Life span

Perennial, moderately long-lived.

Growth form

Solitary herb with basal caudex sparsely covered by marcescent leaf remains, single or branching into a few (rarely numerous), densely to loosely clustered rosettes forming tussocks. Each rosette potentially with one flowering stem (scape), up to 10 or more per tussock. Flowering stems ascending to erect, scarcely elongating during or after flowering, up to 10 cm, never observed with a leaf (except for occasional bracts), glabrous or rarely with sparse, small, multibranched, forked and/or simple hairs.

Leaf

Leaf rosettes usually 3–6 cm broad, sometimes broader. Leaves alternate, up to 10(13) × 5 mm, narrowly to broadly oblanceolate, acute, entire or very rarely with a few teeth, mid vein prominent (especially on withered leaves), dark green. Upper leaf surface glabrous; lower leaf surface distally with minute, multibranched hairs (sometimes very few or absent), sometimes with a few long, simple hairs along the mid vein; margin with long, stout, simple hairs or more rarely a few forked hairs up to 1 mm (sometimes absent). Simple hairs not strongly directed forward (a difference from D. fladnizensis).

Inflorescence

Inflorescence a short raceme with (3)5–8 flowers, more or less corymbose even in the fruit stage, up to 2.5 cm. Pedicels mostly 3–5 mm, more than half the fruit length, slender, attached with the scape at an angle of 40–60° but regularly curved upwards, glabrous.

Flower

Flowers radially symmetric with 4 free sepals and petals. Sepals up to 2 × 1.5 mm (i.e., less than two times as long as broad), ovate or more rarely elliptic, green or tinged red, mostly with white margins. Petals 3.5–4.0 × 1.75–2.0(2.5) mm, 2 times as long as sepals, often contiguous, patent (making the flower more open than in most other white-flowered species of Draba), obovate, slightly notched, milky white.

Fruit

Fruit a silicule 4–8 × 1.75–2.0(2.5) mm, erectopatent or spreading, lanceolate or rarely ovate, mostly acute, glabrous, dark green. Style mostly > 0.3 mm. Seeds 9–10 in each room, dark brown, ca. 1.2 × 0,8 mm.

Reproduction

Sexual reproduction by seeds; no vegetative reproduction. Flowers are comparatively large and adapted to pollination by insects. Draba lacteal is a mixed mater with a certain level of outcrossing (Brochmann 1993). Flowering and seed-set is regular in most years; mature seeds are often observed. Seeds germinate to ca. 70 % (Alsos et al. 2013). Seedling establishment is commonly observed, especially in disturbed sites, but must also be a regular feature in more stable sites.

There are no special adaptations to seed dispersal.

Comparison

Draba fladnizensis, D. subcapitata, D. nivalis, and partly D. lactea and D. norvegica, are the small-grown, white-flowered species of Draba in Svalbard and are often confused. Draba nivalis is, however, easily recognized by being (usually) densely grey-pubescent on leaves and scapes up to the pedicels with minute, regularly stellate hairs less than 0.2 mm broad and with ca. 8 patent to erectopatent branches (strong lens or microscope). The others have either no regularly stellate hairs (D. fladnizensis, D. subcapitata, D. lactea) or may have coarse stellate hairs with much fewer branches (D. norvegica partly). Irregularly multibranched hairs are typical of the stems of D. subcapitata, the leaves and stems of D. norvegica (but see below), and the distal parts of the lower leaf surface of D. lactea. Glabrous stems are typical of D. fladnizensis and D. lactea but the latter species may have stems sparsely pubescent with multibranched hairs. Short petals up to 2.5 mm distinguish D. fladnizensis, D. nivalis and D. subcapitata from the others, in the last-mentioned species also very narrow petals. The two others have longer petals, 3.5–5 mm. Draba norvegica may have pubescent fruits whereas all the others usually have glabrous fruits (pubescent fruits have been observed in a few D. nivalis and D. subcapitata). Draba lactea is usually characterized by leaves with coarse marginal hairs combined with minute multibranched hairs on the lower surface; this combination is unique for that species. The most problematic species to characterize is D. norvegica, probably due to the inclusion of two or more separate species in what currently is treated as a single species. Draba norvegica may be completely glabrous, as may be D. lactea, but then mostly characterized by an elongated infrutescence, fruits lanceolate with acute apex, and mid vein of leaves not conspicuously thickened. In D. lacteal the inflorescence is corymbose, the fruits oblong to lanceolate–elliptic, and the mid vein of the leaves conspicuously thickened.

