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. (svalbardflora.no)
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. (svalbardflora.no)
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. (svalbardflora.no)
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). (svalbardflora.no)