Sorbus

Shrubs or trees, 10–300 dm. Stems 1–8, erect or ascending; bark gray to brown or bronze, usually smooth, becoming scaly with age, with conspicuous horizontal lenticels; long and short shoots present; unarmed; twigs with smell and taste of bitter almond (cyanogenic glycosides); glabrous or hairy. Leaves deciduous, cauline, simple or odd (rarely even) pinnately lobed or divided; stipules usually early deciduous, sometimes persistent, free or short-adnate to petiole, linear, ovate to lanceolate, or flabellate, margins entire or dentate to laciniate; petiole present; blade ovate to oblong, 5–33 cm, membranous to slightly leathery, leaflets 0 or 7–17(–19), usually opposite, rarely alternate, elliptic, ovate, oblong to lanceolate or oblanceolate, margins flat, sharply serrate to nearly entire, venation pinnate, surfaces glabrous or hairy, sometimes glaucous. Inflorescences terminal, 6–400+-flowered, panicles, flat-topped or rounded, glabrous, glaucous, or hairy; bracts present; bracteoles absent. Pedicels present. Flowers opening after leaf expansion, perianth and androecium epigynous or 1/2 epigynous, odor strong, often considered unpleasant or rancid, 5–17 mm diam.; hypanthium green to red, obconic, 2–6 mm, glabrous or hairy; sepals 5, erect or ascending, ovate or triangular; petals 5, white or pink [red], suborbiculate or broadly obovate to broadly ovate, base clawed or not, claw often ± villous; stamens [10–]14–20[–44] in 2 or 3 series, usually slightly longer than petals; carpels 2–5, distinct, partially or wholly connate and adnate to all or proximal 1/2 of hypanthium, usually apically woolly, styles 2–5, terminal, distinct or connate 1/2 of length; ovules 2 or 3[or 4] (all but 1 usually aborting). Fruits pomes, usually orange or red, rarely brown or yellow [green, white, or pink], globose to ovoid, obovoid, ellipsoid, or oblong, [pyriform], 4–19 mm, smooth or with lenticels, shiny, sometimes glaucous, hairy or glabrous; flesh usually with sclereids; hypanthium persistent; sepals usually persistent, rarely deciduous, usually incurved, fleshy; carpels cartilaginous; styles often persistent. Seeds 3–5, brown to reddish brown or yellowish, darkening with maturity, ovoid to lanceoloid, slightly asymmetric and flattened. x = 17.

Distribution

North America, Eurasia, n Africa, Atlantic Islands, Pacific Islands.

Discussion

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.

Selected References

Aldasoro, J. J. et al. 2004. Revision of Sorbus subgenera Aria and Torminaria (Rosaceae—Maloideae). Syst. Bot. Monogr. 69: 1–148.

Jones, G. N. 1939. A synopsis of the North American species of Sorbus. J. Arnold Arbor. 20: 1–43.

McAllister, H. A. 2005. The Genus Sorbus: Mountain Ashes and Other Rowans. Kew.

Lower Taxa

Sorbus subg. Aria, Sorbus subg. Sorbus, Sorbus subg. Torminaria

Keys

Key to sub-genera of Sorbus

1	Leaves pinnately compound at least proximally, petiole nodes 5-lacunate; pome flesh without tanniferous cells.	Sorbus subg. Sorbus
1	Leaves simple, sometimes lobed, petiole nodes 3-lacunate; pome flesh with tanniferous cells. 5!-!Leaves simple, lobed, or proximally pinnately compound (escapes and hybrids). 14!-!Leaves pinnately compound (mostly native species)	> 6
2	Blade margins relatively deeply lobed; pome flesh composed entirely of tanniferous cells; styles connate 1/2 their lengths.	Sorbus subg. Torminaria
2	Blade margins entire, toothed, or relatively shallowly lobed; pome flesh with tanniferous cells in groups; styles distinct.	Sorbus subg. Aria

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

1	Leaves simple, lobed, or proximally pinnately compound (escapes and hybrids).	> 2
1	Leaves pinnately compound (mostly native species).	> 6
2	Leaves simple, without leaflets; styles 2.	> 3
2	Leaves, at least some, proximally pinnate, with 1–3(–5) basal pairs of distinct leaflets; styles 2–5.	> 4
3	Leaves thinly hairy to glabrate abaxially, subpalmately lobed, lobes ± triangular, (1–)1.5–2.5 cm wide, apices acute to acuminate; pomes brown, narrowly obovoid, lenticels abundant; sepal margins prominently glandular (and villous), glands often relatively thick.	Sorbus torminalis
3	Leaves tomentose abaxially, proximally pinnately lobed, lobes ± oblong, 1–1.5(–1.8) cm wide, apices acute to obtuse; pomes bright red, ellipsoid, sometimes narrowly obovoid, lenticels few; sepal margins eglandular (and villous), rarely with inconspicuous glands.	Sorbus intermedia
4	Leaves tomentose abaxially; terminal leaflets 7–10-lobed; styles 2 or 3; pomes red.	Sorbus hybrida
4	Leaves glabrous, glabrate, or sparsely to densely villous abaxially; terminal leaflets 1–3-lobed; styles 3–5; pomes dark purple or red	> 5
5	Terminal leaflet coarsely serrate; pomes subglobose; sepals erect or spreading-ascending, prominent ×Amelasorbus	> 5
5	Terminal leaflet finely serrate; pomes ellipsoid to ovoid; sepals incurved, inconspicuous ×Sorbaronia	> 1
6	Leaflets shiny adaxially (visible on herbarium specimens with strong lighting), green to dark green, never glaucous or blue-green; w North America	> 7
6	Leaflets dull adaxially, green to blue-green or yellowish green, sometimes slightly glaucous; w, e North America	> 9
7	Pomes: sepals erect, prominent; flowering hypanthium plus sepals (4.5–)5–6 mm; petals white to pinkish; w Aleutians (Alaska).	Sorbus sambucifolia
7	Pomes: sepals incurved, inconspicuous; flowering hypanthium plus sepals 3–4 mm; petals white; w North America (except w Aleutians)	> 8
8	Indument primarily rufous on winter buds, leaflet axils, and inflorescences; fruiting pedicels essentially glabrous; leaflets 2.5–4.1(–4.5) cm.	Sorbus californica
8	Indument primarily whitish on winter buds, leaflet axils, and inflorescences; fruiting pedicels sparsely to densely villous; leaflets (3–)4–6.5(–8.7) cm.	Sorbus scopulina
9	Winter buds densely villous, hairs whitish, rarely rufous; hypanthia densely villous, hairs whitish.	Sorbus aucuparia
9	Winter buds glabrous or sparsely to densely villous, hairs primarily rufous; hypanthia glabrous or sparsely villous proximally, hairs whitish or rufous	> 10
10	Leaflet l/w ratios 3.4–5, apices gradually acuminate to long-acuminate; pomes 4–7 mm diam., not glaucous when fresh or dried.	Sorbus americana
10	Leaflet l/w ratios 1.9–3.5(–3.6), apices abruptly short-acuminate, cuspidate, or obtuse to acute; pomes (5–)7–13 mm diam., often glaucous (sometimes only when dried)	> 11
11	Winter buds glutinous, shiny; leaflets (11–)13–17; e North America.	Sorbus decora
11	Winter buds not glutinous, dull; leaflets 7–13; w North America.	Sorbus sitchensis