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.)

Subspecies 2 (2 in the flora).

Populus fremontii hybridizes with P. deltoides subsp. wislizeni where they come in contact in the Colorado Plateau region and trans-Pecos Texas. Hybrids have longer pedicels and narrower discs than does P. fremontii, and less densely hairy twigs in regions where P. fremontii is densely pubescent. It also hybridizes with the two members of sect. Tacamahaca with which it overlaps, P. trichocarpa (P. ×parryi Sargent), and P. angustifolia (P. ×hinckleyana Correll) (synonyms P. ×acuminata Rydberg nothomorph rehderi Sargent and P. ×intercurrens S. Goodrich & S. L. Welsh). Both hybrids have smaller, more numerous teeth on preformed leaves than does P. fremontii, with blades clearly paler abaxially than adaxially, and buds with reddish resin (J. E. Eckenwalder 1984). Preformed leaves are broadly ovate to heart-shaped in P. ×parryi (Eckenwalder 1984b) and usually ovate in P. ×hinckleyana; trees from the type locality in the Davis Mountains, Texas, had round leaves (D. S. Correll 1960). They could be distinguished from P. ×parryi by their smaller leaf blades and hairy petioles. Trees of P. ×parryi have been found in Mohave County, Arizona, east of the present range of P. trichocarpa (Eckenwalder 1992).

Populus ×inopina Eckenwalder is a hybrid between P. fremontii and P. nigra known only from the type locality along Coyote Creek, San Jose, California, where P. nigra was the seed parent (J. E. Eckenwalder 1982). Trees of the same parentage were produced artificially by A. B. Stout and E. J. Schreiner (1933); none appears to have survived. They differ from P. nigra in broader habit, twigs thicker, buds thicker and less red, broader and more cordate leaves with larger teeth, and more numerous stamens. They differ from P. fremontii in narrower habit, twigs more orange, buds without a green cast, longer leaf apices, more numerous teeth, and fewer stamens. If pistillate trees occur, they would be expected to have 2 or 3 carpels and discs 3–5 mm diam., both intermediate between those of the parents.

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