Flora of North America species comparison

Sorbus scopulina is variable, especially in leaflet shape, number, and indument. Plants with narrower leaflets were separated as S. angustifolia; plants with broader leaflets were segregated as S. andersonii and S. cascadensis. Densely hairy forms were named S. dumosa. All represent points on a morphologic continuum, united by their shiny leaflets, whitish indument, and western range. Sparsely hairy forms approach the more southern S. californica but have whitish axillary hairs and larger leaflets than that species, which has rufous axillary hairs and leaflets less than 4 cm. Most S. scopulina have shiny hypanthia and fruits. Occasional collections, all within the range of S. sitchensis, are glaucous-fruited. This may be natural variation, or it could represent introgression from S. sitchensis. H. A. McAllister (2005, pers. comm.) found that individuals of S. scopulina from cordilleran Idaho and Arizona were sexual diploids; plants from the Cascade and Coast ranges of British Columbia and Washington were tetraploid and presumably apomictic. Here, these entities are synonymized, awaiting a larger study that might correlate distinct morphologic characteristics with the ploidy levels. J. J. Aldasoro et al. (1998, 2004) noted that some European Sorbus species are both diploid and tetraploid, suggesting that sexual species occasionally produce spontaneous agamospermous individuals.

The intergeneric hybrid ×Amelasorbus jackii Rehder [Amelanchier alnifolia (Nuttall) Nuttall ex M. Roemer × Sorbus scopulina] has been collected in Idaho and Oregon. Foliage of the Idaho collection is very similar to that of Amelanchier, with basal pinnae inconspicuous and infrequent, suggesting that this hybrid may be overlooked.

The illegitimate later homonym Sorbus alaskana G. N. Jones pertains here.

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

Sorbus decora reputedly flowers about one week later than S. americana. The two are sometimes confused when one is not familiar with the smaller flowers and fruits, and the distinctly longer and notably acuminate leaflets of S. americana. Leaflet length/width ratios greater than 3.2 are sometimes observed on narrower leaflets of S. decora, and a range of representative leaflets need to be sampled before determination. Another species frequently confused with S. decora is S. aucuparia, characterized by ovoid, usually densely whitish-villous and not glutinous winter buds as well as densely villous flowering pedicels and hypanthia, and often abaxially tomentose leaflets at flowering. The teeth on margins of leaves in S. decora are more finely pointed and sharper than those of S. aucuparia, and the apical tooth is often distinctly prolonged in S. decora; it is no more conspicuous than the lateral ones in S. aucuparia (E. G. Voss 1972–1996, vol. 2). The leaflets of S. aucuparia are reported to be minutely papillose abaxially (visible at 100× ); those of S. decora and other members of sect. Commixtae are not papillose (H. A. McAllister 2005). Sorbus decora is reported to be extirpated from Indiana.

Chromosome counts of 2n = 34 were reported by Á. Löve and D. Löve (1965, 1982b) for Sorbus decora. Doubt exists about the identity of the material that they sampled, as the only voucher specimen retraced (Á. & D. Löve 6559, COLO, WIN), although only vegetative and juvenile, is nonetheless referable to S. aucuparia. These doubtful chromosome counts also formed the basis for the recognition of S. groenlandica (Löve and Löve 1965b), reputedly of Greenland, coastal Labrador, and alpine and subalpine regions of northeastern North America, which apparently differed from S. decora with a chromosome count of 2n = 68 (T. W. Böcher and K. Larsen 1950; K. Holmen in C. A. Jørgensen et al. 1958). Reliable S. decora chromosome counts by H. A. McAllister (2005) of three wild collections (Dundas, Ontario; St. Anne de Beaupré, Quebec; Narsarsuaq Greenland) and two of cultivated material rendered this proposition untenable with consistent counts of 2n = 68. Further, critical examination of herbarium specimens from Greenland yielded no consistent morphologic character to separate S. decora from S. groenlandica, though leaflet and fruit size in the latter tended to be relatively smaller.

A naturally occurring putative intergeneric hybrid, ×Sorbaronia arsenii (Britton ex L. Arsène) G. N. Jones, synonym Pyrus ×arsenii (Britton ex L. Arsène) L. Arsène [= Aronia ×prunifolia (Marshall) Rehder × Sorbus decora], is known from New Brunswick, Nova Scotia, Prince Edward Island, St. Pierre and Miquelon, Quebec, and, possibly, Newfoundland. The hybrid is a 10–25 dm shrub with finely serrulate, partially pinnate leaves containing (0 or)1–3 pairs of reduced, distinct leaflets proximally and a much larger, 1–3-lobed terminal segment. The midvein on the adaxial surface of the terminal leaflet may be irregularly glandular or eglandular. A possible variant of this hybrid (parentage unconfirmed), much closer to Sorbus in appearance, is found in southwestern Newfoundland. It possesses finely serrulate, almost fully pinnate leaves, often with four or five pairs of distinct leaflets, the distal first or second leaflet pairs more or less pinnatifid or partially fused to the terminal leaflet. The hypanthium and abaxial surface of the sepals are villous, the anthers are pink or red, and the pomes are usually red, though some are reported to be blackish in age, often with some tomentum persisting on the abaxial surface of the persistent sepals.

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