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 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.)