Flora of North America species comparison

Populus balsamifera has been reported in error from Nebraska based on incorrectly localized specimens and from Idaho, Oregon, and Utah based on vegetative specimens of P. trichocarpa or intergrades. The two species can be difficult to separate vegetatively, particularly in their region of overlap and hybridization along the northern Cordilleran axis from southern Alaska to northwestern Wyoming. Their hybrids, named P. ×hastata Dode, have intermediate leaf shapes and also differ from P. balsamifera in having capsules with 2–4 sparsely hairy or glabrous valves (T. C. Brayshaw 1965). Populus balsamifera also hybridizes and intergrades with another native species of sect. Tacamahaca, P. angustifolia, to form P. ×brayshawii B. Boivin where the margins of their ranges overlap (Brayshaw 1965b). Populus ×brayshawii is intermediate in some respects between its parents. It is most similar to P. angustifolia; it differs in longer petioles and darker twigs, characteristics in which it approaches P. balsamifera. Some trees of P. balsamifera from North Dakota (Bottineau and Divide counties) also seem to show an influence of P. angustifolia, although they are far from the present range of the latter species.

Populus ×jackii Sargent (synonyms P. ×andrewsii Sargent, P. ×bernardii B. Boivin, P. ×dutillyi Lepage, and P. ×gileadensis Rouleau) is an intersectional hybrid with P. deltoides (sect. Aigeiros) and is moderately common in riparian and other wet habitats throughout the broad range of overlap between these two species (Alberta, Colorado, Illinois, Indiana, Iowa, Manitoba, Massachusetts, Michigan, Minnesota, Montana, Nebraska, New York, North Dakota, Ontario, Quebec, Pennsylvania, Saskatchewan, South Dakota, West Virginia, and Wisconsin) (W. G. Ronald et al. 1973; J. E. Eckenwalder 1984). A pistillate clone that probably arose from this hybrid by segregation or backcrossing (‘Balm-of-Gilead’) has been widely cultivated since at least the eighteenth century for its bud resin, used in treating coughs (E. Rouleau 1948). Capsules rarely, if ever, mature and trees do not appear to produce fertile seed, but persist and spread by root sprouts in waste places and at edges of woods. It is cultivated mostly in southeastern Canada and eastern United States to the Great Plains, chiefly in mountains in southeastern United States. This clone is more balsam poplarlike than first generation hybrids and has differences from wild hybrids in North America. It may have arisen in Europe through hybridization and backcrossing between P. balsamifera and southern P. deltoides subsp. deltoides. Most wild hybrids have P. deltoides subsp. monilifera as the cottonwood parent.

Similar to the other North American balsam poplars, Populus balsamifera hybridizes sporadically with the introduced Eurasian P. nigra. That hybrid, P. ×rollandii Rouleau, which was originally thought to have involved P. ×canadensis, itself a hybrid offspring of P. nigra, has been collected in the vicinity of Montreal, Quebec (E. Rouleau 1944). Its leaves are similar in shape to those of P. ×canadensis but have the reddish resin stains and cylindrical petioles of P. balsamifera.

Populus balsamifera does not hybridize naturally with P. tremuloides, as sometimes reported (E. Lepage 1961; F. G. Bernard 1968). Specimens that formed the basis for those reports are either P. ×jackii or slender sucker shoots of P. tremuloides bearing correspondingly narrow leaves.

Populus ×jackii has branchlets that are short-haired or pubescent, petioles densely pubescent, at least distally, preformed blade margins with 20(–45) teeth on each side, sinuses to 1.5 mm deep, pedicels to 3 mm, discs saucer-shaped, 1–4 mm diam., ovaries 3- or 4-carpelled and glabrous, capsules usually 3- or 4-valved, ovoid, (4–)8–11(–16) mm, and seeds (6 or) 7–15(–25) per placenta. It is similar to P. simonii in having winter buds with red resin, petioles to 2 cm, round, cylindrical, or distally slightly flattened in the plane of blade, leaf blades lighter green abaxially, elliptic-rhombate to ovate, (1–)3–20(–27.5) cm, bases acute, cuneate, rounded, truncate, or shallowly cordate, apices obtuse, acute, or acuminate, and surfaces not obviously resin-stained, with abaxial surface glabrous or with short, stiff hairs. The flowers are similar with discs entire, persistent, and not obviously oblique, catkins with floral bracts not ciliate and glabrous abaxially, 10–60(–70) stamens (P. simonii with fewer than 12), anthers usually truncate, stigmas 2–4 and expanded, and ovaries ovoid to spherical. In addition to these traits, P. simonii has terminal buds that are usually less than 12 mm, branchlets whitish tan by the third year, catkins 3–8 cm, and stamens usually fewer than 12.

