Flora of North America species comparison

Clonal aspen groves develop rapidly following fires and other disturbances and may quickly decay in their absence as infections are transmitted through the connecting root system. Populus tremuloides is the most widely distributed tree in North America, found throughout cold and cool-temperate regions from coast to coast and from within the Arctic Circle to the north rim of the Valley of Mexico. It ranges from sea level in the north and east to the north slopes of high mountains in the southernmost part of its range. The southerly locations, like that on Mt. Livermore in the Davis Mountains, Texas, the most southerly stand in the flora area, may be Pleistocene relicts. Groves are often occupied by single clones and show no sexual reproduction but persist and spread by root suckers. Clone formation commonly results also in striking differences in appearance and phenology of adjacent groves or blocks of trees (B. V. Barnes 1969). Some individuals display a particularly rich, yellow autumn coloration that makes them a standout among North American trees, particularly in the West, where this richness was the basis for segregation of P. tremuloides var. aurea. There do not appear to be consistent regional differences within the species that would justify recognition of subspecies or varieties (Barnes 1975). Instead, there is as much variation from clone to clone within a region as there is among regions.

Populus tremuloides hybridizes with both the native P. grandidentata (P. ×smithii B. Boivin) and the Eurasian P. alba (P. ×heimburgeri B. Boivin) in southeastern Canada and the northeastern United States (B. V. Barnes 1961; T. A. Spies and Barnes 1982). Populus ×smithii occurs as far west as the Niobrara River valley, Nebraska, ca. 350 km west of the nearest present populations of P. grandidentata. Preformed leaves are more ovate than those of P. tremuloides and have larger teeth. Populus ×heimburgeri has transiently tomentose twigs, buds, and abaxial leaf surfaces. Contrary to some published reports (E. Lepage 1961; T. C. Brayshaw 1965b; B. Boivin 1966b; F. G. Bernard 1968), P. tremuloides does not hybridize naturally with P. angustifolia, P. balsamifera, or P. deltoides. The correct identification of such specimens is discussed under each of the purported parents.

The closely related Eurasian aspen, Populus tremula Linnaeus, is sometimes cultivated in North America, particularly as a columnar staminate clone (‘Erecta’). Its leaves are very similar in shape to those of P. tremuloides and usually have slightly larger teeth. Buds are often minutely hairy. Artificial hybrids between P. tremula and P. tremuloides, P. ×wettsteinii Hämet-Ahti, are sometimes grown for plantation forestry, particularly in the Great Lakes region.

(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.)