Solitary graminoid herb forming dense tussocks from an often richly branched caudex. Almost all leaves crowded terminally on caudex branches. Culms up to 3–5(10) cm, smooth.

Leaves (1.5)2–3(7) cm long, mostly shorter than the culm, about 1.5–2.5 mm broad at the base, erect and ending in an abrupt acute tip, revolute (slightly M-shaped in cross section), papillose, scabrous in margins and on dorsal mid vein, bright green.

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 1 terminal bisexual spike and 2–4 lateral female spikes. Lowest bract with a 5–8 mm green sheath and a blade of variable length (7–15 mm), not reaching above the inflorescence, upper bracts shorter than lower bracts; the long sheaths of the bracts conceal all or much of the peduncles. Spikes erect and often as pairs at different heights (a peculiar structure, rare if ever seen elsewhere within the genus of Carex). The apical bisexual spike with female flowers at the top (acrogynous), 6–8 × 2 mm, on an erect peduncle 5–9 mm, usually reaching a few millimetres higher than the uppermost female spike. Female spikes 4–7 × 2.5–3 mm, on 4–11 mm long peduncles. Scales obtuse, brown and with a narrow white hyaline margin and green mid vein. Perigynia trigonous, pear-shaped (ovate), brown on the sides and green along the margins and towards the beak, smooth at margins except for the beak; the beak ca. 0.4 mm, distinctly narrowed, often bending sidewards (oblique), scabrous. Stigmas three.

Triangular nut within the perigynium.

Sexual reproduction by seeds; no vegetative reproduction. Seeds are regularly produced and germinate to 67 % (Alsos et al. 2013).

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

Although being rather different, Carex capillaris and C. krausei belong to the same section. Carex krausei differs from C. capillaris in having a bisexual apical spike reaching above the female spikes, the female spikes are more erect and often at very different heights, the scales have a narrow hyaline margin, and the perigynia are dull and with a long, oblique beak. In C. capillaris the apical spike is male and sits between and beneath the female spikes (in ssp. fuscidula, different in other subspecies), the female spikes are nodding and sitting close together (more distant in other subspecies), the scales have a broad hyaline margin, and the perigynia are glossy and with a short, almost straight beak.

Both C. krausei and C. capillaris differ from C. glacialis in the shape of the perigynia (subrotund with an abruptly narrowed, very slender beak in C. glacialis), the length and colour of the bracts subtending the spikes (several millimetres long and green in the two former, about 1 mm long and hyaline in C. glacialis). However, all the three species form very dense and small tussocks with narrow, green leaves.

Growing in rather stable parts of steep, southfacing slopes and only on basic substrates. At both known sites Carex krausei grows in a Dryas octopetala – Carex fuliginosa ssp. misandra community with scattered Salix polaris, Bistorta vivipara and Saxifraga oppositifolia.

Thermophilous. Restricted to the middle arctic tundra zone and the weakly and clearly continental sections. Carex krausei has only fairly recently been discovered in Svalbard. The occurrences are at the southeast side of Stjerthøgda at Dicksonfjorden in James I Land in 1996 (Elven et al. 2001) and at two stands in Ringhorndalen at Wijdefjorden in Ny-Friesland in 2010 (Alsos & Brysting unpubl.). The Svalbard populations are the northernmost known for this species (79°20'N), much farther north than the northernmost ones in E Greenland (71°N) and Polar Ural (67?68°N).

In 2009 the population at Stjerthøgda was estimated to 600 individuals scattered over an area of 150 m along the hill and 60 from the top to the bottom of the slope (starting about 20 m downslope from the cliff and reaching almost to the base of the slope). The population consisted of 177 reproductive and 176 non-reproductive tussock in the largest field, 11 reproductive and 12 non-reproductive tussocks in a drainage channel 40 m farther south, a minimum of 50 reproductive and 50 non-reproductive tussocks 20 m south of the latter one, and some more in another drainage channel even farther south. At the densest parts, about 25 tussocks per square meter were counted (I.G. Alsos, S.W. Steen and B.E. Sandbakk, observ. 2009). The westernmost population in Ringhorndalen consisted of a minimum of 100 individuals within 15 × 15 m and scattered individuals over larger parts of the slope. The central Ringhorndalen population is extensive and very rich in individuals, estimated in 2015 to encompass probably more than 100,000 plants, the majority of these reproductive.

