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.)

Salix discolor is introduced in North Carolina.

Vegetative specimens of Salix discolor can be difficult to distinguish from S. planifolia, but there are two, somewhat variable, characters that can be used. Salix discolor usually has leaves dull adaxially, with arcuate secondary veins widely and irregularly spaced; S. planifolia has leaves slightly or highly glossy adaxially, with straight secondary veins closely and regularly spaced.

Salix discolor in northeastern United States can be difficult to distinguish from widely naturalized S. atrocinerea and S. cinerea. Useful diagnostic characters are: tertiary leaf veins, which are irregular in S. discolor but close and parallel in introduced species, and raised striae on peeled 3–5-year old branches, which are absent or indistinct and relatively short in S. discolor, but long and very prominent in the introductions.

Hybrids:

Salix discolor forms natural hybrids with S. humilis, S. interior, S. myricoides, S. pellita, and S. planifolia. Reports of hybrids with S. candida and S. eriocephala (M. L. Fernald 1950), and S. bebbiana and S. pyrifolia (C. K. Schneider 1921; Fernald), are not based on convincing specimens. Synthetic hybrids with S. bebbiana could not be made (G. W. Argus 1974; A. Mosseler 1990) and those made with S. eriocephala had low seed viability (Mosseler).

Salix discolor × S. humilis has tomentose leaves of S. humilis and longer catkins and styles of S. discolor (G. W. Argus 1986). These species readily hybridize and produce abundant seed (Argus 1974). The hybrids are fertile and backcross. Specimens of S. discolor with densely villous branchlets may be hybrids or introgressants with S. humilis. The two species usually are ecologically isolated; S. discolor occurs in wetland thickets and S. humilis in dry, sandy upland forests. Where the two habitats come into proximity, hybrids occur but large swarms have not been observed.

Salix discolor × S. myricoides (S. ×laurentiana Fernald, syn. S. paraleuca Fernald) usually resembles S. myricoides but has hairy ovaries (R. D. Dorn 1975, 1976). This hybrid was originally described as a species, from lower St. Lawrence River, Quebec. Its most distinctive feature is that hairs appear on ovaries in patches, at the base or, sometimes, only on the stipes. A similar ovary indumentum pattern appears in other hybrids or species of hybrid origin, e.g., S. hookeriana. Characteristics of S. discolor found in S. ×laurentiana include epidermis with gray-margined splits, leaf margins entire or sinuate, leaves with 2–4 teeth per cm, anthers yellow or purple, filaments hairy on proximal half or basally, ovaries hairy, greenish brown or green with red sutures, and adaxial pistillate nectaries ovate. Characteristics of S. myricoides include inner bud-scale membranes separating from the outer ones, stipules more prominent, catkins on distinct flowering branchlets, and longer styles sometimes distinct about half their lengths. This hybrid occurs throughout the area of overlap of the parents. All three taxa often are intermixed but few hybrids seem to produce well-developed seed.

Salix discolor × S. pellita (S. ×pedunculata Fernald) is characterized by juvenile leaves with infolded or sometimes revolute margins, ovaries with patches of hairs relatively short, flattened, crinkled, and refractive, and catkins borne on distinct flowering branchlets 2–10 mm. This sporadic hybrid does not seem to be fertile. It occurs in Newfoundland, Quebec, and Saskatchewan. Although it has been collected at few localities, it probably is more common and should be expected wherever the two species grow together. The type and other collections compare very well with synthetic hybrids (A. Mosseler 1990), which were reported to show a high hybridization success rate, high F1 pollen viability, and high seedling viability. It was suggested that variability within these species may be due to interspecific gene flow. In interpreting the parentage of the wild hybrids it is not possible to rule out hybridization of S. planifolia or S. myricoides with S. pellita, or that these hybrids may be S. myricoides × S. planifolia, as suggested by B. G. O. Floderus (1939). Salix ×pellicolor Lepage is a later synonym of this hybrid.

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