Solitary or slightly mat-forming herb with horizontal creeping and branching rhizome with some distance (typically 2–4 cm) between aerial shoots, thereby growing in loose stands. Aerial shoots erect, either short vegetative shoots or 5–10 cm long (in Svalbard) reproductive shoots, smooth, with 3–5 prophylls (reduced leaves without or with very short blades) at base of culms and 3–4(5) culm leaves with extended blades.

Leaves 2–5 cm, canaliculate, with long, reddish or green sheaths changing gradually into blades (and therefore nearly impossible to measure separately), with no auricles. Blades smooth or slightly ribbed.

Inflorescence of 1–2 heads, when 2 separated on the culm by 0.5–1 cm. Each head with 3–5 flowers at about the same level. Inflorescences subtended by several bracts. The lowermost bract 1–3 cm, firm, with a narrow blade protruding well above the flowers. The other bracts 0.4–1.0 cm, mostly narrowly lanceolate, acuminate with an extended apex, pale brown and hyaline with darker brown veins.

Flowers radially symmetric. Perianth of 6 (3 + 3) tepals. Tepals subequal, 5–7 mm, narrowly lanceolate, acuminate, dark reddish brown with paler apices, nearly as long as fruit. Stamens 6, as long as tepals; anthers narrow, 0.9–1.2 mm long. Gynoecium of 3 carpels with 3 stigmas.

Fruit a one-roomed capsule, 3–5(7) mm, gradually narrowing into an acute to acuminate apex, shiny, golden brown, darker in apex, with style 0.9–1.2 mm, with numerous seeds.

Sexual reproduction by seeds; very local vegetative reproduction by rhizome. Adapted to wind pollination (protogyny with stigmas appearing earlier than stamens). All plants observed in Svalbard are in early flowering, at best, and it is very uncertain (read: rather improbable) whether seeds are produced under the present climatic conditions in Svalbard. This suggests that the present, very restricted range in Svalbard is due to survival of small, now non-reproducing stands in a few favorable sites.

If seeds are produced, they are dispersed ballistically for at best a few decimetres (stiff stems lasting into early winter with capsules arresting the seeds until abrupt movements) and by wind (the seeds are very small and light) or birds.

Juncus leucochlamys differs from the other Juncus of Svalbard in slightly rhizomatous growth (tussocky in J. albescens and J. biglumis, with long rhizomes in J. arcticus) and with most leaves cauline (i.e., on the culm, whereas they are mainly basal in J. albescens and J. biglumis, reduced to prophylls on assimilating stems in J. arcticus).

For differences from the mainland Fennoscandian J. castaneus, see Elven et al. (2010) and Comments below.

Only occurring in a few shallow mires on basic substrates. Always in moist (but not permanently wet) sites, mostly sloping with some downstream percolation of permafrost melt water.

Thermophilous. Middle arctic tundra zone and weakly continental section. Known only from Spitsbergen from two mires near Longyearbyen (Nordenskiöld Land), one of these intact, from one or two mires in the lower parts of the Sassen valley (Sabine Land), and one or two mires in the lower parts of the Gipsdalen valley (Bünsow Land), all these at the inner parts of Isfjorden.

The global range is mainly North American, including Greenland, N Canada and Alaska. It extends into NE Asia and across the North Atlantic to Svalbard and W Iceland (see Comments).

Whether Juncus leucochlamys is a taxon, either a species or a subspecies of J. castaneus, has been disputed. We (H. Solstad & R. Elven) here build on experiences from several arctic regions and refute the treatments of those who do not recognize it (e.g., Brooks & Clemants 2000). Our conclusion is heavily influenced by our experiences with the plants in Iceland, a region where the two nearly meet (see Elven et al. 2010). The Juncus castaneus group occurs in two parts of Iceland, in the northwest and in the east, with very few plants in between. We made a 'blind test' in the two Icelandic herbaria (Reykjavik and Akureyri), where one of us covered the label and asked the other to tell whether the plant came from eastern or western Iceland. All plants were 'correctly' assigned as the differences are visible at a glance (and seen even from a car driving at 60–80 km/h!). As Iceland is the only place in the world where we really have proved a near sympatry, this amateur test has some value. Previous conclusions that the two taxa intergrade (Hultén 1968; Brooks & Clemants 2000) are, in our view, based on a study of only one of them (J. leucochlamys) combined with applying non-diagnostic characters (see Elven et al. 2010).

