Flora of North America species comparison

Subspecies 3 (3 in the flora).

Populus deltoides hybridizes with P. fremontii, the other native species of sect. Aigeiros, in the Colorado Plateau region (Arizona, New Mexico, and Utah) and trans-Pecos Texas. These hybrids involve P. deltoides subsp. wislizeni with both subspecies of P. fremontii and are difficult to distinguish because the parent species are so similar. They have shallowly cup-shaped discs 3–5 mm diam., pedicels 4–6 mm, and, usually, sparsely pubescent branchlets. Populus deltoides hybridizes also with three native members of sect. Tacamahaca. All three hybrids, P. ×generosa A. Henry (P. trichocarpa × P. deltoides), P. ×jackii Sargent (P. balsamifera × P. deltoides), and P. ×acuminata Rydberg (P. angustifolia × P. deltoides), are distinguished from P. deltoides by their buds with reddish resin, fewer triangular leaves with finer teeth, less flattening of the petiole, and a distinctly paler, slightly whitened abaxial leaf surface (J. E. Eckenwalder 1984). Individual hybrids have distinct ranges corresponding to their parental regions of sympatry and may be distinguished from each other by relative leaf width (less than two-thirds as wide as long in P. ×acuminata and more than two-thirds as wide as long in the other two) and base shapes (cordate in P. ×jackii and cuneate in the other two). Hybrids with members of the other two sections are rare or unknown. Hybridization with P. heterophylla (sect. Leucoides) is apparently rare and very local in South Carolina, even though the region of sympatry of these two species occupies essentially the entire range of P. heterophylla. Reported hybrids with P. tremuloides (sect. Populus) named as P. ×bernardii B. Boivin (T. C. Brayshaw 1965b; B. Boivin 1966b) are actually individuals of P. ×jackii (Eckenwalder).

Two related members of sect. Aigeiros, Populus nigra Linnaeus and P. ×canadensis Moench, are often planted as staminate clones, often persist after cultivation, and spread by root suckers but never become naturalized. Most individuals of Eurasian P. nigra cultivated in North America are Lombardy poplars (cv. Italica), an unmistakable, narrowly columnar, staminate clone with heavily buttressed trunk, rhombic preformed leaves, and triangular-ovate neoformed leaves broader than wide. This tree has been known since the eighteenth century and is widely (over-)planted throughout the temperate portion of the flora area as an accent tree. It hybridizes sporadically here with the three native balsam poplars, P. angustifolia, P. balsamifera, and P. trichocarpa; hybrids are discussed under those species. A rare hybrid with P. fremontii (P. ×inopina Eckenwalder) apparently originated from an uncommon pistillate tree of P. nigra (J. E. Eckenwalder 1982). It resembles P. fremontii in leaf shape but has dark reddish brown winter buds.

Populus nigra and P. ×canadensis are both staminate and are similar in having winter buds usually 12+ mm with a balsamic fragrance and orange-red resin. The branchlets are round and bright orange-brown to reddish brown in the first year, turning tan by the third year. The petioles are distally flattened at a right angle to the plane of blade. The margins of the leaf blade are translucent and ciliate; the leaf surfaces are glabrous or glabrate to pubescent but not tomentose. The catkins usually have more than 15 flowers, (4–)7–15 cm. The floral disc is entire, persistent, broadly cup- or saucer-shaped, and not obviously oblique. The anthers are usually truncate. The 2-carpelled ovary is ovoid to spherical and the 2–4 stigmas are expanded. The floral bracts are not ciliate and are glabrous abaxially. The two taxa differ in that P. nigra has branchlets that are nearly parallel, leaf blades without basilaminar glands, preformed blade margins with sinuses no more than 1.2 mm deep, and 12–20(–30) stamens; P. ×canadensis has divergent branchlets, branching at 50º or more, leaf blades with no more than 1 basilaminar gland, preformed blades with the base broadly cuneate and apex gradually acuminate, and (15–)20–30 stamens.

