Flora of North America species comparison

Potentilla norvegica is considered native in both North America and Eurasia, with the American race occasionally recognized as subsp. hirsuta for the stiff hairs on the stems, petioles, and pedicels of most plants in the flora area. This feature and other purported differences between the two races, in addition to being relatively subtle and inconstant, can be found in both America and Eurasia, though undoubtedly at least in part as introductions. Some populations in eastern Canada with glabrous stems have been distinguished as var. labradorica (for example, M. L. Fernald 1950), but such plants typically are intermixed with hairy individuals. The Löves (Á Löve 1954; Á Löve and D. Löve 1966) have argued that all three variants should be treated as distinct species due to chromosomal differences (hirsuta 2n = 56; labradorica 2n = 42; norvegica in the narrow sense 2n = 70) and obligate apomixis. Further research is needed to determine the taxonomic validity and rank of these expressions. If treated as species, P. flexuosa antedates P. labradorica (as noted by J. Soják 1969), while P. monspeliensis, although commonly used for the American race, is based on a European type (as summarized by A. Kurtto et al. in J. Jalas et al. 1972+, vol. 13).

An even greater challenge is determining native versus introduced ranges, especially given the likelihood that both native and Eurasian populations are widespread in North America. Achenes are produced prolifically and easily dispersed, to the extent that Potentilla norvegica is a contaminant in clover and hay fields and considered a weed at least in Canada (P. A. Werner and J. D. Soule 1976).

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

Although now restricted to plants with exclusively (or at least predominantly) cottony hairs on the petioles, the name Potentilla nivea has a long history with an often wider application, sometimes including nearly all of sect. Niveae. As further confusion, J. Soják (1989) noted that the Linnaean type of P. nivea belonged to what is here treated as P. arenosa. Although historical usage of P. nivea has been re-established as a conserved name with a conserved type (B. Eriksen et al. 1999), from 1989 to 1999 the name P. nivea was applied to P. arenosa. During this period, P. prostrata subsp. floccosa was briefly adopted as the correct name for this species (for example, W. J. Cody 1996).

Molecular evidence (B. Eriksen and M. H. Töpel 2006) indicates that populations of Potentilla nivea in the Atlantic regions, including Greenland and eastern Canada, differ from those in the Beringian regions of northwestern North America, suggesting expansion from separate Pleistocene refugia. A comparable pattern was noted by R. Elven and S. G. Aiken (2007) based on morphologic characters. The conserved type of P. nivea is from northern Sweden (Eriksen et al. 1999) and belongs to the Atlantic morphologic group. The variation within each region is large, and racial recognition would accordingly be premature. Plants from sites south of the continental glaciation, which were not included in the analysis by Eriksen and Töpel, deviate in having acuminate leaflet teeth and epicalyx bractlets and in being generally more slender. Epicalyx bractlets of some Washington plants are nearly as narrow as those of P. crebridens.

Additional chromosome numbers have been reported for Potentilla nivea, but it is unknown whether these apply to this species, P. crebridens, some Asian relative, or hybrids.

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