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.)

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.)