Flora of North America species comparison

In habit, Rubus ursinus is often reminiscent of the predominantly eastern North American, non-sympatric R. flagellaris; it is also polymorphic. Rubus ursinus comprises a polyploid spectrum dominated by octoploid and dodecaploid plants. It is an allopolyploid involving phylogenetically distant ancestors with its closest relative being the Hawaiian endemic R. macraei A. Gray. See S. W. Brown (1943) for a discussion of morphological and chromosome number variation in R. ursinus. The widespread, glaucous-stemmed, trifoliate-leaved forms of R. ursinus are vegetatively nearly identical to R. caesius. Of agricultural significance, R. ursinus is a parent of some important cultivars, including loganberry and boysenberry.

There has been uncertainty whether the name Rubus menziesii Hooker and subsequent combinations [Parmena menziesii (Hooker) Greene, R. spectabilis var. menziesii (Hooker) S. Watson, and R. ursinus var. menziesii (Hooker) Focke] pertain to R. spectabilis or R. ursinus. The protologue by Hooker describes a hairy, relatively small, procumbent plant, which is congruent with R. ursinus. The type specimen at Kew also looks like R. ursinus. In the protologue Hooker also claimed the species has red petals and suggested that it may have an affinity with R. spectabilis.

Specimens examined for this study of North American collections attributed to Rubus macrophyllus Weihe & Nees (synonym R. amplificatus Lees) are R. ursinus. If R. macrophyllus was once present in the flora area it is likely absent now; some reports of R. macrophyllus could represent hybrids. There are reports of R. ursinus hybridizing with R. bifrons and R. pensilvanicus in California (T. S. Mallah 1954; L. V. Clark and M. Jasieniuk 2012; L. A. Alice, unpubl.).

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

Rubus bifrons lacks the strongly pruinose stems of the related R. ulmifolius and has usually much larger leaves. The species also often has much broader inflorescences than R. ulmifolius or the related R. vestitus. It is less densely glandular and has longer and more open inflorescences than R. pascuus, and unlike the latter it has spreading, not erect, primocane leaves. Rubus bifrons also lacks the stipitate-glandular hairs and nearly round primocane terminal leaflets of R. vestitus; seedlings of R. bifrons can be densely stipitate-glandular.

L. H. Bailey (1941–1945), M. L. Fernald (1950), Y. Heslop-Harrison (1968), and H. A. Gleason and A. Cronquist (1991) distinguished between Rubus bifrons and another species (referred to as either R. discolor or R. procerus P. J. Müller ex Boulay, both often considered synonyms of R. armeniacus), based on stem shape and indument, prickle shape and angle, leaf shape and margins, inflorescence shape, and petal color. Plants in North America identified as either R. bifrons or the other species can vary broadly in any of these characters, even within individual stems, making it extremely difficult or impossible to distinguish between these species in the flora area. Recent research addressing the genetics or species distinctiveness in this complex in Europe (J. Kollmann et al. 2000) and Australia (K. J. Evans et al. 2007) did not sample plants from North America, and recent work within the flora area (L. V. Clark et al. 2013) addressed this complex only in California, Oregon, and Washington. Until a much-needed genetic study is performed that includes a broad selection of plants of ser. Discolores from throughout North America, Europe, and elsewhere, the precise identities of all of our material in this complex will be elusive. With this degree of uncertainty, a conservative approach is taken here, which is consistent with the treatment herein of the native blackberries. The results of more comprehensive DNA research may show that a different name than R. bifrons is to be used in the flora area. The name R. discolor often has been misattributed as a synonym of R. armeniacus; it is a synonym of R. ulmifolius (H. E. Weber 1985).

In the flora area, Rubus bifrons is most abundant from northern California northward to British Columbia, where it can grow in great density over large areas, often to the exclusion of all other vegetation (for example, K. Williams et al. 2006). There are reports of R. bifrons (as R. armeniacus or R. procerus) hybridizing with R. ursinus (T. S. Mallah 1954; L. V. Clark and M. Jasieniuk 2012; B. Sutherland and L. Alice, unpubl.) and R. laciniatus (R. K. Bammi and H. P. Olmo 1966).

Rubus thyrsoideus Wimmer is an illegitimate name that applies here.

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