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 much larger. The subterranean stems are up to 0.5 cm thick, variable in colour, older parts are dark greyish or reddish brown and younger parts light brown and slimmer. Dormant buds 0.5–1 × 1–1.2 mm, bud scales made from a single leaf, broad, brown and obtuse. Leafy shoots at soil level ascending, short (1–3 cm).

Leaf

Leaves alternate. Petiole (2)3–5(6) mm, much shorter than the blade. Blades up to 1.5(2) × 1.5(2) cm, orbicular, oblong or obovate with crenate or shallowly dentate margins with teeth curved forwards, veins pinnate, conspicuous but not reaching the blade margin, apex usually notched, surfaces glossy, upper surface medium green, lower surface paler green. 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 or loose, 4–5.5 × 2.5–3.5 mm, terminal on short shoots with peduncles 2–3 mm, with 5–7 flowers, appearing at the same time as the leaves. Bracts 1.3–1.5 × 1.5–1.7 mm, ovate or rounded, obtuse, pink or purplish, with a few hairs in the margins and on the ventral surface, dorsal surface glabrous. Anthers bright red before flowering, yellow at flowering. Gynoecium glabrous, dark red; styles dark red, deeply cleft into 4 stigmatic branches.

Fruit

The fruit is a dark reddish-brown capsule.

Reproduction

Sexual reproduction by seeds; local vegetative reproduction by underground stems into mats that may become fragmented with age. Salix herbacea flowers early in the season. It is strictly outcrossing due to being dioecious. By many authors assumed to be wind pollinated but often visited by insects (R. Elven and many others, observ.). Produces large amounts of seeds which ripen unusually early in the season (sometimes late July). Germination may be 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 partly dispersed 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

In heaths, on slopes and in snowbeds where it sometimes grow in pure stands. Also on patterned ground. On mixed to fine textured substrates with good to poor drainage and almost any soil reaction (pH). Most abundant and well developed in sites with snow protection but also quite common in moderately exposed sites. Probably grazed by reindeer.

Distribution

Thermophilous. Middle (Bjørnøya) and northern arctic (Spitsbergen) tundra zones; weakly oceanic (Bjørnøya) to transitional (Spitsbergen) sections; however, we think that the western coastal areas of Spitsbergen have been misclassified and perhaps should be assigned to the weakly oceanic section. Common on Bjørnøya. Documented from a few sites on Spitsbergen in W Sørkapp Land and S Wedel Jarlsberg Land. Belongs to a west coast element with, e.g., Cerastium alpinum, Luzula arcuata, Ranunculus glacialis and Rhodiola rosea.

The general range is amphi-Atlantic in the southern arctic and northern boreal zones (and in the alpine belts farther south), distributed from NE Canada across Greenland, Iceland, and Svalbard to N and C Europe and N European Russia.

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.

An interesting study of the past and future (!) biogeography of Salix herbacea is Alsos et al. (2009).

Literature

Alsos, I.G., Alm, T., Normand, S. & Brochmann, C. 2009. Past and future range shifts and loss of diversity in dwarf willow (Salix herbacea L.) inferred from genetics, fossils and modelling. – Global Ecology & Biogeography 18: 223–239.

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.