Flora of North America species comparison

Variety alaxensis is one of the tallest “trees” in the Canadian Arctic. An extensive stand of willows, some of which reach tree-size, occurs in a valley on deep marine, lake, or river gravels and sands southeast of Deception Bay in northern Ungava, Quebec (P. F. Maycock and J. B. Matthews 1966). The largest of the dominant willows of tree stature, var. alaxensis and Salix planifolia were ca. 5 m tall and 20 cm in diameter. The maximum age for a stem 10 cm in diameter was 60 years, but at this age there was heartwood decay. An outlier stand of tree-sized willows on Victoria Island, Northwest Territories, reached 6–8 m, to 81 years of age (S. A. Edlund and P. A. Egginton 1984).

Hybrids:

Variety alaxensis forms natural hybrids with Salix calcicola, S. drummondiana, S. pellita, and S. planifolia.

Variety alaxensis × Salix calcicola was discovered by M. Blondeau at Kangigsujuak, Quebec. The plants resemble S. calcicola in having broad leaves and stipules, and reddish styles, and var. alaxensis in having densely villous leaves and branchlets, and relatively short pistillate flowering branchlets. The ovaries have hairy beaks or are sparsely hairy throughout.

Variety alaxensis × Salix drummondiana: In the northern Rocky Mountains, plants resembling S. drummondiana but with stipules prominent, linear or lanceolate, and foliaceous, and leaves abaxially densely woolly may be this hybrid.

Variety alaxensis × Salix pellita occurs in the Churchill, Manitoba, region where it grows with var. alaxensis, S. pellita, and S. planifolia. The plants resemble var. alaxensis in having very densely villous branchlets, and leaves with short wavy hairs on abaxial surfaces. The leaf indumentum is sparse, and ferruginous hairs often occur on juvenile and late leaves. Strongly revolute margins suggest S. pellita as the second parent.

Variety alaxensis × Salix planifolia occurs in the Churchill, Manitoba, region where it grows with the two parental species. It resembles var. alaxensis in its very densely villous branchlets and leaves with short wavy hairs on abaxial surfaces. Leaf indumentum is sparse, and ferruginous hairs often occur on juvenile and late leaves.

(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.)