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 grandidentata is a successional species that regenerates after fires by suckering from living rootstocks. The exclusively neoformed leaves on such suckers are much larger than those found on mature trees, are conspicuously pubescent abaxially, and are similar enough to preformed and neoformed leaves of P. heterophylla that they are responsible for incorrect published reports of the latter in upland sites. Once suckers reach their second or third year and begin to branch, they start to bear at least some preformed leaves that clearly identify them as Bigtooth aspen. As far as is known, P. grandidentata and P. heterophylla never grow together at a single site.

Bigtooth aspen hybridizes sporadically with the other native aspen, Populus tremuloides, to form P. ×smithii B. Boivin (synonym P. ×barnesii W. H. Wagner) throughout their large region of sympatry (Frère Marie-Victorin 1930; S. S. Pauley 1956; B. V. Barnes 1961; W. H. Wagner Jr. 1970). Leaves of the hybrids have more numerous, smaller, more rounded teeth than those of P. grandidentata. They may be found as far west as Niobrara River valley, Nebraska, 350 km west of the nearest present station of P. grandidentata.

The related Eurasian white poplar, Populus alba Linnaeus, is commonly and widely planted throughout temperate North America as a pistillate clone with a spreading crown or, less often, as a columnar staminate clone, the Bolleana poplar (‘Pyramidalis’), both of which can persist after cultivation and even spread to a limited extent by root sprouts in old garden sites, roadsides, waste places, hedgerows, and edges of woods. This species differs from P. grandidentata (and all other species of the genus) in having neoformed leaf blades palmately 5-lobed and, along with the petioles, densely white-tomentose abaxially. Unlike all native species of Populus, white poplar has floral bract apices only shallowly cut; these are ciliate like those of native aspens. Populus alba hybridizes commonly with both P. grandidentata, forming P. ×rouleauiana B. Boivin, and P. tremuloides, forming P. ×heimburgeri B. Boivin, in southeastern Canada and the northeastern United States; the hybrids are progressively uncommon southward (E. L. Little Jr. et al. 1957; T. A. Spies and B. V. Barnes 1982). Although their leaves are tomentose abaxially, they differ from P. alba in not having the deeply 5-lobed neoformed leaves. Those of P. ×heimburgeri are shallowly 3-lobed, with apical lobes much larger than lateral ones, and those of P. ×rouleauiana are irregularly and compoundly toothed. With their prominently white-tomentose leaves abaxially, both hybrids are often misidentified as white poplar and are the basis for published reports of naturalized P. alba in sites away from present or former cultivation.

The gray poplar, Populus ×canescens (Aiton) Smith, a natural hybrid between P. alba and the Eurasian aspen, P. tremula Linnaeus, is common and variable in Europe but usually represented by a single pistillate clone in North America. It is widely cultivated, persisting and spreading by root sprouts at former homesites, in waste places, and edges of woods. It is usually less frequent than P. alba, but mostly replaces white poplar in southeastern United States, where it is more widely established under semi-natural conditions. Neoformed leaves have a thin, grayish tomentum abaxially and are irregularly and coarsely toothed rather than 5-lobed, like those of P. alba. Populus ×tomentosa Carrière, from China, is a similar hybrid white poplar with larger leaves and two prominent, cup-shaped basilaminar glands that is rarely planted in southeastern United States in the form of a pistillate clone. It is derived from hybridization between P. alba and a Chinese aspen, P. adenopoda Maximowicz.

Populus alba is similar to P. ×canescens in having resinous winter buds, branchlets and terminal buds that are densely to sparsely tomentose, leaf blade surfaces densely tomentose abaxially when young, retaining their dense tomentum on at least some intervein regions, and usually 5–8(–10) cm. Flowers are similar with discs narrowly cup-shaped, obviously oblique, catkins with floral bracts that are ciliate and apices shallowly cut, 6–12 stamens, the 2 stigmas filiform, and ovaries narrowly ovoid to lanceoloid. Capsules are similar in that they are usually 2-valved, lanceoloid or narrowly ovoid, and 2–7(–9) mm, with seeds (1 or) 2 (or 3) per placenta. The two taxa differ by P. alba having white hairs, neoformed blade margins deeply to shallowly (3 or) 5-lobed, catkin floral bracts densely tomentose, and (1 or) 2 (or 3) seeds per placenta; P. ×canescens has grayish, tannish, or dirty white (tomentose) hairs, neoformed blade margins irregularly toothed, and (3–)5–8 (or 9) seeds per placenta.

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