Sorbus scopulina
Sorbus americana
Cascade mountain-ash, Greene's Mountain ash, mountain ash, Rocky Mountain ash, Rocky Mountain Mountain ash, Rocky Mountain or Cascade Mountain ash, Rocky Mountain or Cascade or Greene's Mountain Ashe, western mountain-ash
American mountain-ash, sorbier d'amérique
1–8;
bark gray, sometimes yellowish or reddish purple, or grayish red;
winter buds olive brown to red-brown, conic, 8–14 mm, shiny, slightly glutinous, glabrous or sparsely or densely whitish-villous.
1–4+;
bark gray to bronze;
winter buds green to purple, shiny, ovoid to conic, 7–20 mm, glutinous, glabrous or hairy along scale margins and at apex, hairs primarily rufous.
pinnately compound;
stipules deciduous or persistent, hairs whitish;
blade paler abaxially, shiny, green to dark green adaxially, leaflets 7–13(–15), opposite or subopposite, lanceolate, oblong, narrowly ovate, oblanceolate, or obovate, (3–)4–6.5(–8.7) × 1.5–2.7 cm, l/w ratio 2.1–4.4, margins finely serrate, apex obtuse, acute, or acuminate, surfaces glabrous, leaf and leaflet axils hairy, hairs whitish.
pinnately compound;
stipules early deciduous, rufous-ciliate, margins sometimes glandular;
blade paler abaxially, dull green to yellowish green adaxially, leaflets 11–17, opposite, lanceolate to oblong-lanceolate, 5–10 × 1–2.5 cm, l/w ratio 3.4–5, margins serrulate to serrate, at least in apical 1/2 and often almost to base, apex acuminate to long acuminate, surfaces glabrous or glabrate at flowering, some hairs occasionally persisting abaxially along midvein;
leaflet axils and petiole bases glabrous or with rufous and/or whitish hairs adaxially.
40–200+-flowered, flat-topped to rounded, 5–15 cm diam.;
peduncles sparsely to densely whitish-villous.
125–400+-flowered, flat-topped or rounded, 6–15 cm diam.;
peduncles glabrous or sparsely villous.
sparsely to densely whitish-villous (in flower and fruit).
glabrous or sparsely villous.
11–13 mm diam.;
hypanthium nearly glabrous or densely villous, hypanthium plus sepals 3–4 mm;
sepals 0.8–2.3 mm, margins lightly to densely whitish-villous, glands absent or sparse and usually inconspicuous;
petals white, ovate, 4–6 mm;
stamens (14–)20;
carpels distinct, apex conic, styles 3 or 4, 2–2.5 mm.
5–7.5(–8.5) mm diam.;
hypanthium glabrous, hypanthium plus sepals 2–2.5 mm;
sepals 0.5–1 mm, margins entire, often with a few thick glands;
petals white, orbiculate to obovate, (2–)2.5–3.5(–4) mm;
stamens 14–20;
carpels 1/2 adnate to hypanthium, apex conic, styles 3 or 4, 1.5–2 mm.
whitish-villous, rarely glabrous with age.
glabrous or sparsely villous.
bright orange to reddish orange, globose, subglobose, broadly obovoid, or broadly elliptic, 8–12.1 × 7.5–12.8 mm, shiny, sometimes lightly glaucous;
sepals inconspicuous, incurved.
bright red to orange-red, globose to subglobose, 4–7 mm diam., shiny, not glaucous;
sepals inconspicuous, incurved.
brown, narrowly ovoid, 3.8–5.4 × 1.7–2.8 mm, slightly asymmetric, slightly flattened.
brown, yellowish when fresh or immature, ovoid to ovoid-lanceoloid, 2.5–3.3 × 1.5–2 mm, asymmetric, slightly flattened.
= 34, 68.
= 34.
Sorbus scopulina
Sorbus americana
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 americana reputedly flowers about one week earlier than S. decora. Some authors have described S. americana as having generally thinner leaflets relative to thicker and firmer ones for S. decora; examination of herbarium specimens appears to support this, though the authors made no systematic study of blade thickness. No confirmed putative hybrids with S. decora are reported, although hybridization may occur. The hybrid ×Sorbaronia jackii Rehder [Aronia ×prunifolia (Marshall) Rehder × S. americana; synonym Pyrus ×jackii (Rehder) Fernald] has been reported from New Brunswick as a fastigiate shrub 20 dm tall with leaves intermediate in appearance between the putative parents. Garden hybrids involving S. americana are occasionally cultivated in North America; none has been reported as escaped. These include: Sorbus ×splendida Hedlund (a hybrid with S. aucuparia), S. ×plantierensis Simon-Louis ex C. K. Schneider (with S. aria), ×Sorbaronia monstrosa (Zabel) C. K. Schneider (with A. arbutifolia), and ×Sorbaronia sorbifolia (Poiret) C. K. Schneider (with A. melanocarpa); synonyms of ×Sorbaronia sorbifolia include A. sargentii (Dippel) Zabel, A. watsoniana M. Roemer, and Pyrus ×mixta Fernald.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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