Life span

Perennial, potentially very long-lived.

Growth form

Solitary dwarf shrub with one central root and numerous prostrate, weakly rooting branches, growing as dense mat through secondary branching, often covering up to a square meter or more.

Leaf

Leaves alternate, ever-green, on short lateral branches. Petioles (3)4–8(19) mm, as long as blades or shorter, slender, with long, linear stipules adherent for 1/3–1/2 of its length. Blades (5)7–10(12) × 2–4.5(6) mm, oblong or ovate; margins crenate in their entire lengths with 4–6 pairs of teeth or subentire at apex but sometimes strongly revolute and seemingly subentire; upper surface dark green, dull or glossy, glabrous or more rarely pubescent, without or with a few pellucid glands, veins indented; lower surface densely covered by white or greyish hairs, on the raised main vein and sometimes on lateral veins with long, branched (feathery) hairs (the so-called "octopetala scales"), sometimes with a few, stalked, red or yellow glands. Often also branched hairs and glands present on petioles.

Inflorescence

Flowers singly on erect pedicels from lateral branch ends.

Flower

Pedicels 1–5 cm, lengthening in fruit stage, sometimes to 10 cm or more, densely pubescent with entangled white hairs, distally also with stout, purplish hairs. Flowers radially symmetric with (6)8(9) free sepals and petals, cup-shaped. Sepals nn × nn mm, 1/2–2/3 as long as petals, linear or narrowly lanceolate, dorsally pubescent with entangled, short, white hairs and stout, purplish hairs. Petals 1–2 × nn cm, obovate, white or cream, rarely pink. Stamens numerous in a ring surrounding a flat discus. Carpels numerous, free.

Fruit

Numerous nutlets, each with a feathery style elongating strongly in fruit stage.

Reproduction

Sexual reproduction by seeds; no vegetative reproduction. Large mats or carpets have rooting branches; however, the roots of the branches, even if they provide stability and additional water and nutrients, do not seem capable of supporting separate individuals if the main root dies. The growth form is therefore to be defined as solitary (see above). Flowering and seed-set may be susceptible to bad weather in the flowering period, resulting in stunted, immature fruit heads. Both fruit maturation and seed germination is temperature sensitive (Wookey et al. 1995; Müller et al. 2011), and seed banks are found only on sites with favourable temperatures (Cooper et al. 2004). Reproduction may also be reduced by reindeer grazing on flowering shoots (Cooper & Wookey 2003). However, frequent presence of seedlings and small, young plants on new river bars suggests that reproduction is efficient to its distributional limits. Seeds (or rather fruits) germinated to ca. 25 % in an experiment (Alsos et al. 2013).

Fruits are easily spread by wind over appreciable distances, due to the retention of their feathery styles.

Comparison

There is nothing similar to this species in Svalbard.

Habitat

Dryas octopetala grows in dry to slightly moist heaths, on patterned ground, old river terraces, steep slopes, and stabilized scree. It is often the dominant plant in its communities, both physiognomically and ecologically. It stabilizes the ground, regulates the percolation of water due to the dense mats, produces large amounts of litter, and is mycotrophic and host of some semiparasites (especially Pedicularis dasyantha). The species is largely indifferent as to soil reaction (pH) in Svalbard but avoids the most acidic areas. The substrate may be fine-grained (loam, sand) or coarse, but usually with good drainage. Due to the root system, the species avoids sites with strong soil movement, both cryoturbation and solifluction.

Distribution

Ubiquitous in the middle and northern arctic tundra zones and a diagnostic species for the limit between the northern arctic tundra and the polar desert. Present in all sections but less dominant to sparse in the clearly continental section (in the arctic steppe vegetation). Found on all major islands of the Svalbard group but not recorded from Bjørnøya.

The general range of Dryas octopetala depends on how widely the species is circumscribed. Some authors consider it widely Eurasian and also as reaching NW North America (and a small enclave in E Greenland). Other authors consider it mainly European and NW Siberian (and in the E Greenland enclave) and as replaced by other species in E Siberia and NW North America. See Comments.

Comments

Dryas is a complicated genus at a circumpolar scale. There has been little agreement as to its taxonomy and subdivision. From three to more than 15 species have been proposed, depending on author (e.g., Hultén 1959, 1971; Yurtsev 1984, 1997; summarized by Elven et al. 2011). Whereas Russian authors mostly have accepted numerous species, European and North American authors have been reluctant to accept more than 3–4 species (Porsild 1947 and Porsild & Cody 1980 being exceptions, accepting many American species). In recent years a compromise solution, accepting many but not all of the Russian proposals, have won favour (see, e.g., Springer & Parfitt 2014).

