Graminoid herb forming dense mats from an extensive system of branched rhizomes. Rhizomes slender, (1.5)2–2.5 mm broad. Length of rhizome segments 1–3.5 cm. Aerial shoots ascending. Culms 4–7 cm, smooth, rarely taller than the leafy shoots.

Leaves 5–10 cm long and often taller than the culms, (1.5)1.6–1.7(1.8) mm broad at the base, ending in pale brown, narrow tips, convolute (M-shaped in cross section), green to bluish green on both surfaces, very slightly papillose. 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 3–4 spikes, the upper one male 7–11 × 2.5 mm, sessile; the others female, 9–11(12) × 2–2.5 mm on 0–0.4 cm long peduncles, erect, with 12–16 flowers. Bracts with black scarious auricles at bases. Bract of the lowermost spike (1.5)1.7–2.3 cm, about equal to or exceeding the spike. Bract of the second lowermost spike usually a scale of 3–3.5 mm, much shorter than the spike. Scales of the female spikes obtuse, dark brown with a narrow mid vein light brown to greenish nearly to the apex. Scales of the male spike more obtuse than on the female ones and with a hyaline margin. Perigynia 1.5–2 × 1.3–1.5 mm, lenticular, with indistinct beak ca. 0.1 mm, smooth at margin, papillose, light reddish brown. Two stigmas.

Lenticular nut within the perigynium.

Sexual reproduction by seeds, theoretically; local vegetative reproduction due to the rhizome. Predominantly or entirely outcrossing due to protandry, and Carex bigelowii shows this feature better than many other Carex. No ripe fruits have been observed in Svalbard C. bigelowii in 1997 (R. Elven observ.) or 2008 (Alsos et al. 2013), see Comments.

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

The section Phacocystis includes three species in Svalbard: Carex bigelowii, C. concolor (synonym: C. aquatilis ssp. stans) and C. subspathacea. Carex subspathacea differs from the two others in very short culms and leafy shoots being clearly longer than the culms, in much fewer flowers in each spike, in reddish leaves and culm, and in reddish brown scales and perigynia (at least in mature stage). Carex concolor differs from C. bigelowii in being much stouter in all parts, especially with much broader rhizomes, with a persistent sheath of leaves from previous years at the base of both leafy and flowering shoots, much broader leaves (3–5 mm), and all bracts much extending above the spikes.

Grows in a mesic mires at its two known sites in Svalbard

Strongly thermophilous. The confirmed stands are located in the middle arctic tundra zone and the weakly and clearly continental sections. A single stand was until recently the only known locality in Svalbard, near Svalbard airport at Longyearbyen (Nordenskiöld Land). The stand was more extensive before the airstrip was build (Elven & Elvebakk 2002). Today, it consists of patches a few meters apart within an area of less than 100 × 100 m (I.G. Alsos, observ. 2009). In 2017 a large stand was reported from Ringhorndalen at Wijdefjorden in northern Spitsbergen (Ny-Friesland). Specimens from this stand have been inspected ans conform to this species and subspecies. There is also a herbarium collection from 2008 from Austfjordneset, farther south at Wijdefjorden, but we have not yet inspected specimens from this site.

The species has been reported from more sites in the Isfjorden area, and also from the Kongsfjorden area (Blomstrandøya), but there is no confirmation by specimens known to us. Several collections previously identified as C. bigelowii (see map in Rønning 1972) have later been re-identified as C. subspathacea.

The general range of Carex bigelowii ssp. arctisibirica reaches from Svalbard and arctic NE European Russia eastwards to Taimyr, but probably with transition to ssp. ensifolia (Turcz. ex Ledeb.) in Siberia. The connection of the Svalbard plant is rather obviously to the east.

