Life span

Perennial, moderately long-lived.

Growth form

Solitary herb with basal caudex densely covered by marcescent leaf remains (mainly mid veins), branching into often numerous, small, very densely clustered rosettes and forming small tussocks. Each rosette potentially with one flowering stem without leaves (scape), up to 15–20 stems per tussock. Flowering stems erect, sometimes very short at the beginning of the flowering, 0–1 cm, but elongating during and after flowering to 3(5) cm, moderately pubescent with a mixture of irregularly branched, forked and simple hairs.

Leaf

Leaf rosettes up to 6 cm but usually much smaller. Leaves alternate, small, up to 8 × 2 mm, narrowly elliptic or oblanceolate, acute (rarely rounded), entire, mid vein very prominent, pale to dark green. Upper leaf surface glabrous or with very sparse simple hairs; lower leaf surface glabrous or pubescent with scattered simple and irregularly branched hairs, mainly distally; margins with dense, long, and stout, simple hairs up to 1 mm, occasionally also with forked hairs distally.

Inflorescence

Inflorescence a short raceme of (2)3–5(6) flowers, not elongating much in the fruit stage (corymbose), up to 1 cm. Pedicels short (mostly < 2 mm), about half as long as fruit, slender, attached with the stem at an angle of 40–60° but regularly curved upwards, with the same type of pubescence as the stem.

Flower

Flowers radially symmetric with 4 free sepals and petals. Sepals 1 × 0.5 mm, (i.e., 2 times as long as broad), elliptic, reddish or purplish green with very narrow white margin. Petals very small and narrow, 2 × 1 mm, about 2 times as long as the sepals, not contiguous, narrowly obovate or spathulate, notched, erectopatent (making the flower half-open), white.

Fruit

Fruit a silicule up to 6 × 3 mm, erect or erectopatent, broadly elliptic, orbicular or sometimes oblong, rarely ovate, glabrous or rarely with sparse simple hairs, dark olive green or brown. Style absent or very short, 0–0.2 mm, but when present often prominent due to the shape of the fruit. Seeds 6–8 in each room, ca. 1 × 0.5 mm, medium brown.

Reproduction

Sexual reproduction by seeds; no vegetative reproduction. Highly selfing due to the very small, half-closed flowers (Brochmann 1993).Flowering and seed-set is regular and abundant in most years; mature seeds are regularly observed. Seeds germinate to 79 % in an experiment (Alsos et al. 2013).

There are no special adaptations to seed dispersal.

Comparison

When in flower, Draba subcapitata differs from all other Svalbard species by its very small and narrow, white petals. The combination of predominantly simple hairs on the leaves and multibranched hairs on the flowering stems is unique among Svalbard Drabas (both whites and yellows). The most similar species (in the flower stage) is D. fladnizensis: Draba fladnizensis has glabrous flowering stems, often with one stem leaf, and slightly broader (broadly obovate or spathulate) petals; whereas D. subcapitata has pubescent flowering stems, always without leaves, and narrower petals. The elliptic, oblong, or orbicular fruits with the short style are also characteristic of D. subcapitata.

Habitat

Dry ridges and outcrops, top and crevices of boulders, dry scree, but also in late (moist) snowbeds and on gravel plains in deltas and along rivers. The presence in both very dry and quite moist sites is surprising but perhaps conditioned by two features of this species: slow growth rate and low competitivity. It is only competitive in extreme sites with little vegetation cover, but quite stable substrates. Such conditions are found at the extreme ends of the moist–dry gradient and the snow cover gradient. Perhaps indifferent as to soil reaction (pH) but seems to be more common in areas with circumneutral or basic substrates.

Distribution

Cryophilous. Present and rather common in all zones and sections. Found on all major islands in the Spitsbergen group (Spitsbergen, Prins Karls Forland, Nordaustlandet, Barentsøya, Edgeøya) and on several harsh, outlying islands (Sjuøyane, Kvitøya, Kong Karls Land). Not recorded from Bjørnøya.

The general range is interrupted circumpolar in the northern arctic zones, only rarely reaching south to the arctic boundary (but present in a very few mountains in N Scandinavia).

Comments

The closest relatives of the diploid (2n = 16), circumarctic Draba subcapitata are two other diploids, the arctic–alpine D. fladnizensis and the C Asian D. altaica. All three differ, however, consistently genetically (Grundt et al. 2004) and represent three different diploid lineages. In spite of assumed predominant inbreeding, D. subcapitata is easily recognizable everywhere in the Arctic and without any discernible geographic structure, neither in morphology nor in genetics.

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.

Brochmann, C. 1993. Reproductive strategies of diploid and polyploid populations of Arctic Draba (Brassicaceae). – Plant Systematics & Evolution 185: 55–83.

Grundt, H.H., Popp, M., Brochmann, C. & Oxelman, B. 2004. Polyploid origins in a circumpolar complex in Draba (Brassicaceae) inferred from cloned nuclear DNA sequences and fingerprints. – Molecular and Phylogenetic Evolution 32: 695–710.