Herb, uncertain whether solitary or ‘social’. Stem subterranean, short and thick, white, with endotrophic mycorrhiza, each year usually producing one thick root and one leaf, rarely several leaves.

Leaves single, erect from the stem, at base with a fibrous sheath, divided near the middle into a vegetative and a reproductive blade (the vegetative blade lateral to the main growth direction), both about 1 cm long from division. Petiole and vegetative blade bright to yellowish green. Vegetative blade oblong with 2—3 pairs of opposite, reniform or fan-shaped pinnae, entire or (outside Svalbard) crenate. Reproductive blade branched with opposite branches carrying sporangia.

Sporangia sessile in two rows along the branches of reproductive blades, usually very few in Svalbard plants, 6–8(10) in total per plant, yellow, opening by a split.

Sexual reproduction by spores; probably some very local vegetative reproduction by detachment from the subterranean stem. Spores are liberated passively when the sporangia split.

Spores are wind dispersed but perhaps mostly over short distances.

The only similar plant in Svalbard is the other species of Botrychium, B. boreale, with triangular vegetative blade that is deeply cut into pinnae, which also are more triangular in shape.

Botrychium lunaria is only found in the moss–Bistorta vivipara–Salix polaris carpet close to the warmest thermal spring in Svalbard, which has a stable temperature around 15—23ºC. Vegetation analyses of four 50 x 50 cm plots show a general high moss cover (81—90 %), lower cover of vascular plants (21—30 %), and hardly any open soil, lichens or stone/sand (Alsos et al. 2011). Five measurements of pH were from 6 to 8.5, with a mean of 7.1 (Alsos et al. 2011).

Very thermophilous. The single locality is located in the middle arctic tundra zone and the weakly continental section. Known only from the close surroundings of the thermal springs at Trollkjeldane, Bockfjorden (Haakon VII Land) in N Spitsbergen (discovered in 1974, see Elvebakk & Spjelkavik 1981). The total area is a stretch of ca. 50 m along the warmest of the springs (23ºC). A total of 21 leaves were counted in 2009 (Alsos et al. 2011), estimated to be ca. 75 % of the population, whereas 35 leaves were counted in 2013 (I.G. Alsos et al. observ.). According to Jonsell (2000: 30), there is no known way of fragmentation in European Botrychium, and the presence of 20—40 leaves should, according to him, mean 20—40 separately germinated individuals. However, both our general experience and the genetic analysis suggest that the underground stem branches and breaks apart and with time may result in large clonal stands.

The general range is nearly circumpolar in the southern arctic, the boreal and the northern temperate zones.

The small population of Botrychium lunaria at Bockfjorden must be a case of fortuitous long distance dispersal and establishment, either spread by birds (geese) or by wind. A restricted sampling of three leaf fragments as far from each other as possible in this small stand showed no genetic variation (Birkeland 2012), suggesting that it is a single clone, possibly originating from branching of subterranean stems.

Recent molecular studies suggest that the Bockfjorden plants do not belong to B. lunaria, but rather to B. onondagense Underw., a species otherwise known from North America.

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.

Birkeland, S. 2012. Rare to be warm in Svalbard: An ecological and genetic snapshot of four red listed plant species. – M.Sc. Thesis University of Oslo, Oslo.

Elvebakk, A. & Spjelkavik, S. 1981. Botanisering blant varme kjelder og vulkanar på nord-Svalbard. – Polarflokken. 5: 104—113.

Jonsell, B. (2000). Ophioglossaceae. – In: Jonsell, B. (ed.), Flora Nordica 1 Lycopodiaceae – Polygonaceae: 29–38.