Solitary herb, either with one flowering stem or small tussocks with up to 10 stems. Flowering stems simple, erect, 10–20(30) cm tall. Stems and leaves with variable but sparse cover of brown, villous hairs.
Leaves alternate. Basal leaves sheathing with petiole usually 2–10 cm long; blades 1.5–2(3) × (1)1.5–2.5(3.5) cm, orbicular in outline, cuneate or truncate at base, and with characteristic deeply lobed margin apically; all lobes pointing forwards; lobes obtuse or acute and with a white apical point (visible also on herbarium material). Stem leaves sessile, palmate or palmatisect with 3–5 linear lobes.
Single, terminal flowers.
Flower radially symmetric with 5 sepals and petals. Sepals 6–10 × 4–6 mm, ovate or obovate, with a dense cover of dark brown, villous hairs. Petals 8–11 × 7–11 mm, orbicular, bright yellow, with strong brown veins when dry. Stamens numerous (> 30), bright yellow, 1–4 mm long. Receptacle taller than broad, with long brown hairs between the nutlets (visible with a hand lens). Carpels numerous, free.
The fruits are nutlets, glabrous or with long brown hairs and with beaks about 1–1.5 mm long and straight to variously curved. Head of nutlets round or elongated, 0.5–1 × 0.5–1 cm.
Sexual reproduction by seeds; no vegetative reproduction. Flowers and fruits regularly and produces numerous nutlets which are assumed to germinate regularly.
The nutlets are partly dispersed by animals as they are hooked and attach easily to fur. Otherwise probably spread by running water and perhaps wind.
The stout native Ranunculus species in Svalbard are R. arcticus, R. nivalis, R. sulphureus, and R. wilanderi. In addition, R. repens, R. acris and R. subborealis are introduced (see these species). Ranuculus suphureus and R. nivalis both have dark brown hairs on the sepals whereas R. arcticus and R. wilanderi have white hairs. The strange leaves of R. arcticus and R. wilanderi, with the broad middle lobe and the very narrow lateral lobes, immediately distinguish them from R. nivalis with regularly deeply lobed leaves, R. sulphureus with broad, shallowly lobed leaves, and from the three introduced species, R. acris and R. subborealis with regularly deeply divided leaves and R. repens with three separate leaflets.
Ranuculus sulphureus and R. nivalis are distinguished by the following characters: In R. sulphureus the basal leaves are nearly orbicular in outline with some deep lobes apically only, the receptacle is pubescent with long brown hairs all over, and the nutlets are glabrous or with long brown hairs; in R. nivalis the basal leaves are pedate or palmate in outline, the receptacle is glabrous or with brown hears apically only, and the nutlets are always glabrous.
Moist sites: snowbeds, slopes with seepage, shallow mires, moist moss tundra, meadows at base of bird cliffs. Usually in well vegetated sites but also sometimes found on bare soil. The substrate varies from clay to stones (all fractions). Ranunculus sulphureus seems to prefer substrates with circumneutral or basic soil reaction (pH), avoiding the most acidic places.
Common in all zones and sections. Present and usually common on all major and most minor islands in the Spitsbergen group (but not found on Kvitøya) and also common on Bjørnøya.
Ranunculus sulphureus is circumpolar, nearly confined to the arctic zones, in Europe southwards to northernmost Norway.
Ranunculus sulphureus is a high polyploid (8–12×, 2n = 64–96) but related to R. pygmaeus (2×; 2n = 16), R. nivalis (6–7×, 2n = 48, 56), and the Siberian, North American and N Greenlandic R. sabinei R.Br. (4×, 2n = 32) (see Elven et al. 2011). It may combine different more low-ploid genomes but is fairly uniform throughout its vast range.
The only variation of possible significance seen in the Svalbard material is dwarf plants, small in both leaves, flowers and fruits, and found as populations in several regions (e.g., Reinsdyrflya). Such plants have sometimes been interpreted as hybrids with R. pygmaeus (several collections), sometimes as belonging to R. sabinei which they superficially resemble. All specimens of putative hybrids that we have inspected, have well developed anthers, pollen and young fruits. Hybrids between the diploid R. pygmaeus and the high polyploid R. sulphureus would be expected to be highly sterile and aborting the anthers and pollen at an early stage. As to R. sabinei , even if there are some similarities between the Svalbard pygmies and that species, the pygmies are different from R. sabinei in nearly all diagnostic characters. All suggested hybrid specimens and plants proposed as R. sabinei have therefore been re-identified as R. sulphureus.
Elven, R., Murray, D.F., Razzhivin, V. & Yurtsev, B.A. (eds.) 2011. Annotated Checklist of the Panarctic Flora (PAF) Vascular plants. http://panarcticflora.org/