Solitary graminoid herb forming dense tussocks from an often richly branched caudex. Almost all leaves crowded terminally on caudex branches. Culms up to 2–5 cm, smooth.

Leaves slightly revolute (M-shaped in cross section), 2–5(6) cm long and mostly shorter than the culm, about 2 mm broad at the base, slightly curving outwards and ending in an abrupt acute tip, papillose, bright green. Leaf margins and dorsal mid vein scabrous.

The flower in Carex is unisexual (either male or female), without perianth, and supported by a scale (the bract of the single flower). The male flower consists of 3 stamens. The female flower consists of a gynoecium of 2 or 3 fused carpels, with a single style and 2 or 3 stigmas, and with a single seed. The gynoecium is surrounded by a perigynium, a container with a narrow apical opening through which the style and stigmas emerge. The perigynia (and nuts) are either lenticular (when two carpels/stigmas) or trigonous (when three). The inflorescences are spikes, one or more per culm. If two or more spikes, all except for the uppermost are supported by more or less leaf-like bracts. Spikes may be unisexual or bisexual, and bisexual spikes may have the female flowers at base (basigynous) or at top (acrogynous). Flowers are wind pollinated and usually cross pollinated because the male flowers reach anthesis before the female flowers (protandry). Cross pollination predominates among sedges investigated in alpine Norway (Berggren & Haugset unpubl.), either due to the protandry or to genetic incompatibility. Seeds are spread inside their perigynia.

Inflorescence of one male spike and 2–3 female spikes. Male spike 7–11 × 1–2 mm, on a slender peduncle 2–11 mm, usually overtopped by female spikes. Female spikes 7–9 × 3–4 mm, on slender peduncles 7–15 mm, spreading or nodding. Bracts with 5–7 mm sheaths (concealing much or all of the peduncles) and blades of variable length, 1.5–2.5 cm, reaching below, up to top of, or above the inflorescence. Scales obtuse and with a white hyaline margin, those of male spikes pale to light brown, those of female spikes shiny reddish brown. Perigynia trigonous, shiny olive brown in their lower part and reddish brown towards the beak, narrowing into a distinct beak 0.2 mm long, smooth at the margins except for the beak which is scabrous. Stigmas three.

Triangular nut within the perigynium.

Sexual reproduction by seeds; no vegetative reproduction. The population differentiation within the main genetic groups of C. capillaris s. lat. (Skjetne 2012) is weaker than in C. atrofusca (Schönswetter et al. 2006) but stronger than in C. bigelowii (Schönswetter et al. 2008). Together, this suggests that C. capillaris may be a mixed mater rather than mainly autogamous as suggested by (Brochmann et al. 1999). Assumed ripe fruits are observed on herbarium material. In 2009, most plants had flowering or fruiting spikes but also non-flowering (probably juvenile) plants were observed in abundance, indicating efficient recruitment from seeds. Also the presence of genetic diversity in the single population, although it was lower in the Svalbard stand than within stands in most other regions, indicates some local recruitment (Skjetne 2012).

Fruits (inside perigynia) have no special adaptation to dispersal but are generally easily dispersed by wind, water, and birds.

Carex capillaris and C. krausei belong to the same section and have been considered one species by many authors, especially by American ones. They may be distinguished by the following characters: In C. capillaris the apical spike is male and does not reach above the female spikes (in ssp. fuscidula, it does in other subspecies of C. capillaris), the female spikes are nodding and at not very different levels, the scales have a broad white hyaline margin, and the perigynia are glossy and with a short, almost straight beak; in C. krausei the apical spike is bisexual (acrogynous) and reaches above the female spikes, the female spikes are more erect and at very different levels, the scales have a very narrow white hyaline margin, and the perigynia are dull and with a long, oblique beak. The two species are rather different and no transitional forms are known (see Comments).

The only possibilities of confusion otherwise are with two only distantly related species from two other sections: Carex fuliginosa ssp. misandra and C. glacialis. The only other Carex species in Svalbard with nodding spikes is the large, cushion forming C. fuliginosa ssp. misandra which differs in much larger size of all plant parts (e.g. culms 15–20 cm, leaf length 5–10 cm, leaf width 4 mm), by having 1–2 bisexual spikes, in acute scales, and in unusually flat perigynia. Carex glacialis differs from C. capillaris (and also C. krausei) in, e.g., the shape of the perigynia (subrotund with an abruptly narrowed, very slender beak) and the length and colour of the sheaths of the bracts subtending the spikes (ca. 1 mm long and hyaline). However, all the three species form very dense and small tussocks with narrow, green leaves.

Growing in sites with high substrate moisture and temperature close to warm springs. It is also found under more moderate conditions in an area influenced by seepage downstream of the springs. Locally dominant but co-occurring with at least 22 species of vascular plants (Skjetne 2012). Soil temperature at 3 and 10 cm depth measured in late July were 12.2–15.0 and 12.1–16.5 °C, respectively, pH ranged from 6.3 to 8.1, slope from 0° to 16°, and covers of vascular plants, bryophytes, and cryptogamic crust were highly variable (Skjetne2012). Carex capillaris and Festuca rubra ssp. richardsonii were often co-dominant. Grazed by geese and reindeers.

