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 angustifolia has been mistakenly reported from eastern California and eastern Oregon based on intergrades with, and narrow-leaved specimens of, P. trichocarpa in those arid regions. It is a characteristic species of the Rocky Mountains, extending onto the plains and overlapping in canyon mouths as they exit the mountains with two North American species of sect. Aigeiros, P. deltoides and P. fremontii, and hybridizing with each of them. It also hybridizes with the other two native species of sect. Tacamahaca, P. balsamifera and P. trichocarpa. The hybrid with P. balsamifera, P. ×brayshawii B. Boivin, differs most obviously in longer petioles, at least 2.5 cm, and is increasingly common from Colorado northwards, largely replacing P. angustifolia in southern Alberta (T. C. Brayshaw 1965b; S. B. Rood et al. 1985). The hybrid with P. trichocarpa (unnamed) is uncommon and largely confined to the Great Basin region, including Montana (W. W. White 1951). Populus angustifolia does not hybridize naturally with P. tremuloides, as sometimes reported (B. Boivin 1966b). The specimens that formed the basis for that report are long shoots of P. angustifolia with relatively coarsely toothed neoformed leaves.

Populus ×acuminata Rydberg is the intersectional hybrid of P. angustifolia with P. deltoides (sect. Aigeiros) that occurs on floodplains of major streams, primarily along the foot of the Rocky Mountain Front Ranges where these species grow together, but also extends onto the plains and Colorado Plateau (Alberta, Arizona, Colorado, Montana, Nebraska, New Mexico, North Dakota, South Dakota, Texas, Utah, and Wyoming) (J. E. Eckenwalder 1984). As with other cloning hybrids, it can often occur without one or both parents. It differs from P. angustifolia in larger, ovate leaves with coarser teeth, less color differentiation between abaxial and adaxial surfaces, and longer petioles that are slightly flattened side to side near the junction with the blade. Because of its frequency and morphological consistency, P. ×acuminata was first described as a species and is often treated as such in local and regional floras. It was long suspected of being a hybrid, and its hybrid origin was amply confirmed by multiple lines of evidence in the 1970s and 1980s (D. J. Crawford 1974; A. G. Jones and D. S. Seigler 1975; S. B. Rood et al. 1985). The name has also been widely misapplied to intersectional hybrids involving other combinations of balsam poplar and cottonwood parents (Eckenwalder).

Populus ×berolinensis Dippel is a similar cultivated, introduced, intersectional hybrid between two Eurasian species, P. laurifolia Ledebour (sect. Tacamahaca) and P. nigra Linnaeus (sect. Aigeiros). It has an upright growth habit and leaves similar in shape to those of P. ×acuminata, but more balsam poplar-like with smaller teeth and greater color differentiation between abaxial and adaxial surfaces. Pistillate individuals may hybridize with native species of both parent sections (W. G. Ronald and J. W. Steele 1974).

Populus ×hinckleyana Correll (synonyms P. ×acuminata nothomorph rehderi Sargent and P. ×intercurrens S. Goodrich & S. L. Welsh) is the intersectional hybrid of P. angustifolia with P. fremontii (sect. Aigeiros) and is moderately common in canyons throughout their range of sympatry (Arizona, Idaho, Nevada, New Mexico, Texas, Utah, and Mexico [Chihuahua, Sonora]). At the type locality in the Davis Mountains, Texas, P. angustifolia is now absent, and the cottonwood parent is P. fremontii subsp. mesetae, but other occurrences have arisen through hybridization with P. fremontii subsp. fremontii (G. C. Bennion et al. 1961; J. E. Eckenwalder 1984; P. Keim et al. 1989). It differs from P. angustifolia in hairy young shoots and a broader floral disc.

A related Eurasian species, Populus simonii Carrière, is moderately commonly cultivated, especially in the form of a broadly pyramidal staminate clone (‘Pyramidalis’). It shares the slender, often 5-angled twigs and relatively small leaves of P. angustifolia, but differs most noticeably in elliptic blades of preformed leaves and obovate blades of neoformed leaves.

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