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.)

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.)