Life span

Perennial, moderately long-lived.

Growth form

Mat-forming herb growing in moderately dense stands with a horizontal woody branched rhizome. Rhizome branches short, ending in a caudex densely covered by black marcescent leaf remains, mainly petioles and stipules. Each caudex branch with a terminal basal rosette. Flowering stems axillary, 0.5–1.5 cm, usually shorter than the leaves, pubescent. Entire plant green or yellowish green.

Leaf

Leaves alternate. Petioles 0.5–0.7(0.8) cm with appressed smooth white hairs. Stipules large, 2.3–2.7 × 1.7–2.2 mm, ovate, glabrous, venation reticulate, pale green with white hyaline margins. Blades ternate (i.e., with three leaflets from one point). Leaflets 7–10 × 3–5 mm, narrowly obovate, cuneate at base, with terete apex with 3 teeth, with subappressed white smooth hairs. Lower leaf surface slightly paler than upper surface.

Inflorescence

Inflorescence a few-flowered cyme. Flowers with bracts ca. 2 × 1 mm, narrowly ovate with stipules of about the same size and shape.

Flower

Flowers radially symmetric, perigynous (perianth at the side of the gynoecium), with 5 free epicalyx bractlets, sepals and petals. Hypanthium ca. 1 × 2 mm, broadly cuneate, with white hairs and minute, yellowish green, sessile glands. Epicalyx bractlets ca. 2 × 0.5 mm, linear or narrowly lanceolate, acute. Sepals 2.7–3.2 × 1.0–1.3 mm, ovate, acute. Both epicalyx bractlets and sepals with white hairs. Petals 1.6–1.9 × 0.7–0.9 mm (i.e., smaller than sepals), narrowly obovate, pale yellow. Stamens (4)5(8). Filaments ca. 0.7 mm. Anthers ca. 0.5 mm broad. Gynoecium of 5–10 free carpels. Styles subbasal (attached near the base of each carpel), slender cylindrical, ca. 0.3 mm.

Fruit

Nutlets, ca. 1.2 × 1 mm, ovoid, olive green or brown, slightly rugulose, glossy.

Reproduction

Sexual reproduction by seeds; very local vegetative reproduction by fragmentation of rhizome. The single known population in Svalbard (Rønning 1961) had abundant flowers and more or less ripe fruits when first discovered in 1960. Ripe fruits were observed also on 3. Aug. 2009 (I.G. Alsos, R. Elven et al. observ.). The small flowers are adapted to self pollination and seed set is probably regular.

There is no special adaptation to dispersal.

Comparison

Sibbaldia procumbens may be mistaken for a Potentilla (and these genera are closely related). The ternate leaves are shared with several species of Potentilla but all these have leaflets with several teeth along the margins, not three in a truncate apex. The small petals, much shorter and narrower than the sepals, are to some degree shared with Potentilla pulchella but that species has pinnate leaves with a dense silvery pubescence and grows in dense tussocks.

Habitat

Only found in depressions in a tussock tundra very close to warm springs (ca. 20ºC). The soil is certainly heated by the spring water. The plant grows in a dense carpet of mosses and Salix polaris but competes successfully with these. The substrate is calcareous.

Distribution

Thermophilous. The site is located in the middle arctic tundra zone and the weakly continental section. The species is only known from the close vicinity of the warm springs Trollkjeldane at Bockfjorden, Haakon VII Land, in N Spitsbergen. In 2009, the species was observed over a stretch of more than 600 m and with at least 1000 plants, nearly 25 % of these flowering, all close to the springs (0.5–20 m). See Alsos et al. (2011).

The general range is circumpolar in the arctic and the northern boreal zones and in mountains at more temperate latitudes. The Svalbard occurrence is by far the most northern one known in the world, isolated by 900–1000 km from the nearest ones known elsewhere.

Comments

The discovery of Sibbaldia procumbens in Svalbard was unexpected due to its extreme northern position. Its isolation was also mentioned by Rønning (1961). The population may theoretically be a remnant from a wider distribution in the warmer parts of the Postglacial (the Hypsithermal), kept alive by its ability to reproduce in the warm spring environment. More probable, however, is that it is due to a fortuitous long distance dispersal by birds in more recent times. The nutlets are of the kind that may be spread in the digestive tracts of snow buntings. The branched rhizome results in clonal growth but the population is too extensive to be explained by this alone. The stands at Trollkjeldene reach for 600 m or more in one direction and must have a certain age, perhaps thousands of years when the annual growth (less than 10 cm horizontally) is considered.

Genetically, the Trollkjeldene population is extremely uniform, with almost no variation (Alsos et al. 2011). This is probably due to local inbreeding.

Literature

Alsos, I.G., Elven, R., Brysting, A.K., Birkeland, S. & Skjetne, I. 2011. Økologiske og genetiske undersøkelser av rødlistearter på Svalbard. – Rapport til Svalbards Miljøvernfond. http://www.sysselmannen.no/Svalbard-environmental-protection-fund/Resultater---rapporter/?q=alsos 45 s.

Rønning, O.I. 1961. Some new contributions to the flora of Svalbard. – Norsk Polarinstitutts Skrifter 124: 1–20.