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.)

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.)