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Perennial, moderately long-lived.
Solitary herb forming small, dense cushions up to 5 cm in diameter. Vertical rhizome branching below ground level into numerous branches covered by several years' withered leaf remains, each branch ending in a dense rosette at ground level. Each rosette potentially with one flowering stem, 0.5–2 cm tall, with mostly two pairs of stem leaves.
Leaves opposite, (3)4–5 mm, crowded, linear or very narrowly lanceolate, with broad, hyaline sheaths with auricles; lamina with prominent mid vein and strengthened margins, acuminate with a hyaline tip, glabrous. Lower pair of stem leaves with short, hyaline sheaths connected between the two leaves, upper pair with hyaline sheaths and margins.
Flowering shoots with a single terminal flowers on glabrous pedicels, 2–3(5) mm long.
Flowers radially symmetric with (4)5 free sepals and petals. Sepals 1.5–2.5 × 0.8–1.0 mm, broadly oblong and boat-shaped, acute, dark green or tinged purplish with narrow hyaline margin. Petals 2.8–3.5 mm long, slightly longer than sepals, obovate or spathulate, entire, white. Stamens usually 10. Gynoecium of (4)5 carpels with (4)5 stigmas.
Fruit a one-roomed capsule that opens apically with (4)5 teeth, with numerous seeds.
Sexual reproduction by seeds; no vegetative reproduction. Flowers very small and not adapted to insect pollination; self pollination assumed to prevail. We assume that the species flowers and sets fruits more or less regularly in Svalbard and at least occasionally produces ripe seeds.
No special adaptations to seed dispersal.
Sagina caespitosa can be confused with S. nivalis and with species of Minuartia, all with small, white flowers. Sagina has 4–5 styles and capsule teeth, whereas Minuartia has 3 styles and capsule teeth. This is not always easily observed but the shapes of buds, sepals, and capsules are usually different. Whereas Sagina has nearly globular buds, short and very concave sepals, and ovoid capsules, Minuartia has elongated buds, lanceolate and more flat sepals, and usually more elongated capsules.
Sagina caespitosa and S. nivalis can be distinguished by the following characters: Sagina caespitosa grows in dense tussocks or cushions of several rosettes from a branching rhizome, has petals longer than sepals, erect flowering stems, and a dense layer of withered leaves retained on its basal parts; S. nivalis grows as one central rosette with lateral, non-rooting branches, has petals equal to or shorter than sepals, procumbent to (rarely) ascending flowering stems, and no dense layer of old, retained leaves.
There is almost no information about habitat in Svalbard. Two of the three finds were made without any notes on ecology. A third find has the following note on the herbarium label: "Wet tundra and polygon field". The main site type of this species in mainland Norway and in Greenland is on frost boils or soil polygons, comparable to Svalbard polygon fields.
Known from two sites at Recherchefjorden (Wedel Jarlsberg Land) in the outer parts of the Bellsund district in S-C Spitsbergen, and one at Nottinghambukta (Wedel Jarlsberg Land) at Hornsund in S Spitsbergen. The sites are located in the northern arctic tundra zone and the transitional section. It is likely that this small-grown and inconspicuous species has been overlooked. Its nearest known localities are in N Norway and E Greenland (and on Jan Mayen).
The global range is restricted amphi-Atlantic with localities in NE Canada, Greenland, Iceland, Jan Mayen, Svalbard, and N and S Norway.
It is highly probable that Sagina caespitosa is overlooked in Svalbard but probably not (or not very much) in the central and northern parts of Spitsbergen where the major parts of botanical exploration have taken place. It may belong to the western coastal element (together with, e.g., Cerastium alpinum, Ranunculus glacialis, Rhodiola rosea, and Salix herbacea) and have its main range along the coast. Here the visits by qualified botanists able to find and recognize this very small-grown plant, have been few.
Westergaard et al. (2011) found support for recognizing Sagina caespitosa as a probable glacial survivalist in Norway. Its major range is in the West Arctic, and its entire range east of the Atlantic is within areas covered by the last glaciation (and previous ones). Its very restricted dispersal abilities makes trans-Atlantic dispersal under present climatic conditions highly unlikely.
Westergaard, K.B., Alsos, I.G., 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.