp to wet soil, humus, logs, and rocks in swamps and moist forests, often along streams and in spray of waterfalls; 30--2100 m; Alta., B.C., N.B., Nfld., N.W.T., N.S., Ont., P.E.I., Que., Yukon; Alaska, Conn., Idaho, Maine, Mass., Mich., Mont., N.H., N.Y., N.C., Oreg., Pa., R.I., Tenn., Vt., Wash., Wis.; Eurasia.
For the past several decades, many Canadian and European bryologists have recognized Rhytidiadelphus subpinnatus as a species distinct from R. squarrosus, whereas American bryologists have held to the traditional concept of a broadly circumscribed R. squarrosus that includes R. subpinnatus. T. Koponen (1971) and H. A. Crum and L. E. Anderson (1981) have provided contrasting opinions and worthwhile discussion on the species problem. In North America R. subpinnatus is by far the more widespread and abundant taxon. The two taxa are distinct ecologically, but intergrade somewhat morphologically. The most troublesome specimens are those from a few isolated subalpine sites of the Canadian West. The plants grow in wet areas on the forest floor or along the margins of lakes and hot springs in alpine meadows or in open Abies lasicarpa stands. Although their leaf morphology is clearly that of R. subpinnatus, these specimens are less branched and their leaves are more strongly squarrose and crowded. As W. B. Schofield and S. S. Talbot (1991) noted, within one colony plants resembling both taxa can be found.