Low shrub up to 10—20 cm tall, prostrate (in Svalbard) with rooting branches, forming large mats (clones). The shoot system consists of long and short shoots. Long shoots are terete to slightly angled, 2—4(10) mm broad, with dark grey bark. Young branches and twigs are densely pubescent with patent, white hairs up to 0.3 mm, the youngest twigs with red glands becoming black and warty on older branches, older branches becoming glabrous. Short shoots are up to 10 mm long, with numerous persistent, brown bud scales with hairy margins, with several leaves, and on some shoots with an inflorescence. Buds and very young shoots sticky.

Leaves alternate, deciduous. Petiole finely pubescent and very short, occasionally to 5 mm. Blade 5—9 × 5—9 mm (occasionally to 20 × 17 mm), orbicular or flabellate in outline, base broadly cuneate to (more rarely) truncate, margin crenate or shallowly lobed with ca. 1.5 mm long, obtuse, often angled lobes, pale green on lower surface, medium green on upper surface, turning intensely red in autumn, veins distinctly raised on lower surface, slightly raised on upper surface, both surfaces with yellow, very sticky glands, glabrous or with a few hairs at base towards the petiole.

Inflorescences are catkins with densely packed, 2—3-flowered cymes, each cyme supported by a 3-lobed scale formed from the bract and the two bracteoles. Catkins are unisexual but both sexes are present on the same plant. Male catkins occur along most of the twig, 5—10 mm, erect or patent, yellowish, each flower with 2 scale-like tepals and 2 bifid stamens. Female catkins are concentrated at the distal end of the twig, 6—12 × 3—5 mm, erect on 3—5 mm peduncles, deep red, flowers without perianth, gynoecium of 2 carpels with 2 stigmas.

Fruit a lens-shaped nut, 1.3—2 mm wide, with a narrow wing on each side.

Sexual reproduction by seeds; very local vegetative reproduction by layering and rooting of branches. Wind pollination. Plants are regularly observed with both male and female catkins, and fruits develop in Svalbard to a certain stage but mature nuts have not been observed. Immature nuts collected did not germinate in an experiment (Alsos et al. 2013). It is therefore uncertain whether the species reproduces by seeds in Svalbard under current climatic conditions. However, the stands may persist for very long periods (centuries or millennia) due to the rooting branches.

Fruits are wind dispersed (wings).

The only other Svalbard shrub or dwarf shrub with flat, deciduous leaves is Vaccinium uliginosum, different in almost all aspects (leaves entire, bluish or purplish green, very small, flowers single and bell-shaped, fruit a berry). For a comparison with mainland B. nana, see Comments.

Confined to heaths in gentle slopes with a dense and stable vegetation cover and almost no frost disturbance of the soil. As the plant is unable (or nearly unable) to reproduce under current climatic conditions in Svalbard, it is at present restricted to places with stable vegetation and soil conditions. Disturbance in the soil by frost movement, or landslides, will over time kill the plants. The substrate is usually rich in humus and the active layer in summer relatively deep. Moisture is available throughout the growth season due to melt-water from the permafrost; however, the species is never found in soaked or submerged sites and is sensitive both to winter ice and winter frost and abration (it demands a stable snow cover). It may be largely independent upon soil reaction (pH); however, within its distribution area in Svalbard, very acidic soils are almost absent. On the other hand, the humic soil it occurs in is never basic.

Thermophilous. Resticted to the middle arctic tundra zone and the weakly continental section. The entire range in Svalbard is confined to the Colesdalen—Adventdalen area on the north side of the peninsula between Isfjorden and Van Mijenfjorden (Nordenskiöld Land), within an area of 50 × 10 km. Betula nana is circumpolar in the southern arctic and the boreal zones in two major races. Subspecies nana is broadly amphi-Atlantic and reaches from easternmost Canada across Greenland, Iceland, Svalbard, and mainland Europe to NW Siberia. It is replaced in NE Siberia and most parts of North America (except for NE Canada and Greenland) by a rather different ssp. exilis (Sukaczev) Hultén. For the affinity and possible taxonomy of the Svalbard plant, see Comments.