The species has an interrupted circumpolar distribution, mainly in arctic and northern boreal regions. In the North Atlantic regions, it is present in Iceland, NE Greenland and NE European Russia, besides Svalbard.

Carex krausei and C. capillaris belong to the quite small section Chlorostachyae where only four species are recognized from arctic regions. Several authors have been reluctant to accept C. krausei as different from C. capillaris at species level, but these authors seem mainly to be researchers little familiar with the two in the field. Field researchers never have had any problems keeping them apart and have never reported any dubious population or collection whatsoever. Several morphological studies (see Elven et al. 2011) and a genetic analysis (Skjetne 2012) shows that C. krausei and C. capillaris are separate species. The problem is rather the variation within each of these species (for C. capillaris, see that species).

A classical study of the Carex capillaris group is the revision by Löve et al. (1957). Several later investigators have been very reluctant to accept their results, especially as to chromosome numbers distinguishing between taxa (see the acerbic comments of P.W. Ball and R. Elven in Elven et al. 2011). Whereas C. capillaris is well documented as a polymorphic species with several defendable races, this is not the case with C. krausei. The races and species that have been proposed in its affinity (C. boecheriana Á.Löve, D.Löve & Raymond, C. capillaris var. major Drejer ex Blytt and var. robustior (Drejer ex Lange) Böcher, C. capillaris var. porsildiana Polunin = C. krausei ssp. porsildiana (Polunin) Á.Löve & D.Löve) are very difficult to defend when the supporting cytological evidence of Löve et al. (1957) is considered unreliable. The experiences of some of us who have seen the plant in several regions (R. Elven & H. Solstad in Siberia, Alaska, Canada, Greenland, and Svalbard) is that it is a rather monomorphic species and not readily divisible into races. Still, the effects of the faulty study of the Löves have had repercussions as late as in 2011; see Rune (2011) where C. krausei and C. capillaris var. robustior (or C. porsildiana) are treated as two independent taxa; we consider these names full synonyms.

It is surprising that a species of Carex with such a limited distribution in Svalbard has such a high level of germination of seeds. The majority of the rare and thermophilous species in Svalbard have poor or no recruitment (Alsos et al. 2003; Alsos et al. 2013). The high germination percentage suggests that its distribution is not limited by recruitment. As it has a circumpolar distribution, dispersal abilities are also unlikely to limit the distribution. Rather, the distribution may be limited by a narrow ecological amplitude as this species occurs with small, disjunct populations throughout its entire range (Hultén & Fries 1986).

Alsos, I.G., Müller, E. & Eidesen, P.B. 2013. Germinating seeds or bulbils in 87 of 113 tested Arctic species indicate potential for ex situ seed bank storage. – Polar Biology 36: 819–830. Doi 10.1007/s00300-013-1307-7.

Alsos, I.G., Spjelkavik, S. & Engelskjøn, T. 2003. Seed bank size and composition of Betula nana, Vaccinium uliginosum, and Campanula rotundifolia habitats in Svalbard and northern Norway. – Canadian Journal of Botany 81: 220–231. Doi 10.1139/b03-018.

Elven, R., Hansen, K.T. & Steen, S.W. 2001. Islandsstarr Carex krausei ny for Svalbard, og litt om arktisk hårstarr Carex capillaris ssp. fuscidula. – Blyttia 59: 186–189. Doi

Hultén, E. & Fries, M. 1986. Atlas of North European Vascular Plants North of the Tropic of Cancer. – Königstein: Koeltz Scientific Books.

Löve, Á., Löve, D. & Raymond, M. 1957. Cytotaxonomy of Carex section Capillares. – Canadian Journal of Botany 35: 715–761.

Rune, F. 2011. Wild flowers of Greenland. – Gyldenlund Naturforlag, Copenhagen.