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 eriocephala sometimes is very difficult to separate from S. myricoides. Some of the confusion may be due to hybridization but no definite hybrids have been seen.

Salix eriocephala can be distinguished from S. myricoides by having stipules on early leaves foliaceous, apices acute to rounded, largest medial blades 4.9–23.3 times as long as petiole, abaxial surface usually thickly glaucous (stomata can be seen through the grayish wax), margins serrulate or serrate, floral bracts 0.8–1.6 mm, moderately to very densely hairy, styles 0.3–0.6 mm, and stigmas 0.16–0.28 mm; S. myricoides has stipules on early leaves rudimentary or foliaceous, apices acuminate or acute, largest medial blades 4.7–13.4 times as long as petiole, abaxial surface usually with very thick wax (stomata cannot be seen through the wax), margins crenulate to serrulate, floral bracts 1.2–3 mm, sparsely to moderately densely hairy, styles 0.3–1.3 mm, and stigmas 0.28–0.56 mm.

Hybrids:

Salix eriocephala forms natural hybrids with S. candida, S. famelica, S. humilis, S. interior, S. lasiandra, S. petiolaris, and S. sericea. Hybrids with S. amygdaloides, S. bebbiana, S. myricoides, and S. pedicellaris have been reported (M. L. Fernald 1950) but no convincing specimens have been seen. Controlled pollinations made with S. discolor had low success and many seedlings were abnormal (A. Mosseler 1990). In controlled pollination using S. eriocephala as the maternal parent, seeds were rarely produced due to pollen-stigma incompatibility (Mosseler 1989).

Salix eriocephala × S. famelica: Hybrids and intergrades occur in the area of overlap (R. D. Dorn 1995). Specimens from a population in Douglas County, Nebraska, which included successive collections and cultivated specimens, have branches with yellow-mottled coloration of S. famelica and villous indumentum of S. eriocephala; they may be this hybrid.

Salix eriocephala × S. petiolaris: Controlled pollinations (A. Mosseler 1990) had low seed-set but a high percent of seed germination and seedling survival. Because reproductive barriers between these species are weak, it was suggested that their morphological variability may be due to interspecific gene flow (Mosseler). Natural hybrids are known from Illinois, Maine, Massachusetts, Michigan, Missouri, New York, Ontario, Quebec, and West Virginia.

Salix eriocephala × S. sericea: This hybrid is relatively common wherever the ranges of the parents overlap. It has been studied in the southeastern United States (G. W. Argus 1986) and in eastern Canada. The results of a molecular study (T. M. Hardig et al. 2000) have been discussed already under the genus. In general, the hybrids resemble S. eriocephala but have leaves that are sparsely to moderately densely short-silky on abaxial surfaces and ovaries hairy as in S. sericea. Foliaceous stipules are often present on late leaves and sometimes even on early leaves, as in S. eriocephala, but they are not as prominent. In S. sericea stipules usually are lacking or rudimentary, but on late leaves they may be foliaceous. Petioles and branchlets of hybrids are finely velvety as in S. sericea. This hybrid was described from Maine (O. W. Knight 1907), where it was noted that the catkins were usually abortive but sometimes produced one or two fertile seeds.

Salix eriocephala is distinguished from S. sericea in having stipules on early and late leaves foliaceous, 4–6.2–8.3 × 2.5–3.6–4.6 mm, 1.5–2 times as long as wide, ovaries glabrous, juvenile blades glabrous or sparsely hairy, hairs white, largest medial blade abaxial surfaces glabrous, puberulent, sparsely pubescent, or short-silky, stipes 1.2–2.8 mm, and capsules 3.5–7 mm; S. sericea has stipules on early leaves absent or rudimentary, on late leaves rudimentary to foliaceous, 1.1–1.6–2.1 × 0.4–0.6–0.8 mm, 2.3–3 times as long as wide, ovaries short-silky, juvenile blades very densely short-silky, hairs white, sometimes also ferruginous, largest medial blade abaxial surfaces densely short-silky, stipes 0.6–1.5 mm, and capsules 2.5–4 mm.

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