Plants apparently without sterile culms; strongly cespitose, often with rhizomes 1-4 cm long, 1-2 mm thick. Culms (10) 30-80 cm, usually unbranched, occasionally branched, usually slightly to strongly scabrous, sometimes puberulent beneath the panicles; nodes (1) 2 (3). Sheaths scabrous; collars usually scabrous or hairy, rarely smooth; ligules (1.5) 2-4 (9) mm, usually truncate and entire, sometimes lacerate. Blades (4) 5-17 (30) cm long, 2-5 (6) mm wide, flat or involute, stiff, abaxial surfaces scabrous, adaxial surfaces usually densely long-hairy, rarely sparsely hairy. Panicles 4-13 (15) cm long, 0.9-2 (2.8) cm wide, erect, contracted, infrequently interrupted near the base, often red or purple-tinged; branches 1.3-3.5 cm, scabrous, prickles long, almost hairlike, spikelet-bearing to the base. Spikelets (4.5) 5.5-6.5 (8) mm; rachilla prolongations about (1) 2 mm, hairs about 2 mm. Glumes keeled, usually scabrous, rarely scabrous on the keels only, lateral-veins obscure to prominent, apices acute; callus hairs (0.9) 1.2-1.5 (2.4) mm, 0.2-0.4 (0.6) times as long as the lemmas, sparse; lemmas (3.5) 4-4.5 (5) mm, usually 1-2.5 mm shorter than, rarely equal to, the glumes; awns (4.5) 6-7 (9) mm, attached to the lower 1/10-1/3 of the lemmas, usually exserted, stout, easily distinguished from the callus hairs, bent; anthers (1.3) 1.7-2.5 (2.9) mm. 2n = 42-58, 84.
Colo., N.Mex., Wash., Utah, Alaska, Minn., Idaho, Mont., Nev., S.Dak., Wyo., Alta., B.C., Greenland, Man., Nfld. and Labr. (Labr.), N.W.T., Nunavut, Ont., Que., Sask., Yukon, Calif., Oreg.
Calamagrostis purpurascens grows in alpine tundra, on subalpine slopes, in grasslands, sand dunes, meadows, coniferous and deciduous forests, and disturbed soils, usually on rocky ridgetops and slopes and, infrequently, on valley floors. It prefers well- to moderately-drained, medium- to coarse-textured substrates, including scree and talus, that are often calcareous, at elevations from 15-4000 m. Its range extends from Alaska through Canada to Greenland and Newfoundland, including the islands of the Canadian arctic, and south in the western mountains to California and northern New Mexico. It does not occur near the open coast except in the Aleutian Islands, the Arctic, and the Olympic Peninsula in Washington. In Asia, it ranges from eastern and central arctic Siberia south to the Kamchatka Peninsula and Sakhalin Island.
The hairy adaxial leaf surfaces are a reliable diagnostic characteristic for Calamagrostis purpur¬ascens. Many specimens from Washington and Oregon currently identified as C. purpurascens belong to C. tacomensis (p. 716). In addition to differing in its leaf vestiture, C. purpurascens has shorter awns and panicle branches, and more scabrous glumes, than C. tacomensis. Plants of C. purpurascens that have short awns barely projecting beyond the lemma margins have been mistaken for C. montanensis (p. 724), but that species does not have hairy adaxial leaf surfaces.
Calamagrostis purpurascens var. laricina Louis-Marie supposedly has shorter glumes and awns, with the awns barely exserted, if at all. The variety is not recognized here because the range in variation in these two characters is continuous; plants that match the description of var. laricina are widely distributed throughout the range of the species. Some collections having both short and long awns are on the same sheet.
Calamagrostis lepageana Louis-Marie, collected only from Mont-Commis, Quebec, is here included within C. purpurascens. It differs from the typical form in that the panicle is more open; the branches are sparsely short-scabrous; and the glumes are at the shortest limit for the species, and are smooth or sparsely scabrous only on the keels. Other characteristics, including the densely hairy adaxial leaf surfaces, fit C. purpurascens.
According to Norwegian botanist Reidar Elven (pers. comm.), five specimens at ALA, collected from four sites on the Seward Peninsula in Alaska, are distinct from Calamagrostis purpurascens but are related to it, or are possibly a hybrid between C. purpurascens and C. sesquiflora (p. 714). They differ from typical C. purpurascens in having bristly (versus hairy) upper leaf surfaces, and acuminate, dark purple (versus acute, pale pink or lilac) glumes. They can be called C. purpurascens subsp. arctica (Vasey) Hulten; in eastern Russia this taxon is recognized at the species level as C. arctica Vasey.
"decumbent" is not a number.