Strongly thermophilous. Only found in one locality in the middle arctic tundra zone and weakly continental section, at Trollkjeldene by Bockfjorden (Haakon VII Land) in N Spitsbergen, where it was discovered in 1960 growing in "quite considerable quantities" close to the thermal springs (Rønning 1961). In 2009, it was found in an area of ca. 100 × 600 m, reaching from north of the thermal springs to the riverbank in south. It was among the most abundant species in the area, and the population was estimated to more than 2000 individuals (Skjetne 2012).

The presence of Carex capillaris ssp. fuscidula by Bockfjorden needs some explanation. The plant is strongly isolated here, and the occurrence can be explained in the two ordinary ways: fortuitous long distance dispersal by birds, or a relic from earlier and warmer postglacial times. Warm springs are known, from other parts of the Arctic (e.g., Greenland, Alaska), both as favored resting places for many birds and as places of survival of thermophilous plants (and animals) during harsher times. See Comments.

The general range is circumpolar, mainly in the arctic zones but also found in mountains in the boreal zones.

Carex capillaris is highly polymorphic at a circumpolar scale with at least three major northern races. Subspecies capillaris is present in Europe and W Siberia and reaches across the North Atlantic to Iceland and probably also to S Greenland and perhaps E Canada (Labrador and Newfoundland). This is the main subspecies of Scandinavia. Subspecies fuscidula is circumpolar (Egorova 1999) and the only subspecies in the northernmost parts, including the northern half of Greenland and Svalbard. It is also common in Iceland and molecular (Skjetne 2012) and morphological evidence supports its presence as rather frequent in the Scandinavian mountains. The third subspecies or variety is unresolved as to name (perhaps var. elongata Olney ex Fernald) but does not concern us as it is mainly temperate North American.

In the best investigated part of the range of C. capillaris, in E Beringia (Alaska and Yukon), the morphological pattern is clear with some overlap in the ranges among the two races present (ssp. fuscidula and var. elongata) but no intermediates or transitions. Whether the same is the case in the North Atlantic regions is not known. There is no modern study of the group. An earlier study based on cytology and morphology (Löve et al. 1957) is suspected to build on partly fictive (constructed) evidence and is now discounted by most researchers in Carex (see comments by P.W. Ball and R. Elven in Elven et al. 2011).

The population in Svalbard is genetically depleted and forms a distinct genetic subgroup of ssp. fuscidula, most closely related to N Norwegian plants among those investigated (Skjetne 2012). It is therefore highly unlikely that the Bockfjorden population is due to any recent dispersal event.

See also Comments to Carex krausei.

Brochmann, C. & Steen, S.W. 1999. Sex and genes in the flora of Svalbard - implications for conservation biology and climate change. – Det Norske Videnskaps-Akademi. I. Matematisk Naturvitenskapelig Klasse, Skrifter, Ny serie 38: 33–72. Doi

Egorova, T.V. 1999. The sedges (Carex L.) of Russia and adjacent states. – St.-Petersburg State Chemical–Pharmaceutical Academy, St.-Petersburg, and Missouri Bot. Gard. Press, St. Louis.

Elven, R., Murray, D.F., Razzhivin, V.Y. & Yurtsev, B.A. (eds.) 2011. Annotated Checklist of the Panarctic Flora (PAF) Vascular plants. http://panarcticflora.org/ Löve, Á., Löve, D. & Raymond, M. 1957. Cytotaxonomy of Carex section Capillares. – Canadian Journal of Botany 35: 715–761.

Rønning, O.I. 1961. Some new contributions to the flora of Svalbard. – Norsk Polarinstitutts Skrifter 124: 1–20. Doi

Rønning, O.I. 1972. The distribution of the vascular cryptogams and monocotyledons in Svalbard. – Det Kongelige Norske Videnskabers Selskabs Skrifter 1972-24. 63 pp.

Schönswetter, P., Elven, R. & Brochmann, C. 2008. Trans-Atlantic dispersal and large-scale lack of genetic structure in the circumpolar, arctic-alpine sedge Carex bigelowii s. l. (Cyperaceae). – American Journal of Botany 95: 1006–1014. Doi 10.3732/ajb.2007196.

Schönswetter, P., Popp, M. & Brochmann, C. 2006. Central Asian origin of and strong genetic differentiation among populations of the rare and disjunct Carex atrofusca (Cyperaceae) in the Alps. – Journal of Biogeography 33: 948–956. Doi

Skjetne, I. 2012. Conservation genetics and ecology of four red listed vascular plant species in the high arctic archipelago of Svalbard. – M. Sc. Thesis, Univ. Oslo, Oslo.