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Herb, mostly with an extensive underground system of very slender (ca. 0.5 mm broad), white, 3—10 cm long rhizome branches with numerous pairs of scaly (vestigial) leaves at 4—6 mm distance, more rarely growing as small tussocks. Aerial shoots short, 0.5—1.5 cm, with 3—4 pairs of crowded leaves. Stems pubescent with characteristic, very short (less than 0.05 mm) and thick, down-pointing (retrorse) white hairs. Plants in moss tundra are bright green and mat forming, whereas plants on a gravel plain are more purple and cushion forming. The latter growth form has only been seen one place in Svalbard, at Bockfjorden (near a stand with mat forming plants)
Leaves opposite, minute, 3—5 × 2—2.5 mm, broadly ovate or obovate, obtuse to rounded, with a distinct mid vein, glabrous except for a few minute, thick hairs at base, fleshy, dark green or with a reddish or purplish tinge.
Flowers singly at the end of aerial shots, very rarely 2—4 flowers in a dichasial cyme.
Flowers on pedicels 1—2 mm, radially symmetric, 6—8 mm in diameter, with 5 free sepals and petals. Sepals 3—4 × 1.2—1.4 mm, oblong, subacute, with indistinct mid vein, glabrous, dark green or purplish with very narrow (up to 0.2 mm) pink hyaline margins. Petals 4—5 × 1.5—2.5 mm, slightly longer than sepals, elliptic, obtuse, spreading, translucent white. Stamens 10; filaments ca. 3 mm; anthers very short, 0.2—0.3 mm, pale pink. Gynoecium of 3 carpels with 3 stigmas.
Capsule 4—5 mm, longer than sepals, protruding from flower when mature, pear-shaped (pyriform), olive brown, slightly glossy with one room and several seeds, opening by 6 teeth. Seeds 0.6—0.8 mm, brown.
Sexual reproduction by seeds; very local vegetative reproduction by runners. No adaptation to insect pollination; probably self pollinated. Seed production probably regular in Svalbard.
Seeds are not adapted to any special kind of dispersal, probably passively dispersed along the ground over very short distances.
Arenaria differs from other small Caryophyllaceae by having shorter and broader leaves and short, thick hairs on stems and in leaf margins. The two species of Arenaria are rather different. Whereas A. humifusa is rhizomatous and normally with scattered aerial shoots (except for one stand at Bockfjorden), A. pseudofrigida is densely caespitose. The flowers of A. humifusa are very small, 6—8 mm in diameter, and the sepals have 1 vein; the flowers of A. pseudofrigida are larger, 10—12 mm in diameter, and the sepals have 3 veins.
The information from the very few finds suggests that frost patterned ground, or disturbed moist heath, is the main site type in Svalbard, but it has also been found on a gravel plain at Bockfjorden. All finds are made in areas with calcareous or at least basic substrates. This species is obviously an extremely weak competitor easily overgrown by taller plants.
Thermophilous. Restricted to the middle arctic tundra zone and the weakly continental section. Known from three localities in NW Spitsbergen: Engelskbukta south of Brøggerhalvøya (Oscar II Land), Ossian Sarsfjellet at the head of Kongsfjorden (Haakon VII Land), and Bockfjorden in the Liefdefjorden area (also Haakon VII Land). This small-grown species is rather hard to discover. The finds in Svalbard are fairly recent (in the last 50 years) and we suspect that Arenaria humifusa may be much overlooked.
The general range is amphi-Atlantic, reaching from NE Canada and W Greenland (where it is rather frequent) across to E Greenland, Svalbard, N Fennoscandia, and NW European Russia (where it is very rare). It is replaced by the vicariant A. longipedunculata Hultén in the Beringian regions. However, recent genetic studies suggest that the range of A. longipedunculata must be extended east to Greenland and overlaps that of A. humifusa (Westergaard et al. 2011).
Arenaria humifusa has scattered populations in Canada and W Greenland and only very few populations in E Greenland, Scandinavia and Svalbard. This distribution pattern is referred to as a west-arctic pattern and is shared by only about 30 species. As their entire range lies within the previously glaciated area, and as they all usually lack special adaptation for seed dispersal, these species have been inferred as a strong proof of glacial survival (see especially Nordhagen 1936). The arguments from the discussion are referred by Westergaard et al. (2011) who found molecular support for the hypothesis of survival during glaciations in this (and one more) species among those very few yet investigated. Despite the populations being small and scattered in Svalbard, the plants there have higher levels of genetic diversity than found in most of the range of the species. This may be explained in two ways: either as several immigrations or as the current distribution being relict from a previously more continuous distribution in earlier and warmer Postglacial times (the Hypsithermal).
Nordhagen, R. 1936. Om Arenaria humifusa Wg. og dens betydning for utforskningen av Skandinavias eldste floraelement. — Bergens Museums Årbok, Naturvidenskapelig Rekke 1935(1). 183 pp.
Westergaard, K.B., Alsos, I., Popp, M., Engelskjøn, T., Flatberg, K.I. & Brochmann, C. 2011. Glacial survival may matter after all: nunatak signatures in the rare European populations of two west-arctic species. — Molecular Ecology 20: 376—393.