Carex supina ssp. spaniocarpa is a small tufted perennial sedge that occurs in dry sites in the mountains of Denali, north of the Alaska Range crest including rocky places and discontinuous tundra. This diminutive species has few-flowered spikes and grows 8-15 cm tall from long reddish-brown rhizomes. Stems are scabrous and three-angled. Leaves are basal, thin (1-1.5 mm wide), scabrous margined and typically shorter than the stem. Plants produce male and female flowers in separate spikes, but they are sometimes tightly clustered enough to appear as one spike. The inflorescence consists of 2-3 spikes: a terminal staminatespike and the lateral spikes sessile, pistillate and few-flowered. The pistillate scales are ovate, acute and reddish brown with scarious margins and a pale midvein. The perigynia are ovate, shiny brown with a prominent beak. The fruits are achenes with 3 stigmas. Another small alpine sedge is Carex rupestris, that species is smaller, has a single spike with both male and female flowers, and usually has strongly curled leaves.
This species flowers in early summer in Denali.
Carex supina subsp. spaniocarpa 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 wind and water dispersed.
Carex supina subsp. spaniocarpa has a circumpolar distribution ranging from Eurasia, across Alaska and Canada to Greenland and in occasionally localities in the Canadian Arctic Archipelago. In North America, this species occurs across all Canadian provinces and reaching northernmost Minnesota in the lower 48 states. Carex supina subsp. spaniocarpa occurs in eastern and northern Alaska but is absent from the Yukon-Kuskokwim delta, Alaska Peninsula and southeastern panhandle. This species occurs in occasional localities north of the Alaska Range crest in Denali, primarily in the northeastern quadrant of the park.
Carex supina subsp. sapaniocarpa is found from 638-1169 with an average plot elevation of 990 m. It is observed almost exclusively on south facing slopes. It strongly prefers steep slopes as the average plot slope is 23 degrees.