Life span

Perennial, potentially very long-lived.

Growth form

Mat-forming prostrate dwarf shrub with extensive branched subterranean stems, may form stands of 1 m² or possibly much larger. The subterranean stems are up to 0.5 cm thick and variable in colour: older parts dark greyish or reddish brown and younger parts light brown and thinner. Dormant buds 0.5 × 1 mm, bud scales made from a single leaf, broad, brown and obtuse. Leafy shoot at soil level ascending, short (1–3 cm).

Leaf

Leaves alternate. Petioles (2)3–6(8) mm, shorter than the blades. Blades up to 1.5(2) x 1.5(2) cm, rounded (obovate or orbicular) or slightly oblong, with entire margins, veins pinnate, not conspicuous and not reaching the margin, apex often notched, both surfaces dark green, glossy. Largely glabrous but sometimes with sparse white hirsute hairs on petioles, in blade margins and on blade surface.

Inflorescence and Flower

The inflorescences of Salix are more or less dense, pedunculate, many-flowered spikes. The plants are unisexual. The flowers are without any perianth but supported by bracts. The male flower consists of 2 stamens (in all arctic species, 1–5 or rarely more in some southern species) and a scale-like gland between the stamens and the spike mid axis. The female flower consists of a sessile or short-stalked, fused gynoecium of 2 carpels, with a style and a stigma with 2–4 branches, and a gland similar to that of the male flower. The fruit is a one-roomed capsule with numerous seeds with long hairs facilitating dispersal.

Spikes small, compact, 7–10 × 5–6 mm, with 6–11 flowers, on peduncles 3–10(15) mm terminal on short shoots, appearing at the same time as the leaves. Bracts 1.5–2.0 × 1.7–2.2 mm, obtuse, dark reddish brown on the distal half, pubescent with dense, long, hirsute hairs on the margins and ventral surface, dorsal surface sparsely pubescent. Anthers bright red before flowering, yellow at flowering. Gynoecium densely pubescent with white, hirsute hairs; stigmas dark red, deeply cleft into 4 stigmatic branches.

Fruit

The fruit is a dark reddish-brown capsule but often seeming grey due to its dense pubescence.

Reproduction

Sexual reproduction by seeds; local reproduction by underground shoots resulting in extensive mats that may break into fragments with age. Flowers early in the season. Wind pollinated, probably also insect pollinated, and strictly outcrossing. Produces large amounts of seeds which mature unusually early in the season (sometimes late July). Germination 80 % at 20°C and 10–30 % in the field (Alsos et al. 2013; Müller et al. 2011). Germination is drought and temperature sensitive. Seeds of Salix are rumoured to retain their germination ability for a very short time, days rather than weeks.

Seeds are dispersed partly by wind, due to the long hairs, partly by attaching to animals.

Comparison

Three dwarf willows are present in Svalbard: Salix herbacea, S. polaris and S. reticulata. Salix reticulata differs from the two others by being larger, having thick leaves with a network of veins impressed on the upper surface (‘reticulata’) and sharply raised on the lower surface, and by an obvious colour difference between the dark green upper leaf surface and the nearly white lower surface. Salix herbacea and S. polaris are most easily distinguished by the leaves of the former being crenate–dentate, whereas those of the latter are entire, and by the gynoecia and fruits of S. herbacea being glabrous, those of S. polaris white hirsute. Note, however, that there are hybrids between S. herbacea and S. polaris with leaves slightly dentate in the lower parts of their margins.

Habitat

Common and often dominant in heaths and slopes but particularly in snowbeds where it sometimes occur in pure stands. Also common on patterned ground and nearly all kinds of open ground. On mixed to fine textured substrates with good to poor drainage and almost any soil reaction (pH). More abundant and well developed in sites with snow protection but also quite common in moderately exposed sites. Probably grazed by reindeer.

Distribution

Common to dominating in the middle and northern arctic tundra zones. Stated to delimit the northern arctic tundra zone from the polar desert zone but there are many records from areas classified as polar desert, probably single plants and collected because they were rare in the sites. Occurs in all sections. One of the most widely distributed of all Svalbard plants and present on all major islands and many smaller ones.

The general range is Eurasian from Scandinavia and Svalbard east to E Siberia, in the arctic zones and in mountains in the boreal zones, in Europe south to S Norway.

Comments

Salix polaris and S. herbacea are related but differ in their ploidy levels: S. herbacea is diploid (2n = 38), S. polaris hexaploid (2n = 112), see Elven & Karlsson (2000) and Elven et al. (2011).

Intermediate forms between S. herbacea and S. polaris are commonly found on Bjørnøya and scattered along the west coast of Spitsbergen from Sørkapp Land at least north to Magdalenafjorden (Albert I Land), far north of the range of S. herbacea. These hybrids are recognized from S. polaris (which they often grow together with) by having leaves with a few teeth in the proximal parts of the blade margins and often in less hairy gynoecia and fruits. They are assumed to be remnants of hybridization that took place in the Holocene warm period, the Hypsithermal, 7000–4000 years BP, when S. herbacea had a wider distribution in Svalbard (Birks 1991). Similar hybrids in mainland Scandinavia have normal fruit-set and have a high level of seed germination (above 90 %; Elven 1974). They have been counted as tetraploid (2n = 76; Borgen & Elven 1983) and probably reproduce independent of their diploid and hexaploid parents. They also have a range outside that of the less frequent of their parents in Scandinavia, that is south and west of S. polaris, giving additional support for their independence. They have been given a binomial name as Salix nothula Notø.

Literature

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 changes in west Spitsbergen – plant macrofossils from Skardtjørna, an Arctic lake. The Holocene 1: 209–218.

Borgen, L. & Elven, R. 1983. Chromosome numbers of flowering plants from northern Norway and Svalbard. – Nordic Journal of Botany 3: 301–306.

Elven, R. 1974. Artsinnvandring og vegetasjonsutvikling på resente morener i Finseområdet. – Cand. real. Thesis, Univ. Oslo, Oslo.

Elven, R. & Karlsson, T. 2000. Salix L. – In: Jonsell, B. (ed.), Flora Nordica. 1. Lycopodiaceae to Polygonaceae: 117–188.

Elven, R., Murray, D.F., Razzhivin, V. & Yurtsev, B.A. (eds.) 2011. Checklist of the Panarctic Flora (PAF).

Hagen, D. 2002. Propagation of native Arctic and alpine species with a restoration potential. – Polar Research 21: 37–47.

Müller, E., Cooper, E.J. & Alsos, I.G. 2011. Germinability of arctic plants is high in perceived optimal conditions, but low in the field. – Botany 89: 337–348.