Habitat

Occurs most frequently in rather moist growth sites such as next to water bodies (seepage, along brooks and rivers, or on lake shores), on moist patterned ground, in snowbeds, on moist tussocks in mires, but also quite common in less wet heaths among dwarf shrubs and graminoids. Observed to be a common species on moist road verges in the settlements of Longyearbyen, Barentsburg and Ny-Ålesund. Usually on fine textured sediments with poor to intermediate drainage and circumneutral to basic soil reaction (pH), but also sometimes reported from weakly acidic substrates. Growth sites range from moderately protected to slightly exposed. Probably little grazed by reindeer and geese.

Distribution

Common to frequent in all zones and sections. One of the most hardy of all Svalbard plants. The documented range is, however, restricted to Spitsbergen, Nordaustlandet and Barentsøya.

The general range is circumpolar in the arctic zones and in mountains in the northern boreal zones.

Comments

Draba lactea is a polymorphic, polyploid species shown by Brochmann et al. (1992) with fixed heterozygosity, indicating more than one, probably several, origins from hybridizations among diploid species (this study based on Svalbard and Scandinavian plants). The species has been reported to be uniformly hexaploid (2n = 48) but cytological and genetic studies have shown that also tetraploids (2n = 32) must be included in this species (Zhukova & Petrovsky 1984; Grundt et al. 2005). Russian authors have suggested that there are some morphological difference between the tetraploids and hexaploids, and they have partly assigned the former under the name D. pseudopilosa Pohle, based on Siberian plants. Grundt et al. (2005) did not find support for such a separation.

Several authors concerned with the American plants have claimed transitions between D. fladnizensis and D. lactea to be common (e.g., Hultén 1945, 1968; Scoggan 1978). Such claims are unfounded; natural hybrids are unknown and artificial hybrids sterile. The claims are based on an erroneous morphological concept of the two species (i.e., many plants of D. lactea misidentified as D. fladnizensis). Other authors have assumed the diploid D. fladnizensis to be parental, together with the diploid D. nivalis, in the predominantly hexaploid D. lactea (e.g., Knaben 1966). Also this claim seems to be unfounded and based on a provincial viewpoint that the hybridization behind the hexaploid D. lactea must have taken place among species present in the North Atlantic regions. Grundt et al. (2004) rather point towards an origin of D. lactea from the Beringian diploid and tetraploid (2n = 16, 32) D. palanderiana.

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., Soltis, D.E. & Soltis, P.A. 1992. Electrophoretic relationships and phylogeny of Nordic polyploids in Draba (Brassicaceae). – Plant Systematics & Evolution 182: 35–70.

Grundt, H.H., Kjølner, S., Borgen, L., Rieseberg, L.H. & Brochmann, C. 2006. High biological species diversity in the arctic flora. – Proceedings of the National Academy of Sciences 103: 972–975.

Grundt, H.H., Obermayer, R. & Borgen, L. 2005. Ploidal levels in the arctic–alpine polyploid Draba lactea (Brassicaceae) and its low-ploid relatives. – Botanical Journal of the Linnaean Society 147: 333–347.

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.

Hultén, E. 1945. Flora of Alaska and Yukon. V. Dicotyledoneae. Rhoeadales, Sarraceniales, Rosales I (Crassulaceae, Saxifragaceae). – Acta Universitas Lundensis, n. s., sect. 2, 41(1): 797–978.

Hultén, E. 1968. Comments on the flora of Alaska and Yukon. – Arkiv för Botanik, ser. 2, 7(1). 147 pp.

Knaben, G. 1966. Cytotaxonomical studies in some Draba species. – Botaniska Notiser 119: 427–444.

Scoggan, H.J. 1978. The Flora of Canada. 3. Dicotyledoneae (Saururaceae to Violaceae) (pp. 547–1115). – National Museum of Natural Sciences, National Museums of Canada, Ottawa.

Zhukova, P.G. & Petrovsky, V.V. 1984. Tsitotaksonomicheskoe izuchenie nekotorykh vidov krestotsvetich (Brassicaceae) iz severnoi Azii. – Botanicheskii Zhurnal 69: 236–240.