Salix athabascensis
Salix ×fragilis
Athabasca willow
brittle willow, crack willow, hybrid white willow
erect;
branches gray-brown, hairy;
branchlets red-brown, sparsely or moderately densely pubescent, (buds alba-type).
stipules absent or rudimentary on early ones, usually rudimentary, rarely foliaceous, on late ones;
petiole (shallowly grooved adaxially), 3–10 mm, (puberulent or villous);
largest medial blade oblong, narrowly elliptic, elliptic, oblanceolate, or obovate, 17–50 × 8–18 mm, 1.9–3.2 times as long as wide, base cuneate or convex, margins flat or slightly revolute, entire, apex acuminate or convex, abaxial surface glabrescent or sparsely silky, hairs appressed or somewhat spreading, (usually white, sometimes also ferruginous), straight or wavy, adaxial dull or slightly glossy, glabrous, glabrescent, pilose, or sparsely long-silky along midribs and margin, (hairs usually white, sometimes also ferruginous, appressed);
proximal blade margins entire;
juvenile blade sparsely to moderately densely villous or long-silky abaxially (hairs usually white, sometimes also ferruginous).
abaxial nectary (0–)0.3–0.6 mm, adaxial nectary oblong or ovate, 0.4–1.2 mm, nectaries distinct;
filaments distinct, hairy basally or on proximal 1/2;
anthers globose, 0.4–0.6 mm.
abaxial nectary absent, adaxial nectary oblong, 0.4–1.3 mm, shorter than stipe;
stipe 0.8–1.3 mm;
ovary pyriform, very densely long-silky, beak gradually tapering to or slightly bulged below styles;
ovules 6–14 per ovary;
styles 0.5–1 mm;
stigmas flat, abaxially non-papillate with rounded tip, or broadly to slenderly cylindrical, 0.28–0.35–0.48 mm.
5.6–7.2 mm.
staminate 14–31 × 8–18 mm, flowering branchlet 1.5–9 mm;
pistillate loosely flowered, stout to globose, 10–58 × 7–25 mm, flowering branchlet 3.5–26 mm;
floral bract tawny, 1–1.6 mm, apex rounded, entire, abaxially sparsely hairy, almost glabrous, hairs wavy.
×fragilis Linnaeus: The hybrid white willow, S. alba Linnaeus × S. euxina I.
, a European introduction, is the most commonly cultivated and naturalized tree-willow in the flora area.
is characterized by: trees, 3–20 m, stems erect or drooping;
branches highly brittle at base;
petioles with spherical or foliaceous glands distally, pilose or villous adaxially;
largest medial leaf blade amphistomatous, very narrowly elliptic or narrowly elliptic, margins uniformly serrate or serrulate, abaxial surface glaucous, both surfaces sparsely long-silky to glabrescent, adaxial surface slightly glossy or dull;
juvenile leaves at first densely long-silky soon glabrous;
pistillate bract deciduous after flowering;
stamens 2;
anthers yellow;
pistillate adaxial nectary shorter than or equal to stipe;
stipe 0.3–0.5 mm;
ovary pyriform, glabrous;
ovules 6–12 per ovary;
styles 0.4–1 mm;
capsules 4.5–6 mm; 2n = 57, 76.
= 76, 95, 114.
Salix athabascensis
Salix ×fragilis
The three polyploid chromosome numbers reported for Salix athabascensis, as well as the presence of leaves with ferruginous hairs, otherwise unknown in sect. Myrtilloides, are indicators of allopolyploidy.
Hybrids:
Salix athabascensis forms natural hybrids with S. pedicellaris. These hybrids combine the characteristics of the parents. The ovaries may be moderately densely villous or glabrous, but commonly have hairs in patches, or the stipes may be hairy and the ovaries glabrous; juvenile blades, and sometimes mature leaves, are hairy with white and ferruginous hairs. Some plants that resemble S. athabascensis have leaves glaucous adaxially, as in S. pedicellaris. The ovaries often appear to be infertile.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Individual trees can persist for years by trunk suckering and spread vegetatively by shoot fragmentation along stream margins, shingle and sand beaches, sedge meadows, hardwood forests, and sand pits.
A study of Salix ×fragilis in Colorado (as S. ×rubens) showed that 2172 of 2175 trees were pistillate. Occasionally seed was set, possibly fertilized by S. alba (P. B. Shafroth et al. 1994). There are at least five clones of S. ×fragilis (as S. ×rubens) in cultivation (T. Berg in B. Jonsell and T. Karlsson 2000+, vol. 1); the pistillate are sterile but the staminate produce viable pollen. The hybrid plants are often misidentified as S. “fragilis” or as S. nigra. In the flora area, reproduction of the hybrid seems to be mainly by stem fragmentation.
Prior to the lectotypification of Salix fragilis Linnaeus and the description of S. euxina (I. V. Belyaeva 2009), the name S. “fragilis” was often inadvertently used for both the pure species and for its hybrids with S. alba. Thus all herbarium specimens under the names “fragilis” and “×rubens” need to be revised.
Salix ×fragilis can be separated from S. euxina by having branches and branchlets hairy or glabrescent in age versus glabrous; leaf blades not glaucous abaxially versus glaucous; leaves amphistomatous versus hypostomatous or with stomata only along veins and at apex; and pistillate catkins slender and loosely flowered versus stout and moderately densely flowered.
Several molecular studies have been designed to understand the nature of this hybrid. H. Beissmann et al. (1997), using AFLP markers, were able to recognize three clusters: Salix alba, S. euxina (as S. fragilis), and S. ×fragilis (as S. ×rubens); but a study by K. De Cock et al. (2003), also using AFLP markers, was unable to resolve S. alba and S. ×fragilis (as S. ×rubens). They recommended the use of experimental hybridization to study the genesis of this hybrid. Molecular and genetic studies by L. L. Triest (2001) and coworkers concluded that in modern open agricultural situations in Belgium, hybridization was of low occurrence, and that morphologically intermediate plants were not necessarily genetically intermediate. These studies saw different facets of the question. Clearly there are three entities, S. alba, S. euxina, and their hybrid but, because S. euxina may be rare outside of cultivation, natural hybridization may not occur and the question of whether S. ×fragilis can be backcrossed with S. alba remains to be studied. The specimens used in these molecular studies require reidentification.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
BC
PNW Herbaria
WildflowerSearch
iNaturalist
USDA Plants Database
OR
CalFlora
Flora NW
IL Wildflowers
MD Biodiversity
MI Flora
MN Wildflowers
