Flora of North America species comparison

Species 3–18 (3 in the flora).

Comprehensive taxonomies of Empetrum are relatively few and none is recent (e.g., R. Good 1927; V. N. Vassiljev 1961). Empetrum in North America has been treated regionally, especially in northeastern North America, without consideration of the problems faced continent-wide, and without a unified taxonomy that addresses the variation elsewhere. In his circumpolar review, E. Hultén (1971) wrote of Empetrum: “A...complex, where different authors rarely, if ever, arrive at the same conclusion.” He remarked that the genus Empetrum could be considered to comprise a single, variable species. Vassiljev, on the other hand, proposed 18 species worldwide, ten for North America including Greenland.

Empetrum is monophyletic (A. A. Anderberg 1994c; Li J. H. et al. 2002; M. Popp et al., unpubl.). Analyses of morphology (Anderberg) and molecular genetics (Li et al.; Popp et al.) have shown relationships to other closely related ericads, congeners, and to some extent within Empetrum. Using molecular methods, V. Mirré (2004) using amplified fragment length polymorphism (AFLP) and Popp et al. using plastid trnS–trnfM and trnS–trnG and nuclear RPB2 and RPC2 sequences have, in preliminary studies, evaluated relationships among taxa, but without finding sufficient structure to create a new taxonomy. However, these studies tell us that assumptions about key characters in the treatment by V. N. Vassiljev (1961), for example, are not well supported by molecular data, and we cannot, therefore, simply accept and repeat entirely what is in previous, albeit monographic, treatments.

A. A. Anderberg (1994c) and V. Mirré (2004) found good separation of the red-fruited Southern Hemisphere plants (Empetrum rubrum Vahl ex Willdenow), although Li J. H. et al. (2002) and M. Popp et al. (unpubl.) did not. Nevertheless, we are treating E. rubrum as distinct from all North American taxa. Popp et al. did find that red-fruited, diploid E. eamesii was well separated from other taxa in the region, whereas, in terms of molecular genetics, the Southern Hemisphere red-fruited diploid E. rubrum has its closest connections in the Northern Hemisphere not to E. eamesii but to black-fruited Northern Hemisphere plants treated here as E. nigrum in the broad sense. The solution is to recognize the diploid E. eamesii, the diploid and tetraploid E. nigrum, and E. atropurpureum as a possible allotetraploid from diploid E. eamesii and a diploid E. nigrum. This leaves a great deal of variation within E. nigrum in the broad sense unaccounted for. Various hybrid combinations were alleged by D. Löve (1960); these require confirmation. For the most part, as there are exceptions, diploid taxa are normally dioecious and tetraploids are normally synoecious but sometimes polygamous.*

* The authors kindly wish to acknowledge the unpublished information provided by John Maunder, Pierre Morrisett, and Peter Zika on the northeastern endemic taxa of Empetrum for the flora area.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

All black-fruited plants of the genus in North America are here considered to be Empetrum nigrum in the broad sense. There is great variation within the species, and various names have been proposed. Boreal and arctic diploids (2n = 26, E. Hultén 1968) with unisexual flowers, dioecious plants, have been treated as E. nigrum, and tetraploids (2n = 52) with bisexual flowers, synoecious plants, as E. hermaphroditum.

Empetrum nigrum is typified on European, dioecious, diploid plants, and in this strict sense, there has been a question whether the name should be applied to North American plants (Á. Löve and D. Löve 1959) in areas with synoecious tetraploids, where diploids were thought to be absent.

Dioecious, diploid, black-fruited crowberries are now well known from Maine, New Brunswick, Newfoundland, and Nova Scotia (J. Maunder, pers. comm.; P. Morisset, pers. comm.). There appears to be some ecological separation between the dioecious Empetrum nigrum and the synoecious E. nigrum; the dioecious plants in Newfoundland are reported to be replaced by the synoecious ones at higher elevations or in very exposed localities (J. Maunder, pers. comm.).

Dioecious plants have been long known in the Pacific Northwest (J. A. Calder and R. L. Taylor 1968) and Alaska (E. Hultén 1968), and diploids have now been confirmed for the Cascade Range, Washington; Prince Rupert, British Columbia; and the Kenai Peninsula, Alaska (V. Mirré 2004). Some of these western diploids have chloroplast DNA more closely related to South American accessions (diploid Empetrum rubrum) than to any Northern Hemisphere Empetrum investigated with chloroplast trnS-trnfM and trnS-trnG sequences (M. Popp et al., unpubl.). The nuclear sequence data show a more complex pattern wherein the alleles found in the South American accessions represent a subset of the alleles found in the Northern Hemisphere accessions (nuclear low copy RPB2 and RPC2 sequences).

Empetrum hermaphroditum, a name in prominent usage, is typified on a Greenlandic, synoecious tetraploid, and this name and combinations of it have been applied to virtually all black-fruited synoecious plants in North America and wherever else black-fruited, synoecious crowberries are found (cf. Á. Löve and D. Löve 1959). In Europe, in addition to the diploids, there are synoecious tetraploids of E. nigrum.

V. Mirré (2004) concluded that tetraploid synoecious plants have arisen multiple times from different combinations of diploids resulting in a large pool of unstructured genetic variation. We conclude that synoecious tetraploids do not necessarily represent a distinct lineage and must, for the present, be subsumed by Empetrum nigrum as understood here.

The population of Empetrum nigrum reported from Montauk Point, Long Island, New York, has been extirpated (S. E. Clemants, pers. comm.).

(Discussion copyrighted by Flora of North America; reprinted with permission.)