Russians accept two species, Americans generally do not. We have inspected plants from NE Asia (Chukotka), North America, Greenland, and N Europe in the field and all specimens in main herbaria of Alaska (ALA), Canada (CAN & DAO), Iceland (AMNH & ICEL), and Norway (O, TRH & TROM) and have reached a conclusion that may explain the divergence between the Russian and North American treatments (Elven et al. 2010). We accept two species: J. castaneus and J. leucochlamys. We find all material we have inspected from NE Asia, North America, Greenland, and Svalbard to differ in some consistent features (referred by Elven et al. 2010, 2011) from the material from northern mainland Europe and NW Siberia: In J. leucochlamys the tepals are longer and subequal, the tepals and capsule are of subequal length, and the capsule is uniformly much longer and tapering and distinctly beaked; in J. castaneus s. str. the tepals are shorter and the inner ones much shorter and subobtuse, and much shorter than the capsule. Furthermore, the capsule is much shorter and abruptly narrowed and with a short beak. Other reported characters do not differentiate fully as reliably as those above. Long bracts are more typical of J. leucochlamys than of J. castaneus s. str.; inflorescences with several levels are more common in J. leucochlamys as are paler tepals and capsules, but dark and one-level inflorescences are common also within what we now accept as J. leucochlamys and several levels may occur in J. castaneus s. str.; leaf width does not separate even if some specimens of J. leucochlamys are conspicuously broad-leaved. Following our criteria, the two taxa become nearly allopatric (i.e., non-overlapping in their ranges). The reason why the Russians accept them is that they really have two species. The problems of separation in North America (as discussed by Hultén 1968) are then resolved as only one species is present and no separation is needed.

Juncus castaneus s. str. is restricted to N Europe (including E Iceland) and NW Siberia east to the Taimyr Peninsula, whereas the geographically major taxon is J. leucochlamys, distributed in NE Siberia eastwards from Taimyr, the Russian Far East, North America, Greenland, NW Iceland, and Svalbard. We have not yet seen intermediates in the inspected material. However, we have not seen much material from the possible meeting zone in N Siberia. The ranges we report for the two species deviate strongly from those mapped by Kirschner et al. (2002, for two subspecies).

There may be two hypotheses for the presence of J. leucochlamys in Svalbard: either repeated introductions or reduction from a larger range in earlier, climatically more favourable parts of the Postglacial (the Hypsithermal 7000–4000 years ago). The places where the species has been found are among the climatically most favorable shallow mire sites known in Svalbard. The chances that seeds were dispersed from somewhere else (probably Greenland) and should hit just these places, are neglectable. We therefore recline to the hypothesis of relict populations in favourable sites in Svalbard.

Brooks, R.E. & Clemants, S.E. 2000. Juncus Linnaeus. – In: Flora of North America Editorial Committee (eds.), Flora of North America north of Mexico. 22. Magnoliophyta: Alismatidae, Arecidae, Commelinidae (in part), and Zingiberidae: 211–255.

Elven, R., Murray, D.F. & Solstad, H. 2010. To arter av kastanjesiv (Juncus castaneus og Juncus leucochlamys), begge i Europa. – Blyttia 68: 128–131.

Hultén, E. 1968. Comments on the flora of Alaska and Yukon. – Arkiv för Botanik, ser. 2, 7(1). 147 pp.

Kirschner, J. et al. 2002. Species plantarum: flora of the world. Part 7. Juncaceae: Juncus subg. Juncus. – Australian Biological Resources Study, Canberra.