Flora of North America species comparison

Subspecies 3 (3 in the flora).

Populus deltoides hybridizes with P. fremontii, the other native species of sect. Aigeiros, in the Colorado Plateau region (Arizona, New Mexico, and Utah) and trans-Pecos Texas. These hybrids involve P. deltoides subsp. wislizeni with both subspecies of P. fremontii and are difficult to distinguish because the parent species are so similar. They have shallowly cup-shaped discs 3–5 mm diam., pedicels 4–6 mm, and, usually, sparsely pubescent branchlets. Populus deltoides hybridizes also with three native members of sect. Tacamahaca. All three hybrids, P. ×generosa A. Henry (P. trichocarpa × P. deltoides), P. ×jackii Sargent (P. balsamifera × P. deltoides), and P. ×acuminata Rydberg (P. angustifolia × P. deltoides), are distinguished from P. deltoides by their buds with reddish resin, fewer triangular leaves with finer teeth, less flattening of the petiole, and a distinctly paler, slightly whitened abaxial leaf surface (J. E. Eckenwalder 1984). Individual hybrids have distinct ranges corresponding to their parental regions of sympatry and may be distinguished from each other by relative leaf width (less than two-thirds as wide as long in P. ×acuminata and more than two-thirds as wide as long in the other two) and base shapes (cordate in P. ×jackii and cuneate in the other two). Hybrids with members of the other two sections are rare or unknown. Hybridization with P. heterophylla (sect. Leucoides) is apparently rare and very local in South Carolina, even though the region of sympatry of these two species occupies essentially the entire range of P. heterophylla. Reported hybrids with P. tremuloides (sect. Populus) named as P. ×bernardii B. Boivin (T. C. Brayshaw 1965b; B. Boivin 1966b) are actually individuals of P. ×jackii (Eckenwalder).

Two related members of sect. Aigeiros, Populus nigra Linnaeus and P. ×canadensis Moench, are often planted as staminate clones, often persist after cultivation, and spread by root suckers but never become naturalized. Most individuals of Eurasian P. nigra cultivated in North America are Lombardy poplars (cv. Italica), an unmistakable, narrowly columnar, staminate clone with heavily buttressed trunk, rhombic preformed leaves, and triangular-ovate neoformed leaves broader than wide. This tree has been known since the eighteenth century and is widely (over-)planted throughout the temperate portion of the flora area as an accent tree. It hybridizes sporadically here with the three native balsam poplars, P. angustifolia, P. balsamifera, and P. trichocarpa; hybrids are discussed under those species. A rare hybrid with P. fremontii (P. ×inopina Eckenwalder) apparently originated from an uncommon pistillate tree of P. nigra (J. E. Eckenwalder 1982). It resembles P. fremontii in leaf shape but has dark reddish brown winter buds.

Populus nigra and P. ×canadensis are both staminate and are similar in having winter buds usually 12+ mm with a balsamic fragrance and orange-red resin. The branchlets are round and bright orange-brown to reddish brown in the first year, turning tan by the third year. The petioles are distally flattened at a right angle to the plane of blade. The margins of the leaf blade are translucent and ciliate; the leaf surfaces are glabrous or glabrate to pubescent but not tomentose. The catkins usually have more than 15 flowers, (4–)7–15 cm. The floral disc is entire, persistent, broadly cup- or saucer-shaped, and not obviously oblique. The anthers are usually truncate. The 2-carpelled ovary is ovoid to spherical and the 2–4 stigmas are expanded. The floral bracts are not ciliate and are glabrous abaxially. The two taxa differ in that P. nigra has branchlets that are nearly parallel, leaf blades without basilaminar glands, preformed blade margins with sinuses no more than 1.2 mm deep, and 12–20(–30) stamens; P. ×canadensis has divergent branchlets, branching at 50º or more, leaf blades with no more than 1 basilaminar gland, preformed blades with the base broadly cuneate and apex gradually acuminate, and (15–)20–30 stamens.

Populus ×canadensis (P. ×euramericana Guinier [illegitimate name]; B. K. Boom 1957) is an intercontinental hybrid that first arose spontaneously between P. deltoides and P. nigra after the former was introduced into Europe in the late seventeenth century. Deliberate new hybrids of this parentage are one of the mainstays of Europe’s growing commercial poplar plantations. They are also important in eastern North America but are often replaced by P. ×generosa (P. trichocarpa × P. deltoides) in commercial plantations in British Columbia, Oregon, and Washington. Only one clone is commonly, and very widely, grown horticulturally, the Carolina poplar (‘Eugenei’), a staminate clone with a fairly narrow habit inherited from its staminate parent, the Lombardy poplar. It is often confused with P. deltoides, with narrower preformed leaves, often slightly longer than wide, with more numerous, smaller teeth, and with bases obtuse or rounded, rather than truncate or subcordate. It differs further from P. deltoides subspp. deltoides and monilifera in having 0–1 basilaminar glands rather than 2–6.

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

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.)