Populus ×canadensis (P. ×euramericana Guinier [illegitimate name]; B. K. Boom 1957) is an intercontinental hybrid that first arose spontaneously between P. deltoides and P. nigra after the former was introduced into Europe in the late seventeenth century. Deliberate new hybrids of this parentage are one of the mainstays of Europe’s growing commercial poplar plantations. They are also important in eastern North America but are often replaced by P. ×generosa (P. trichocarpa × P. deltoides) in commercial plantations in British Columbia, Oregon, and Washington. Only one clone is commonly, and very widely, grown horticulturally, the Carolina poplar (‘Eugenei’), a staminate clone with a fairly narrow habit inherited from its staminate parent, the Lombardy poplar. It is often confused with P. deltoides, with narrower preformed leaves, often slightly longer than wide, with more numerous, smaller teeth, and with bases obtuse or rounded, rather than truncate or subcordate. It differs further from P. deltoides subspp. deltoides and monilifera in having 0–1 basilaminar glands rather than 2–6.

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

Genera 50+, species ca. 1000 (4 genera, 123 species in the flora).

Taxonomic placement of the Salicaceae and the genera included in it have varied greatly. Some botanists (H. G. A. Engler and K. Prantl 1887–1915) treated it as a primitive member of the Dicotyledoneae and grouped it with other families having simple, apetalous, unisexual flowers arranged in catkins, the “Amentiferae.” At about the same time, others (C. E. Bessey 1915) took a different view, regarding the simple flowers as the result of reduction, and placed the taxa in Caryophyllales. As early as 1905, H. Hallier could see that there were similarities between Salicaceae and Flacourtiaceae; at the time, he was vigorously challenged by E. Gilg (1915). A. D. J. Meeuse (1975) summarized evidence for a close relationship between these families, including wood anatomy, phytochemistry, host-parasite relationships (including rust fungi), and morphology. He concluded that the Salicaceae could be combined with the Flacourtiaceae, “perhaps as a tribe.” A. Cronquist (1988) and R. F. Thorne (1992b) placed the Salicaceae, in a narrow sense, in Violales near Flacourtiaceae.

Molecular studies support a close relationship between Salicaceae and Flacourtiaceae in Malpighiales and show that Flacourtiaceae, in a broad sense, is paraphyletic. Based on a study of plastid rbcL DNA sequences, Salix and Populus were nested within a subset of 52 genera of Flacourtiaceae (M. W. Chase et al. 2002). Chase et al. proposed moving some genera from broadly circumscribed Flacourtiaceae to Salicaceae. Other studies, based on different gene sequences, came to the same conclusion (O. I. Nandi et al. 1998; V. Savolainen et al. 2000; K. W. Hilu et al. 2003; Angiosperm Phylogeny Group 2003). The discovery of the extinct fossil genus Pseudosalix (L. D. Boucher et al. 2003), from the Eocene Green River Formation of Utah, provided further support for placing some members of Flacourtiaceae in Salicaceae. The well-preserved Pseudosalix fossils, in which reproductive structures are directly associated with the leaves, occur intermixed with Populus fossils. The leaves are slender and have salicoid teeth, inflorescences are cymose, flowers are unisexual, pedicellate, tetrasepalous, and 3- or 4-carpellate, and seeds are comose, i.e., having characteristics intermediate between Salicaceae and Flacourtiaceae.

The presence, in both families, of salicoid teeth is often cited in support of their close relationship (W. S. Judd 1997b; O. Nandi et al. 1998; M. W. Chase et al. 2002; H. P. Wilkinson 2007). Salicoid teeth were first recognized and defined as having the tip of the medial vein (seta) of the tooth retained as a dark, but not opaque, non-deciduous spherical callosity fused to the tooth apex and were reported to occur in Salicaceae and Idesia of the Flacourtiaceae (L. J. Hickey and J. A. Wolfe 1975). Nandi et al. reported that a broad survey of angiosperm leaves showed that salicoid teeth occur outside of Flacourtiaceae and Salicaceae only in Tetracentraceae.

Isozyme and cytological evidence show that Populus and Salix are ancient polyploids (D. E. Soltis and P. S. Soltis 1990; Wang R. and Wang J. 1991). All Salix and Populus species contain salicin (R. T. Palo 1984).

The genera often included in Salicaceae, in the narrow sense, are Chosenia, Populus, Salix (A. K. Skvortsov 1999), and, sometimes, Toisusu. Molecular studies (E. Leskinen and C. Alström-Rapaport 1999; T. Azuma et al. 2000) show that Chosenia is nested within Salix. H. Ohashi (2001) treated Toisusu as Salix subg. Pleuradinea Kimura and Chosenia as Salix subg. Chosenia (Nakai) H. Ohashi.

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