The rather strange, very restricted current range of Betula nana in Svalbard may be explained in the two usual ways: either as a remnant from a previously larger range in the warmer parts of the Postglacial, or as result of a single successful dispersal event and establishment and subsequent spread to its present range. The compact range and the absence of this very easily found plant (sticking out in the landscape in August and September due to its beautiful autumnal colours, easily observed even from helicopter) in numerous assumed equally suitable valleys, could be arguments for the single establishment explanation. However, here palynological data tell another story. There are now numerous studies showing that Betula, and probably B. nana, were part of the vegetation over larger areas in western Spitsbergen in early Postglacial times (e.g., Birks 1991; Birks et al. 1994; numerous subsequent studies).

Several observers have noted that the Svalbard plant differs from the mainland Fennoscandian one, and not only in its smaller size. The mainland plant has more rounded leaves with a cordate to truncate base and shorter, more rounded lobes or teeth. It also lacks the red glands on the very young twigs. Jonsell (2000) noted these characters as being in common with ssp. exilis; however, ssp. exilis differs in many more characters. The Svalbard plant obviously belongs to ssp. nana if only two races are recognized. The special characters of the Svalbard plant are in common with plants from Novaya Zemlya and the Russian mainland in the surroundings of the Polar Ural. From these areas, Perfilev (1963) described a Betula tundrarum Perfil, later recombined as B. nana var. tundrarum (Perfil.) Elven in Elven et al. (2011). Variety tundrarum might be a recognizable race in arctic Europe and NW Siberia. Czerepanov (1966) accepted B. tundrarum from Svalbard (‘Spitsbergen’), whereas he rejected (justifiably, in our opinion) Perfilev's reports from Greenland and the Murman area. Molecular results (AFLP: Alsos et al. 2006; Eidesen 2007) connect the Svalbard plants most closely to the arctic Russian ones and not to the Fennoscandian ones (strengthening the ‘tundrarum’ connection). Both these groups are, however, much more similar to each other than they are to ssp. exilis in AFLP markers. The only reason we have here for not entering var. tundrarum as name for the Svalbard plant is that we don’t like several levels of subspecific taxa, and the differences between ssp. nana and ssp. exilis are at a higher taxonomic level than that between Svalbard–Russian var. tundrarum and mainland European plants.

Alsos, I.G., Eidesen, P.B., Ehrich, D., Skrede, I., Westergaard, K., Jacobsen, G.H., Landvik, J.Y., Taberlet, P. & Brochmann, C. 2006. Multiple and complex long-distance plant colonization of arctic islands. – Poster, Global Environmental Change: Regional Challenges, Beijing, 9–12. Nov. 2006.

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.

Birks, H.H. 1991. Holocene vegetational history and climatic change in west Spitsbergen – plant macrofossils from Skardtjønna, and Arctic lake. – The Holocene 1: 209–218.

Birks, H.H., Paus, Aa., Svendsen, J.I., Alm, T., Mangerud, J. & Landvik, J.Y. 1994. Late Weichselian environmental change in Norway, including Svalbard. – Journal of Quaternary Science 9: 133–145.

Czerepanov, S.K. 1966. Betulaceae. – In: Tolmachev, A.I. (ed.), Flora Arctica URSS. V. Salicaceae–Portulacaceae: 119–138.

Eidesen, P.B. 2007. The unfaithful birches – what is a species in phylogeography? – Paper in: Eidesen, P.B., Arctic–alpine plants on the move – Individual and comparative phylogeographies reveal responses to climate change. – Dr. Sci. thesis, Univ. Oslo, Oslo.

Elven, R., Murray, D.F., Razzhivin, V. & Yurtsev, B.A. (eds.) 2011. Annotated Checklist of the Panarctic Flora (PAF) Vascular plants. http://panarcticflora.org/

Jonsell, B. 2000. Betula L. – In: Jonsell, B. (ed.), Flora Nordica. 1. Lycopodiaceae to Polygonaceae: 197–203.

Perfiljev, I.A. 1963. Karlikovaya berezka Arkhangelskikh tundr. — Botanicheskii Zhurnal 48: 1138—1150.