Populus x canadensis Moench
(James E. Eckenwalder)
Synonyme : Populus x euramericana Guinier, nom. illeg.
Populus x canadensis Moench includes all natural and artificial hybrids between any races of P. nigra L. and P. deltoides Marsh., including backcrosses and advanced generation hybrids.
These were the first intercontinental poplar hybrids to be used in plantation culture and among the first tree hybrids of any kind to be so used. They first attained prominence in the 18 th century, when P. canadensis was named, and many individual combinations of different races of the two parents were given species names in the 19 th and early 20 th centuries. Botanical nomenclature subsumes all combinations between any two species under a single epithet, the earliest described one that conforms to the rules of nomenclature, in this case P. x canadensis. The frequently used P. x euramericana Guinier has no botanical standing and is considered a technically illegitimate renaming of P. x canadensis, although there is no impediment to referring to these clones informally as Euramerican hybrids.
The numerous Euramerican hybrids as a group share a typical set of characteristics that set them apart from and in between the parent species. They are vigorous and many possess a relatively narrow crown of upwardly angled branches inherited through the Lombardy poplar. Most have grayish mature bark and first-year twigs with an orange cast, contrasting with the darker bark and redder twigs of P. nigra and the browner bark and yellowish or tan twigs of P. deltoides. Many of the hybrids inherit 5-angled twigs from P. deltoides, and such twigs are more prominently expressed in the most vigorous shoots.
On the other hand, the reddish resinous winter buds are derived from P. nigra and contrast with the browner buds and paler resin of P. deltoides. The leaves usually readily distinguish Euramerican hybrids from their parents, but this applies especially to the spring flush of leaves that overwinter in the bud (preformed, or early, or short-or spur-shoot leaves) rather than those that develop during the course of the growing season (neoformed, or late or long-shoot leaves), which can be difficult to distinguish from those of P. deltoides. Both leaf forms of P. deltoides (at least in those race that have contributed to P. x canadensis) usually have 1-3 pairs of glands at the base of the leaf on the upper side near the attachment to the petiole (basilaminar glands), and these glands are completely lacking in P. nigra, while Euramerican hybrids may have a single gland of the pair. The teeth on the preformed leaves of Euramerican hybrids are fairly coarse, intermediate between the very coarse toothing of P. deltoides, with the largest teeth up to 7 mm deep, and the much finer toothing in P. nigra. The base of the performed leaves can be particularly diagnostic. Euramerican hybrids, like cv. Eugenei, often have a tooth-less, broadly wedge-shaped base, with an abrupt shoulder linking it to the toothed portion of the margin.
Flowering characteristics also set Euramerican hybrids apart from the parent species. Female clones have seed capsules opening with 2-3 valves, unlike the 4 valves of P. deltoides and uniformly 2 valves of P. nigra. Although the two parent species have different modal numbers of stamens in the male flowers, they both can have about 30, as do many hybrids. However, the longer pedicels (flower stalks), at least some of them longer than about 4 mm, distinguish Euramerican hybrids from P. nigra, while the bud color distinguishes them from P. deltoides.
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