Solitary herb with short subterranean rhizome, mostly extensively branched at or below ground level from a single root, densely to laxly caespitose and forming from a single rosette to large tussocks with several tens (perhaps hundreds) of rosettes at ground level. Each rosette potentially with one flowering stem with reduced leaves. Flowering stems erect, from very short to 10 cm tall or more. Leaves, stems, and calyces have short, white hairs tipped by red glands, increasing in density upwards on the plant.
Leaves alternate. Basal leaves with indistinct transition from a winged petiole into a blade (0.5)0.8–1.5(2.5) × (0.3)0.4–0.6(0.8) cm. Blade divided for more than 50 % of its length into three broadly linear or oblong, obtuse or rounded lobes, the two lateral lobes often with a narrower secondary lobe on the outward side. Stem leaves much reduced, small, oblong or linear, obtuse. Leaves green or red, sometimes purple, but very often with a conspicuous attack of a yellow or orange rust fungus, Melampsora epitea.
Flowers terminal on stem, single or more often 2–5 in a short or elongated cyme.
Flowers radially symmetric with 5 free sepals and petals. Sepals 2.5–3.5 × 1.5–2.5 mm, appressed in both flower and fruit stage, broadly ovate, obtuse or rounded, green or more often tinged red or purple. Petals varying much in size but mostly 4–8 × 2.5–5 mm, patent or erectopatent (flowers fully open or half open), oblong, obovate or narrowly oblong/obovate, rounded, contiguous or non-overlapping, white or pale yellow, rarely pale pink. Some populations have yellow and narrow petals associated with half open flowers, whereas the majority of populations have white and broader petals associated with fully open flowers. Stamens 10. Ovary semi-inferior, of two carpels with two rooms, split at the top.
Fruit a capsule with numerous seeds.
Sexual reproduction by seeds; no vegetative reproduction. The entire tussock has one root in common. This plant flowers profusely every year, from very early in the season, and starts flowering already at the one-rosette stage. It may flower two times within a season (in exceptional cases perhaps three times). Seeds ripen regularly, sometimes even after a second flowering (R. Elven observ., Sassendalen). Flowers are adapted to insect pollination but self pollination is assumed to be common (Brochmann et al. 1999). Seeds germinate to near 100 % in the laboratory and to low percentages (6–10 %) in the field (Alsos et al. 2013; Müller et al. 2011).
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).
Saxifraga cespitosa is characteristic and usually easily recognized. If in doubt, look at the two-beaked gynoecium or capsule (in common for the genera Saxifraga and Micranthes) and the basal 3(5)-lobed leaves, a combination not found in any other Svalbard plant. However, plants with half-closed, yellow flowers, or with heavy attacks by the rust fungus, often raise questions.
Confined to dry or only slightly or temporarily moist sites, in heaths and early snowbeds, on ridges, river terraces and bars, and in scree and on cliffs, especially bird cliff meadows where the plants may become very large. Substrates are mostly sand, gravel or coarser kinds, because silt and clay retain too much water for this xerophilous species. Probably indifferent as to soil reaction (pH).
Common to frequent in all zones and sections. Present on all major islands, incl. Bjørnøya.
The general range is circumpolar and arctic–alpine, very widespread.
Saxifraga cespitosa is an extremely polymorphic species, globally but also within Svalbard. The variation has been described as eight or more species (see, e.g., Elven et al. 2011). One geographically restricted molecular study, now fairly old, is undertaken to date: Tollefsrud et al. (1998). These authors documented significant variation but did not resolve it in any way.
Three taxa have been proposed present in Svalbard: a main one with usually two or more white flowers per stem (S. cespitosa s. str.); a compact one with one white flower per stem, corresponding to S. uniflora R.Br. (S. cespitosa ssp. uniflora (R.Br.) A.E.Porsild) described from arctic Canada; and the yellow-flowered one with half-open flowers, described from Svalbard as S. cespitosa var. aurea Hadač (S. aurea (Hadač) Rønning, an invalid name for several reasons). These proposed taxa occur as distinct populations and are often easily recognized and commented on. However, they occur here and there within the general range, without any geographical or ecological pattern. The yellow-flowered one occurs also in S Norway (e.g., the Finse area) and N Norway (e.g., Båtsfjord). The compact one-flowered one is part of the polymorphy in Canada, Greenland, Svalbard, and also in the higher mountains of Scandinavia. At present, the majority of authors do not recognize taxa beneath the species level in S. cespitosa in the North Atlantic regions (e.g., Brouillet & Elvander 2009; Elven et al. 2011) but two races in the Beringian regions (ssp. cespitosa and ssp. monticola (Small) A.E.Porsild).
Saxifraga cespitosa is a highly polyploid species (mainly decaploid, 2n = 80) and has several close relatives, mainly at lower ploidy levels, e.g., S. hypnoides L. and S. rosacea Moench in W Europe, both reaching as far north as N Iceland. Whereas S. cespitosa has a very wide, circumpolar range, all its close relatives are more temperate Europeans. It is not inconceivable that our S. cespitosa includes more than one combination of more low-ploid genomes from these relatives.
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. & Steen, S.W. 1999. Sex and genes in the flora of Svalbard - implications for conservation biology and climate change. – Det Norske Videnskaps-Akademi. I. Matematisk Naturvitenskapelig Klasse, Skrifter, Ny serie 38: 33–72.
Brouillet, L. & Elvander, P.E. 2009. Saxifraga Linnaeus. – In: Flora of North America Editorial Committee (eds.), Flora of North America north of Mexico. 8. Magnoliophyta: Paeoniaceae to Ericaceae: 132–146.
Elven, R., Murray, D.F., Razzhivin, V. & Yurtsev, B.A. (eds.) 2011. Checklist of the Panarctic Flora (PAF) Vascular plants. http://panarcticflora.org/
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.
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 Polarinstitutts Skrifter 181: 87–117.
Savile, D.B.O. 1972. Arctic adaptations in plants. – Canada Department of Agriculture Research Branch Monograph 6. 81 pp.
Tollefsrud, M.M., Bachmann, K., Jakobsen, S. & Brochmann, C. 1998. Glacial survival does not matter – II: RAPD phylogeography of Nordic Saxifraga cespitosa. – Molecular Ecology 7: 1217–1232.