Flora of North America species comparison

Species ca. 130 (10 in the flora).

The taxonomy of Sorbus is complicated by apomixis, polyploidy, and hybridization among sections and genera, especially in Eurasia. Hybrids with other genera in Maleae are generally distinguished by their incompletely divided, deeply lobed leaves, and include ×Amelasorbus Rehder (Sorbus × Amelanchier), ×Crataegosorbus Makino (Sorbus × Crataegus), ×Sorbaronia C. K. Schneider (Sorbus × Aronia), ×Sorbocotoneaster Pojarkova (Sorbus × Cotoneaster), ×Sorbopyrus C. K. Schneider (Sorbus × Pyrus), and ×Tormimalus Holub (as Torminalis Medikus [= Sorbus subg. Torminaria] × Malus). Only ×Amelasorbus and ×Sorbaronia are known to occur naturally in North America. Parentages of some reported ×Sorbaronia nothospecies are difficult to verify, especially for densely hairy hybrids involving either Aronia arbutifolia, A. ×prunifolia (Marshall) Rehder, or Sorbus aucuparia; all these possibilities are claimed. In addition to their partially pinnate leaves, some ×Sorbaronia hybrids may be recognized by the presence of pink to red anthers and black or purple fruits; these are reported also to be somewhat sterile.

Diploid Sorbus are often obligate outcrossers; some apomictic triploids and tetraploids are self-compatible (C. S. Campbell et al. 1991; H. A. McAllister 2005). Some species have morphologically indistinguishable diploid and tetraploid races, with sexual and apomictic individuals. North American species are in need of biosystematic review.

Sorbus is treated here in the broad sense, as a provisional arrangement to accommodate more than 70 natural hybrids between simple- and pinnate-leaved subgenera (J. B. Phipps et al. 1990; E. B. Nelson-Jones et al. 2002; C. Kalkman 2004; A. Robertson et al. 2004, 2004b; J. J. Aldasoro et al. 2004, 2005). Additional hybrid taxa continue to be discovered in Europe (G. Aas et al. 1994; M. F. Fay et al. 2002; T. C. G. Rich and L. Houston 2006; A. Robertson and C. Sydes 2006). Nonetheless, the argument has been made to revive generic ranking for the simple-leaved subgenera Aria and Torminaria, based on flower and fruit characters (K. R. Robertson et al. 1991), with some support from molecular investigations (C. S. Campbell et al. 1995, 2007; D. Potter et al. 2007). Subgenus Torminaria is anomalous in Sorbus, having a unique flavonoid chemistry, leaves with relatively few broad lobes, and scented brown fruits, thick-skinned as in subg. Cormus (Spach) Duchartre and some Pyrus, and being dispersed by carnivorous mammals as well as birds (C. M. Herrera 1987, 1989). Eurasian hybrids between subgenera frequently involve S. torminalis of subg. Torminaria. Subgenera Aria and Torminaria are likely to be recognized at generic rank once molecular studies can consistently resolve their placement within the Pyrinae, overcoming current difficulties with interfertility, reticulate relationships, rapid radiation, and small samples (Campbell et al. 2007; Potter et al.).

Some species of Sorbus are cultivated as ornamentals. The colorful fruits of native and introduced species persist after the leaves drop and are consumed by birds across North America (A. C. Martin et al. 1951). One exotic species disseminated by birds is invasive (S. aucuparia). The pomes of S. americana, collected after a frost, can be sweetened for preserves. The pulp of some strains of S. aucuparia is reported to be less bitter. It may be eaten fresh or dried or used as flour. The fruits of a number of Eurasian species are made into vinegar, spirits, or medications. Sorbus domestica and S. torminalis pomes are sometimes eaten after being bletted (allowed to autodigest and become soft and mushy in texture). The fine-grained wood of Sorbus is used for carving or furniture in China. The bark of some species is antiseptic, or used to tan leather. Sorbus sambucifolia pomes show some potential in early testing for treating tumors (Y. Yoshizawa et al. 2000, 2000b).

In order to identify intergeneric hybrids involving Sorbus as one of the parents, the following key includes leads for ×Amelasorbus and ×Sorbaronia.

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

Sorbus hybrida is considered an apomictic tetraploid producing fully fertile pollen and seed and subject to dispersal. An almost undistinguishable European taxon, S. ×thuringiaca (Ilse) Fritsch, often confused with S. hybrida, has been reported for New Hampshire and Vermont. Described as an unstable diploid hybrid between S. aria and S. aucuparia, S. ×thuringiaca has oblong leaves, 1–3(–5) pairs of free leaflets, 10–12(–13) pairs of lateral veins, and produces sterile pollen and relatively few viable seeds. Uncritical reports of escaped S. ×thuringiaca in North America should be considered doubtful; they probably refer to the more likely S. hybrida. The authors have not seen a specimen to document a report of S. hybrida from Montana.

Sorbus hybrida is placed in subg. Sorbus following J. J. Aldasoro et al. (2004), based on leaf division and pome characteristics, including the lack of tanniferous cells in parenchyma, lack of starch, and relatively small sclereid groups.

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