Flora of North America species comparison

Variety monica applies to the diminutive alpine form that sometimes is recognized in the southern Rocky Mountains (S. J. Brunsfeld and F. D. Johnson 1985); it occurs at higher elevations (2200–4000 m) and is characterized by low growth form (0.14–1 m) and smaller, slightly broader leaves. Although it can be distinctive, it is morphologically confluent with the typical species. B. G. O. Floderus (1939) may be correct in characterizing it as an alpine ecotype.

Salix planifolia and S. pulchra are closely related. Their ranges overlap in northwestern Canada, from northern British Columbia across the southern quarter of the Yukon and northeastward into the Great Bear Lake area. Specimens identified as S. pulchra occur as far northeastward as Coppermine and northeast of Bathurst Inlet; S. planifolia has been recognized in the Mackenzie Delta and Eskimo Lake regions, Northwest Territories. Outlying records should be treated with caution because identification of individual specimens out of context may not be definitive. G. W. Argus (1969, 1973) treated these taxa as subspecies based on their intergradation in northwestern British Columbia, their tetraploid chromosome number, and their similar leaf flavonoid chromatographic patterns, but this taxonomy needs reconsideration.

The primary differences between the two species are stipule size, shape, and persistence and the pubescence on juvenile leaves. Stipules of Salix planifolia are oblong to narrowly elliptic or obovate, 0.8–3 mm (or –4.5 mm at Back River, Northwest Territories), distinctly shorter than petioles, and rarely marcescent for more than one year; stipules of S. pulchra are linear to narrowly oblong, 3–32 mm, usually longer than petioles, and usually marcescent for two or more years. Juvenile leaves of S. planifolia are usually more densely hairy, but vary from glabrescent to sparsely or very densely pubescent or long-silky, whereas juvenile leaves of S. pulchra are usually glabrous or, sometimes, sparsely hairy. The occurrence of rhombic mature leaf blades in S. pulchra sometimes is distinctive, but overlap in leaf shape between the two taxa is very great.

The area of geographic overlap in Yukon and western Northwest Territories is large, but evidence suggests that there the two species may be separated by elevation. In the vicinity of Whitehorse, Yukon, Salix pulchra occurs at higher elevations (1400–1900 m) than S. planifolia (ca. 1000 m); no mixed populations were seen. In Nahanni National Park, Northwest Territories, where S. planifolia is more common than S. pulchra, the latter occurs only in alpine and subalpine habitats (1200–1400 m). Evidence from both localities indicates an elevational separation of the two taxa. Within the area of overlap there is little evidence of intergradation except that S. planifolia has stipules that tend to be more marcescent (40% are marcescent) and sometimes longer (2–3.5 mm) than is usual outside the area of overlap. Nevertheless, specimens from the area of overlap can be easily assigned to one taxon or the other with only a few apparent intermediates. The problem in recognizing intermediacy is that there are only a few, variable characters that separate the two. In contrast, in 1973, G. W. Argus described evidence of hybridization and introgression along the Haines Road in northwestern British Columbia. This was based on variation in stipule size, presence, and persistence in what appeared to be a hybrid swarm. Further data are needed to answer questions about actual hybridization. Are the species separated by habitat or elevation, and are there reproductive barriers? Answers could be gained by population studies and controlled hybridization. Until that is done it is best to treat these taxa as species.

See 76. Salix discolor and 95a. S. alaxensis var. alaxensis for comparative descriptions.

Hybrids:

Salix planifolia forms natural hybrids with S. alaxensis var. alaxensis, S. argyrocarpa, S. brachycarpa var. brachycarpa, S. candida, S. drummondiana, S. humilis, S. pellita, S. pulchra, and S. scouleriana. Hybrids with S. glauca var. cordifolia have been reported (C. K. Schneider 1921) but no convincing specimens have been seen.

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

Comparisons of genetic variation in Salix alaxensis var. alaxensis from British Columbia and S. silicicola from Lake Athabasca sand dunes, Saskatchewan, based on isozyme loci, fit predicted relationships between progenitor and derived taxon (B. G. Purdy and R. J. Bayer 1995). Allelic diversity of S. silicicola was a subset of that of S. alaxensis, there was less genetic variation in S. silicicola than in S. alaxensis, and interspecific genetic variation within the two species was similar and relatively very high. This suggested a recent origin for the derived S. silicicola.

Salix silicicola is a uniform population that differs from S. alaxensis in its very densely villous or tomentose leaves and branchlets. These characters seem to be an adaptation to reduce sand abrasion and water loss in a sand dune environment. It is unlikely that it would have evolved in situ but probably derived from a pre-adapted source such as the one represented by specimens of putative S. silicicola from Pelly Lake, Nunavut. The isozyme study did not include specimens from that population or of S. alaxensis from Northwest Territories from which S. silicicola is likely to have been derived. Occurrence of S. silicicola-like plants in northern continental Nunavut suggests that during the late Pleistocene, it had a wider range, which now is represented by two disjunct populations. The question of appropriate taxonomic rank for the derived taxon is still unresolved. Although S. silicicola is different from S. alaxensis in its general appearance, they are very similar genetically, and argument could be made for treating them as varieties (B. Boivin 1966b).

Hybrids:

Salix silicicola forms natural hybrids with S. brachycarpa var. psammophila.

of conservation concern

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