Even in Svalbard, three taxa have been proposed present (Yurtsev 1997): Dryas octopetala ssp. subincisa Jurtz., D. punctata Juz. ssp. punctata, and their hybrid D. x vagans Juz. Yurtsev suggested that D. octopetala ssp. subincisa could have its origin in a hybrid between D. octopetala s. str. and the Siberian D. incisa Juz. He assumed D. octopetala in a strict meaning to be absent from Svalbard. The matter depends (in the main) on the interpretation of the glands and hairs on the leaves. The pellucid upper surface glands and the stipitate lower surface glands are, according to Yurtsev, diagnostic for D. punctata, whereas the feathery hairs and absence of glands are diagnostic for D. octopetala s. str. Where these characters are combined, Yurtsev assumed hybrids. Later investigators have confirmed the morphological traits indicated by Yurtsev but have not been able to find any pattern of genetically or morphologically distinctive entities (Siegismund & Philipp 1999; Skrede et al. 2006). Skrede et al. (2006) showed that Eurasian Dryas divided on two genetic lineages. Svalbard Dryas was connected eastwards to arctic Russia, whereas nearly all Scandinavian Dryas was connected southwards to C and W Europe (the exception being some plants in Finnmark, NE Norway). The two lineages do, however, not correspond to the taxa as circumscribed by Yurtsev. Consistent morphological differences between the lineages have not yet been found. The current morphological evidence supports only one species in Europe (including Svalbard) – D. octopetala – and the presence of glands together with feathery hairs seems to be gene geography rather than taxonomy.

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.

Cooper, E.J., Alsos, I.G., Hagen, D., Smith, F.M., Coulson, S.J. & Hodkinson, I.D. 2004. Recruitment in the Arctic: diversity and importance of the seed bank. –  Journal of Vegetation Science 15: 115–124.

Cooper, E.J. & Wookey, P.A. 2003. Flora herbivory of Dryas octopetala by Svalbard reindeer. – Arctic, Antarctic, and Alpine Research 35: 369–376.

Hultén, E. 1959. Studies in the genus Dryas. – Svensk Botanisk Tidskrift 53: 507–542.

Hultén, E. 1971. The circumpolar plants. II Dicotyledons. – Kungliga Svenska Vetenskaps-Akademiens Handlingar, ser. 4, 13(1). 463 pp.

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.

Porsild, A.E. 1947. The genus Dryas in North America. – Canadian Field-Naturalist 61: 175–192.

Porsild, A.E. & Cody, W.J. 1980. Vascular plants of Continental Northwest Territories, Camada. – National Museum of Natural Sciences, National Museums of Canada, Ottawa.

Siegismund, H. & Philipp, M. 1999. The genus Dryas on Svalbard: morphological and genetic evidence for a single species, Dryas octopetala L. – Skrifter Norske Videnskaps-Akademi. I. Matematisk Naturvitenskapelig Klasse, n. s. 38: 269–279.

Skrede, I., Eidesen, P.B., Portela, R.P. & Brochmann, C. 2006. Refugia, differentiation and postglacial migration in arctic-alpine Eurasia, exemplified by the mountain avens (Dryas octopetala L.). – Molecular Ecology 15: 1827–1840.

Springer, J.C. & Parfitt, B.D. 2014. Dryas Linnaeus. – In: Flora of North America Editorial Committee (eds.), Flora of North America North of Mexico. 8. Magnoliophyta: Picramniaceae to Rosaceae: 326–331,

Wookey, P.A., Robinson, C.H., Parsons, A.N., Welker, J.M., Press, M.C., Callaghan, T.V. & Lee, J.A. 1995. Environmental constraints on the growth, photosynthesis and reproductive development of  Dryas octopetala at high Arctic polar demi-desert, Svalbard. – Oecologia 102: 478–489.

Yurtsev, B.A. 1984. Dryas L. – In: Yurtsev, B.A. (ed.), Flora Arctica URSS. IX, 1. Droseraceae–Rosaceae: 250–286.

Yurtsev, B.A. 1997. Analysis of evolutionary differentiation in some key arctic–alpine taxa: Dryas, Oxytropis sect. Arctobia and Taraxacum sect. Arctica. – Opera Botanica 132: 27–37.