The section Phacocystis is among the more complicated groups of Carex and its species are mostly polymorphic, often both regionally and at a local scale. Carex bigelowii s. lat. is circumpolar and arctic–alpine and found, under different names, in the majority of northern regions of the world. Schönswetter et al. (2008) found three main clades in a molecular study. One clade is distributed mainly in NE North America and Greenland but reaches Iceland and probably some parts of Scandinavia. This clade includes the type region of C. bigelowii s. str. (White Mountains in New Hampshire) and is ssp. bigelowii (C. hyperborea Drejer, C. bigelowii ssp. hyperborea (Drejer) Böcher) if subspecies are recognized. Another clade is mainly European and constitutes the majority of the plants in European mountains, including Scandinavia. This clade is ssp. dacica (Heuff.) T.V.Egorova and also includes the type regions of C. rigida Gooden. (C. bigelowii ssp. rigida (Gooden.) W.Schultze-Motel, Scotland) and ssp. nardeticola Holub (the Carpathian Mountains). We suspect that this clade may be present also in Iceland, E Greenland and Jan Mayen. The third clade is the most widespread, reaching from NE Europe across all of N Asia, Beringia, and into W Canada. A multitude of names at levels of species and subspecies belong in this clade, among them the two most widely used ones: C. ensifolia (Russian–Siberian) and C. lugens Holm (Beringian–American). The Svalbard plant belongs in this third clade. Its correct name as subspecies should probably be ssp. ensifolia but there may be problems with the priority of that name. For now, we apply the name ssp. arctisibirica based on a type from Taimyr in N Siberia. Both C. ensifolia s. str. and C. bigelowii ssp. arctisibirica differ markedly from the two other European subspecies in their very narrow leaves (the reason for the name 'ensifolia') and some details in the spikes and perigynia. Also the very short rhizome segments, making the Svalbard plant tussocky rather than mat-forming, is a difference from both ssp. bigelowii and ssp. rigida but in common with Siberian and Beringian plants (among these especially C. lugens, a main constituent of the infamous tussock tundra of Beringia).

The origin of the Svalbard populations is most probably as a relic from 8000–4000 BP, when the climate in Svalbard was 1–2°C warmer than today. The earliest herbarium vouchers from the Hotellneset site date from 1898, so its origin is not related to construction of the airstrip (and its affinity to NE Russian plants is incompatible with a recent introduction by Russian miners from W Russia or Ukraine). The lack of seed ripening under current climatic conditions suggests that the discontinuous local populations are results of either several independent initial germination events or fragmentation of a previously large clone; we strongly suspect the latter and that the absence of seed-set is due to genetic incompatibility. Clones of C. bigelowii s. lat may become well over 3000 years old (Jónsdóttir et al. 2000). Fragmentation of large clones has been observed in other thermophilous relic plant species in Svalbard (Alsos et al. 2002; Alsos et al. 2007) and we find it likely that also C. bigelowii is a relic from the warm periods of the Holocene.

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., Eidesen, P.B., Ehrich, D., Skrede, I., Westergaard, K., Jacobsen, G.H., Landvik, J.Y., Taberlet, P. & Brochmann, C. 2007. Frequent long-distance colonization in the changing Arctic. – Science 316: 1606–1609. Doi 10.1126/science.1139178

Alsos, I.G., Engelskjøn, T. & Brochmann, C. 2002. Conservation genetics and population history of Betula nana, Vaccinium uliginosum, and Campanula rotundifolia in the arctic archipelago of Svalbard. – Arctic, Antarctic, and Alpine Research 34: 408–418.

Elven, R. & Elvebakk, A. 2002. Sibirstarr Carex bigelowii ssp. arctisibirica på Svalbard – og noe om stivstarr-komplekset C. bigelowii coll. – Blyttia 60: 50–58.

Jónsdóttir, I.S., Augner, M., Fagerström, T., Persson, H. & Stenström, A. 2000. Genet age in marginal populations of two clonal Carexspecies in the Siberian Arctic. – Ecography 23: 402–412. Doi 10.1111/j.1600-0587.2000.tb00297.x.

Rønning, O.I. 1972. The distribution of the vascular cryptogams and monocotyledons in Svalbard. – Det Kongelige Norske Videnskabers Selskabs Skrifter 1972-24: 1–63.