Carex membranacea is a sedge with inflated black perigynia, found in wetlands across Denali from the lowlands high into the alpine zone, which is most common in poorly-drained subalpine areas. Plants grow singly from long rhizomes. Stems are stout, 10-55 cm tall. The leaves are about the same length as the stem, flat to V-shaped, with somewhat revolute margins, 3-6 mm wide. Plants produce male and female flowers in separate spikes, but on the same stem. The 1-3 staminatespikes are terminal and narrow. The 1-3 pistillatespikes are lateral, sessile or on a short, upright stalk. The pistillate scales are ovate to lanceolate, black, shorter and narrower than the perigynia. The perigynia are shiny, black and membranaceous, 2-5 mm long. Fruits are 3-angled achenes with 3 stigmas. C. rotundata and C. saxatilis have similar large, black spikes. C. rotundata is easily distinguished by its rounded stems. C. saxatilis has 2 stigmas, and the pistillatespikes are long-peduncled.
Carex membranacea is perennial and flowers in mid-summer (July).
C. membranacea 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 water dispersed.
Carex membranacea is an amphi-Beringian species that primarily occurs in North America, barely reaching into eastern Eurasia in the Russian Far East. This species occurs in Alaska and across northern Canada, is found throughout Alaska and northern Canada to Quebec and Labrador in the east, although it is absent from southern Canada and the continental U.S. In Alaska, occurs widely across the mainland part of the state from the Arctic Coast southwards, but is absent from the Aleutians and the southeastern panhandle region. In Denali, this species is widespread in localities scattered across the park, on both sides of the Alaska Range.