Flora of North America species comparison

Sometimes Salix interior is treated as a subspecies of S. exigua (R. D. Dorn 1998). Salix exigua and S. interior hybridize and apparently intergrade in the western Great Plains; because the area of overlap is relatively small and distinctiveness of the two taxa is not compromised by hybridization and introgression, it is best to treat them as separate species.

Leaves on sylleptic shoots are usually very densely silky. Salix interior sometimes has shoots that arise from buds on either side of the normal axillary bud. They do not seem to be directly related to the stipules because they are enclosed by the petiole. Catkins with both staminate and pistillate flowers are rare in S. interior, but a Quebec specimen had some catkins predominantly pistillate and others staminate; most were a mixture. The flowers were not teratological, but a mature capsule contained aborted ovules.

Hybrids:

Salix interior forms natural hybrids with S. exigua var. exigua. Controlled pollinations using S. interior (as S. exigua) from southern Ontario (A. Mosseler 1990) successfully produced F1 hybrids with S. bebbiana, S. discolor, S. eriocephala, and S. petiolaris. Seed production was usually relatively low, except in crosses with S. discolor. In general, F1 viability was relatively low in crosses with these members of subg. Vetrix. No seeds were produced in crosses with members of subgenera Protitea or Salix. Morphology of the hybrids usually was intermediate between the two parents, but when S. petiolaris was used as the maternal parent, the F1s more closely resembled that species. J. Salick and E. Pfeffer (1999) extended these findings to show that, although crosses between S. interior (as S. exigua) and S. eriocephala are partially sterile, their clonal growth parameters (sprouting, shoot length, and biomass production) are strong and thus permit these partially sterile hybrids to exist as successful individuals and perhaps to “... make a contribution to interspecific gene flow over time.” Of particular taxonomic interest is that, in this cross, the staminate parent has a significant influence on leaf shape, whereas in the cross S. eriocephala × S. petiolaris it is the pistillate parent that is significant for leaf shape. Relatively few hybrids resembling those produced by Mosseler have been recognized in nature, but it is possible that the unusually broadly leaved plants named S. interior var. exterior and var. wheeleri, from northern Maine, Nebraska, New York, and West Virginia, and probably elsewhere, may be hybrids. Phenological isolation may be strong enough to prevent crosses in nature (A. Mosseler and C. S. Papadopol 1989) with the earlier flowering S. eriocephala and S. petiolaris, a barrier that even an occasional period of overlap cannot breach.

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

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.)