Solitary herb with more or less vertical caudex branch(es) from the roots, with 1 to several flowering stems. Stems simple, 5–8 cm, without scales at base. Basal leaves 2–3. The only stem leaves are bracts crowded as a whorl beneath the inflorescence. Stems and leaves glabrous.
Leaves alternate. Basal leaves with short or long sheaths (0.5–1.5 cm) with more or less strengthened veins, sometimes remaining as stiff, dark brown fibres at the base of the stem for 1–2 years. Petioles slender, (3)3.5–5.5(6) cm, 3–4 times as long as the blades. Blades 1.0–1.4 × 1.6–2.0(2.2) cm, pedate or palmate with three main segments: a narrow central segment with 3(5) apical teeth or lobes, and two larger and broader lateral segments divided again into 3–5(7) lobes. Stem leaves sessile, palmatisect with several narrow, linear lobes.
Single, terminal flower or 2(3) flowers in a short cyme.
Flower radially symmetric with 5 free sepals and ca. 5 free petals. Sepals ca. 5 × 2.5 mm, ovate, oblong or obovate, with more or less dense, villous hairs, pale yellowish green. Petals ca. 8 × 6 mm, broadly obovate, overlapping, not plicate, yellow. Stamens 10–15; filaments ca. 2.5 mm; anthers ca. 0.8 mm, yellow. Receptacle ovoid, about as tall as wide, with short, white hairs. Carpels numerous, free.
The fruits are nutlets, glabrous, and with beaks curved 180–200°. Head of nutlets globular.
Asexual reproduction by seeds; very local vegetative reproduction by fragmentation of caudex. Produces numerous nutlets which are assumed to ripen more or less regularly (but see Distribution below). The Ranunculus auricomus complex, to which R. wilanderi is assumed to belong (Ericsson in Jonsell 2001), is pseudogamous apomictic (meaning that it needs pollen to stimulate asexual seed production). No germinable fruits were found in 2008, and there are few if any such fruits on the herbarium vouchers. We assume, however, that fruits are produced now and then and fruits collected later germinated to ca. 28 % in an experiment (Alsos et al. 2013). Agamospermous plants are usually much more regular in fruit production than seminiferous plants.
Fruits are adapted to animal dispersal due to the hooks on the nutlets. The dispersal must be inefficient as the species is located to one restricted patch, within a large area with much the same vegetation pattern.
The tall, 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 5-lobed leaves and R. repens with three separate leaflets. The plicate petals are unique for R. arcticus in Svalbard.
The species most similar to R. wilanderi is R. arcticus. They have in common the strangely dissected leaves (albeit with some differences in details) but they differ in the following characters: In R. wilanderi the sepals are sparsely pubescent, the pedicels are not elongating much, and the nutlets have evenly curved beaks not pointing downwards. Ranunculus wilanderi does not have the plicate petals of R. arcticus; in R. arcticus the sepals are densely pubescent, the pedicels elongate strongly during flowering, and the nutlets have strongly hooked beaks with tip pointing downwards.
Ranunculus wilanderi is found in drainage and seepage areas in a gently sloping moss tundra, usually deeply embedded in the moss cover. The substrate in the area is circumneutral or basic. In the Norwegian Red List 2021, the habitat is characterized as: "A moist to wet patterned ground rich in mosses, transitional to wetland, on a gentle slope with seepage on moderately basic ground. This habitat type is very sensitive to changes in precipitation and temperature, and we consider the locality as potentially threathened by any significant change in climate". (Solstad et al. 2021, translated here.)
Probably thermophilous. The only known locality of Ranunculus wilanderi in the world is on Kapp Thordsen (Dickson Land), on the north side of Isfjorden in the central parts of Spitsbergen, where the species has been known since 1871. The population has been evaluated as "fairly extensive" (Elven & Elvebakk 1996) or as consisting of less than 20 plants (Engelskjøn et al. 2003). In 2008 and 2009, a total of 85 individuals in four groups were observed, within one km² (R. Elven & I.G. Alsos observ.).
The Ranunculus auricomus group is one of the larger European agamospermic complexes with more than 400 described species or microspecies (see, e.g., Ericsson 2001), mainly in Europe. A few species reach the Arctic in mainland N Europe and Asia, one reaches Iceland (R. islandicus (Fagerstr. & G.Kvist) Ericsson, one or two Novaya Zemlya (e.g., R. glabratus (Lynge) Ericsson), a few Greenland (e.g., R. boecheri (Fagerstr. & G.Kvist) Ericsson, and also R. glabratus), and one, very characteristic species Svalbard (R. wilanderi). Even if R. wilanderi obviously belongs to this group (Ericsson 2001), it differs in several characters and in its general appearance, in its ecology, and it does not resemble any of the species known from Novaya Zemlya or Greenland. The background of this species in Svalbard is then a mystery. From where has it arrived? Or has it evolved in isolation in Svalbard, and from what?
In a molecular study, Birkeland (2012) found two different genotypes in this small population, surprising in a very local agamospermic plant.
Birkeland, S. 2012. Rare to be warm in Svalbard: An ecological and genetic snapshot of four red listed plant species. – Msc. Thesis University of Oslo, Oslo. 78 p.
Elven, R. & Elvebakk, A. 1996. A catalogue of Svalbard plants, fungi, algae, and cyanobacteria. Part 1. Vascular plants. – Norsk Polarinstitutts Skrifter 198: 9–55.
Engelskjøn, T., Alsos, I.G. & Lund, L. 2003. Twenty of the most thermophilous vascular plant species in Svalbard and their conservation status. – Polar Research 22: 317–339.
Ericsson, S. 2001. Ranuculus p.p. – In: Jonsell, B. (ed.), Flora Nordica II. Chenopodiaceae – Fumariaceae: 237–256.
Solstad, H., Elven, R., Arnesen, G., Eidesen, P.B., Gaarder, G., Hegre, H., Høitomt, T., Mjelde, M. & Pedersen, O. 2021. Karplanter: Vurdering av polarnyresoleie Ranunculus wilanderi for Svalbard. – Rødlista for arter 2021. Artsdatabanken. https://www.artsdatabanken.no/lister/rodlisteforarter/2021/26866