Flora of North America species comparison

Populus trichocarpa has been mistakenly reported from North Dakota based on specimens of P. ×jackii (P. balsamifera × P. deltoides). It hybridizes with P. balsamifera to form P. ×hastata Dode along the northern Rocky Mountain axis (Alaska, Alberta, British Columbia, and Idaho). Hybrids have capsules with 2–4 glabrous or sparsely hairy valves. The extent of hybridization has led to treatment of P. trichocarpa as a subspecies of P. balsamifera (T. C. Brayshaw 1965; L. A. Viereck and J. M. Foote 1970); these two balsam poplars are more closely related to Asian members of sect. Tacamahaca than they are to each other (J. E. Eckenwalder 1996). Mountain ranges of the Intermountain Region (California, Idaho, Nevada, Oregon, and Utah) have trees intermediate between P. trichocarpa and P. angustifolia. These hybrids have narrower leaves with shorter petioles and sparsely hairy capsules with 2–3 valves.

In addition to hybridizing with other North American species of sect. Tacamahaca, Populus trichocarpa also hybridizes with both native species of sect. Aigeiros. Populus ×generosa A. Henry (synonym P. ×interamericana van Broekhuizen), a hybrid between P. trichocarpa and P. deltoides, is rare in the far western area of the range for P. deltoides subsp. monilifera, where it overlaps with the more drought-tolerant inland P. trichocarpa (Idaho, Montana, Washington, and Wyoming) (J. E. Eckenwalder 1984). This hybrid has also been grown artificially, and such hybrids between coastal P. trichocarpa and P. deltoides subsp. deltoides are becoming increasingly important plantation trees in the Pacific Northwest from northern Oregon to British Columbia, as well as in Europe. They are perhaps the fastest growing of all poplars in volume, with the rapid height growth of P. trichocarpa added to the steady diameter growth of P. deltoides (R. F. Stettler et al. 1988).

Populus ×parryi Sargent, a hybrid between P. trichocarpa and P. fremontii, is commonly found in a wide variety of mesic habitats throughout the region of sympatry between its parents in California and Nevada (and beyond the range of P. trichocarpa in Mohave County, Arizona; J. E. Eckenwalder 1992). It can be found particularly in canyons where its parents are elevationally separated but overlap as permanent streams spill out into lower elevations (Eckenwalder 1984, 1984b). A morphologically and ecologically distinctive race of P. trichocarpa in coastal southern California with heart-shaped leaves may have arisen through this kind of hybridization (Eckenwalder 1984c). This race includes the type of P. trichocarpa from Ventura County.

Populus maximowiczii A. Henry is an Asian balsam poplar that is sometimes cultivated as an ornamental, but usually as a plantation tree or parent of plantation hybrids. It is distinguished from P. trichocarpa and P. balsamifera by its elliptic leaves with rugose adaxial surfaces.

(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.)