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

Populus angustifolia has been mistakenly reported from eastern California and eastern Oregon based on intergrades with, and narrow-leaved specimens of, P. trichocarpa in those arid regions. It is a characteristic species of the Rocky Mountains, extending onto the plains and overlapping in canyon mouths as they exit the mountains with two North American species of sect. Aigeiros, P. deltoides and P. fremontii, and hybridizing with each of them. It also hybridizes with the other two native species of sect. Tacamahaca, P. balsamifera and P. trichocarpa. The hybrid with P. balsamifera, P. ×brayshawii B. Boivin, differs most obviously in longer petioles, at least 2.5 cm, and is increasingly common from Colorado northwards, largely replacing P. angustifolia in southern Alberta (T. C. Brayshaw 1965b; S. B. Rood et al. 1985). The hybrid with P. trichocarpa (unnamed) is uncommon and largely confined to the Great Basin region, including Montana (W. W. White 1951). Populus angustifolia does not hybridize naturally with P. tremuloides, as sometimes reported (B. Boivin 1966b). The specimens that formed the basis for that report are long shoots of P. angustifolia with relatively coarsely toothed neoformed leaves.

Populus ×acuminata Rydberg is the intersectional hybrid of P. angustifolia with P. deltoides (sect. Aigeiros) that occurs on floodplains of major streams, primarily along the foot of the Rocky Mountain Front Ranges where these species grow together, but also extends onto the plains and Colorado Plateau (Alberta, Arizona, Colorado, Montana, Nebraska, New Mexico, North Dakota, South Dakota, Texas, Utah, and Wyoming) (J. E. Eckenwalder 1984). As with other cloning hybrids, it can often occur without one or both parents. It differs from P. angustifolia in larger, ovate leaves with coarser teeth, less color differentiation between abaxial and adaxial surfaces, and longer petioles that are slightly flattened side to side near the junction with the blade. Because of its frequency and morphological consistency, P. ×acuminata was first described as a species and is often treated as such in local and regional floras. It was long suspected of being a hybrid, and its hybrid origin was amply confirmed by multiple lines of evidence in the 1970s and 1980s (D. J. Crawford 1974; A. G. Jones and D. S. Seigler 1975; S. B. Rood et al. 1985). The name has also been widely misapplied to intersectional hybrids involving other combinations of balsam poplar and cottonwood parents (Eckenwalder).

Populus ×berolinensis Dippel is a similar cultivated, introduced, intersectional hybrid between two Eurasian species, P. laurifolia Ledebour (sect. Tacamahaca) and P. nigra Linnaeus (sect. Aigeiros). It has an upright growth habit and leaves similar in shape to those of P. ×acuminata, but more balsam poplar-like with smaller teeth and greater color differentiation between abaxial and adaxial surfaces. Pistillate individuals may hybridize with native species of both parent sections (W. G. Ronald and J. W. Steele 1974).

Populus ×hinckleyana Correll (synonyms P. ×acuminata nothomorph rehderi Sargent and P. ×intercurrens S. Goodrich & S. L. Welsh) is the intersectional hybrid of P. angustifolia with P. fremontii (sect. Aigeiros) and is moderately common in canyons throughout their range of sympatry (Arizona, Idaho, Nevada, New Mexico, Texas, Utah, and Mexico [Chihuahua, Sonora]). At the type locality in the Davis Mountains, Texas, P. angustifolia is now absent, and the cottonwood parent is P. fremontii subsp. mesetae, but other occurrences have arisen through hybridization with P. fremontii subsp. fremontii (G. C. Bennion et al. 1961; J. E. Eckenwalder 1984; P. Keim et al. 1989). It differs from P. angustifolia in hairy young shoots and a broader floral disc.

A related Eurasian species, Populus simonii Carrière, is moderately commonly cultivated, especially in the form of a broadly pyramidal staminate clone (‘Pyramidalis’). It shares the slender, often 5-angled twigs and relatively small leaves of P. angustifolia, but differs most noticeably in elliptic blades of preformed leaves and obovate blades of neoformed leaves.

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