The Flora of Svalbard

Local vegetative reproduction by runners is efficient. The plant flowers and fruits regularly and produces numerous seeds. Saxifraga rivularis is strongly autogamous (Brochmann et al. 2001) with an autodeposition efficiency of 1.0 (Molau 1993). Seeds germinate to about 13% (Alsos et al. in prep.), and germination from seed bank is common (Cooper et al. 2004). The capsules have apical opening which ensures that the seeds only are dispersed at a minimum wind speed. Seed dispersal is often after the first snow fall, which increases the dispersal distance as the seeds are blown across a smooth surface (Savile 1972). Seeds are also dispersed by animals, e.g. geese, that selectively feed on seed capsules (Prop et al. 1984). Secondary dispersal by water or wind

Saxifraga rivularis and S. hyperborea are similar in most features and have been confused. They are most easily separated by the former regularly having subterranean runners (seen when digging the plants, but often lost in herbarium specimens), the latter never. Saxifraga rivularis therefore often grows in diffuse tussocks or small mats, whereas S. hyperborea always forms small, dense tussocks. The flowering stems of S. hyperborea are more often erect than those of S. rivularis, and more often reddish to purplish coloured, but these differences are not constant. Both species are easily separated from the related S. cernua and S. svalbardensis by their lack of bulbils in the axils of stem leaves and their much smaller flowers.

Saxifraga rivularis is characteristic of moist to wet snowbeds, seepage areas, shallow mires, shores (also upper parts of seashores), and moist manured sites, often below bird cliffs. The substrate may be fine-grained or coarser. The species is largely indifferent as to soil reaction (pH), but perhaps found more often in areas with acidic substrates than in those with basic ones.

Saxifraga rivularis is tetraploid (2n = 52) and is an allopolyploid species from the two diploid (2n = 26) species S. hyperborea and S. bracteata D.Don (Jørgensen et al. 2006). Saxifraga bracteata is restricted to the coasts of the North Pacific and the Bering Strait, and Jørgensen et al. (2006) assumed the origin of the allotetraploid to have taken place by hybridization of S. bracteata and S. hyperborea, followed by polyploidization, in the Beringian region. As S. bracteata has subterranean runners, S. rivularis seems to have inherited this diagnostic feature from that parent. 

Jørgensen et al. (2006) accepted two subspecies of S. rivularis: the Atlantic ssp. rivularis and the Beringian ssp. arctolitoralis (Jurtz. & V.V.Petrovsky) M.H.Jørg. & Elven. Subspecies arctolitoralis has subsequently been proved (by molecular markers and morphology) to reach across arctic America to W and E Greenland, but not to Svalbard or mainland Europe (Westergaard et al. 2010). Even if S. rivularis and S. hyperborea are superficially similar, they differ distinctly in morphology, ploidy level, and molecular markers (Guldahl et al. 2005; Jørgensen et al. 2006; Westergaard et al. 2010) and deserve rank as two independent species.

Molecular studies suggest that S. rivularis might have survived the last glaciation in Svalbard (Westergaard et al. 2010).

Alsos, I.G., Müller, E. & Eidesen, P.B. In prep. Germinability of 87 arctic species stored in Svalbard Global Seed Vault.

Brochmann, C. & Håpnes, A. 2001. Reproductive strategies in some arctic Saxifraga (Saxifragaceae), with emphasis on the narrow endemic S. svalbardensis and its parental species. – Biological Journal of the Linnean Society 137: 31−49.

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.

Guldahl, A.S., Gabrielsen, T.M., Scheen, A.-C., Borgen, L., Steen, S.W., Spjelkavik, S. & Brochmann, C. 2005. The Saxifraga rivularis complex in Svalbard: Molecules, ploidy and morphology. – Flora 200: 207–221.

Jørgensen, M.H., Elven, R., Tribsch, A., Gabrielsen, T.M., Stedje, B. & Brochmann, C. 2006. Taxonomy and evolutionary relationships in the Saxifraga rivularis complex. – Systematic Botany 31: 702–729.

Molau, U. 1993. Relationship between flowering phenology and life history strategies in tundra plants. – Arctic and Alpine Research 25: 391−402.

Prop, J., van Erden, M.R. & Drent, R.H. 1984. Reproductive success of Barnacle Goose Branta leucopsis in relation to food exploitation on the breeding grounds, western Spitsbergen. – Norsk Polarinstitutt Skrifter 181: 87−117.

Savile, D.B.O. 1972. Arctic adaptations in plants. – Canada Department of Agriculture Research Branch Monograph 6. 81 pp.

Westergaard, K.B., Jørgensen, M.H., Gabrielsen, T.M., Alsos, I.G. & Brochmann, C. 2010. The extreme Beringian/Atlantic disjunction in Saxifraga rivularis (Saxifragaceae) has formed at least twice. – Journal of Biogeography 37: 1262−1276.