Carex atrosquama is a tall, stout sedge with prominent black flowering head that occurs primarily in floodplains and meadows in the subalpine zone north of the Alaska Range crest in Denali. This species has a single stem bearing a terminal cluster of spikes. Plants are tufted, growing 20-80 cm tall from a short rhizome. The leaves are 2-5 mm wide, with reddish sheaths and are much shorter than the stem. The stem is scabrous. The spikes (3-4) are erect, aggregated, and short-stalked, the inflorescence 2.5-3 cm long. The lowest bract is leaf-like and more or less equal in length to the inflorescence. The terminal spike is bisexual with the female flower above, male flowers below. Lateral spikes contain only female flowers. The pistillate scales are black, lanceolate in shape and shorter than the perigynia. The perigynia are green (aging to brown), papillose, purple-beaked, and elliptic in shape, 2.5-3.5 mm long. The fruits are achenes, with 3 stigmas. Similar species in Denali include C. media, in that species the pistillate scales have a transparent margin, and the spikes and perigynia are smaller.
Carex atrosquama generally flowers around mid-summer in Denali.
C. atrosquama is monoecious, but male and female flowers are produced in separate spikes. Species in the genus Carex are wind pollinated, but usually also self-pollinate to some degree (Bertin 2007; Friedman and Barrett 2009). Seeds are gravity and wind dispersed. It can also spread vegetatively through rhizomatous growth.
Carex atrosquama occurs only in North America, where it is endemic to the western Cordillera, ranging from Alaska east to the Northwest Territories and south through British Columbia/Alberta into the Rocky Mountain states and Pacific Northwest. In Alaska, C. atrosquama occurs primarily in the mountains south of the Yukon River, with isolated stations to the north. In Denali, this species occurs most frequently in the northeastern quadrant of the park, but with stations in the mountains on both sides of the Alaska Range crest.
In Denali, C. atrosquama is observed growing at elevations from 608 m to 1160 m with an average elevation of 844 m. This species is observed equally on north and south facing sites, but occurs over a wider range of elevations on south-facing slopes. It is found on sites that averaged 10 degrees in slope.