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Mostly a solitary dwarf shrub with extensively branched shoot system below and above ground, subterranean branches rarely rooting and the entire plant usually depends on a single, central root. Forming large, dense or loose mats, often several metres in diameter. Central branches up to 5 mm broad or more, with flaking bark on the older, dark grey parts; leafy branches slender, terete, up to 1 mm broad, reddish yellow or red, densely pubescent with short, stumpy, glandular hairs.
Leaves alternate, 3–5 × 1.1–1.4 mm, ever-green (lasting for at least 3–4 seasons), narrowly oblong, obtuse or subacute, with a minute petiole, patent or moderately retrorse, apparently with an upper and lower side but actually densely folded backwards so that the upper leaf surface makes up most of the leaf’s exterior parts. The lower surface (with stomata) is hidden inside the fold and opening towards the outside (and air) with a narrow slit on the lower side of the leaf (a so-called ericoid leaf). Except for the slit, the leaf is flat and shiny, the apparent lower side being more rounded, and the apparent margin has minute, stalked glands (sometimes also elsewhere), green, yellowish green or reddish.
In strictly morphological terms, a short, bud-like raceme in leaf axils on the distal part of a branch, with 3 minute bracts, one of which bears a sessile flower. Visually, flowers appear to sit singly in leaf axils but with too many bracts and bracteoles to really be a single flowers.
Flowers bisexual (in this subspecies), radially symmetric, with 3 free sepals and petals. Sepals and petals 1–1.5 x 0.3–0.5 mm, oblong, obtuse, usually pink or purple. Stamens 3; filaments 4–5(6) mm, reaching far beyond the perianth; anthers short and broad, ca. 0.5 × 1 mm, without spurs (whereas many other genera of Ericaceae have spurs on the anthers, facilitating efficient deposition of pollen on visiting insects), opening by pores, purple. Gynoecium of 6–9 fused carpels with one ovule per carpel; stigma sessile, with 6–9 stigmatic rays, purple. The entire flower is thereby purple and, seen in a lens, quite beautiful in its prime.
Fruit a juicy, globular drupe, 5–8 mm in diameter, with 6–9 stones (pyrenes), black or purplish black, shiny or with a bloom. Stamens are usually retained in the fruiting stage, showing that the flowers have been bisexual.
Sexual reproduction by seeds; restricted local vegetative reproduction by layering and rooting of shoots. Pollination is assumed to be affected by wind but the purple colour attract some insects (flies often observed on Empetrum flowers). Fruit set is common in Svalbard, but the degree of ripening varies among sites. Ripe fruit from Longyeardalen germinated to ca. 30 % (Alsos et al. 2013).
Fruits are dispersed by birds feeding on them.
There is nothing resembling this plant in Svalbard. The only other dwarf shrubs with ever-green leaves (Cassiope tetragona and Harrimanella hypnoides) have scale-like or needle-shaped, forward-pointing leaves and white, bell-shaped flowers with a fused corolla.
Only found on heath slopes with dry substrate, good snow protection, and exposure towards south, west or east. The substrate may be rather coarse (probably not occurring on fine-grained substrates with good water capacity). Common associated species are Cassiope tetragona and Salix polaris, occasionally with Betula nana found in the vicinity. Empetrum is found on substrates with circumneutral to basic soil reaction (pH).
Thermophilous. In the middle arctic tundra zone but slightly transgressing into the northern arctic tundra zone, and in the clearly and weakly continental sections, slightly transgressing into the transitional section. Only found in Spitsbergen and mainly in the fjord districts in the west and north from Recherchefjorden (Wedel Jarlsberg Land) and the entrance of Van Keulenfjorden (Nathorst Land) north to Brucepynten at Raudfjorden (Haakon VII Land).
The global distribution depends on how the subspecies is circumscribed. Empetrum nigrum is circumpolar in the boreal and arctic zones; subspecies hermaphroditum may be northern amphi-Atlantic. See Comments.
The very small genus Empetrum has been considered by some authors to consist of only one or perhaps two species (e.g., Good 1927), by others with up to 18 species (Vassiljev 1961). It is bipolar, i.e., with clearly separated distribution areas in the northern and southern hemispheres. The northern distribution area is very large, comprising the entire circumboreal and circumarctic zones; the southern hemisphere area is restricted to southernmost South America, the Falkland Islands, and some South Atlantic islands (e.g., Gough and Tristan da Cunha). The southern plants are all red-fruited (Empetrum rubrum Good) but otherwise quite similar to the northern ones. The northern plants vary from black-fruited (the vast majority) to purple- or pink-fruited (in E Canada). An extensive study by Mirré (2004) based on AFLP markers, followed by Popp et al. (2011) based on plastid and nuclear sequences, showed a very complicated structure with several intermingled diploid and tetraploid lineages, probably with a reticulate structure due to polyploidizations from several diploid combinations. For the relations between plants with red, purple and black fruits, see Murray et al. (2007) and Popp et al. (2011).
Another complication is the sex relationships. Linnaeus' European Empetrum nigrum s. str. (ssp. nigrum) is diploid (2n = 26, x = 13) and has unisexual plants, i.e., male and female organs are well separated and the female plants therefore set much less fruit than those of ssp. hermaphroditum present in Svalbard. Empetrum nigrum s. str. is a boreal plant common in mainland Europe and probably in E North America, and also present in Iceland. Unisexual plants are also known from coastal areas surrounding the North Pacific, and these plants seem to be the closest relatives of the red-fruited South American plants (see Popp et al. 2011). The bisexual plants are all tetraploid (2n = 52) and are known from alpine and arctic areas throughout the northern hemisphere. Traditionally, these have been treated collectively as ssp. hermaphroditum (or as a species, E. hermaphroditum because there is a strong reproductive barrier towards the diploids), but this view may have to be modified. Mirré (2007) and Popp et al. (2011) showed that the tetraploids in the mountains on the European mainland probably have a parental background different from the tetraploids in Greenland and elsewhere. As the name 'hermaphroditum' is based on a Greenland plant, this name may be less appropriate for European tetraploids, e.g., in Fennoscandia where it has been uniformly used until now. The genotypes corresponding to the Greenland and true 'hermaphroditum' may be restricted to the arctic areas surrounding the North Atlantic, including Svalbard.
Alsos, I.G., Müller, E. & Eidesen, P.B. 2013. Germinating seeds or bulbils in 87 of 113 tested Arctic species indicate potential for ex situ seed bank storage. – Polar Biology 36: 819–830.
Good, R.D.O. 1927. The genus Empetrum L. – Botanical Journal of the Linnaean Society 47: 489–523.
Mirré, V. 2004. Phylogeny, migration and evolution of a bipolar model group: the genus Empetrum (Crowberries). – Cand. scient. Thesis, Univ. Oslo, Oslo.
Murray, D.F., Mirré, V. & Elven, R. 2009. Empetrum Linnaeus. – In: Flora of North America Editorial Committee (eds.), Flora of North America north of Mexico. 8. Magnoliophyta: Paeoniaceae to Ericaceae: 486–489.
Popp, M., Mirré, V. & Brochmann, C. 2011. A single Mid-Pleistocene long distance dispersal by a bird can explain the extreme bipolar disjunction in crowberries (Empetrum). – Proceedings of the National Academy of the Sciences of the United States of America 108: 6520–6525.
Vassiljev, V.N. 1961. Rod Empetrum. – Akad. Nauk SSSR, Bot. Inst. V.L. Komarova, Moscow.