Flora of North America species comparison

The Virginia plants of Salix lucida are introduced (G. W. Argus 1986).

The Salix lucida complex is a group of three weakly delimited taxa, S. lasiandra var. caudata, S. lasiandra var. lasiandra, and S. lucida. The morphological characters used to separate them (leaves amphistomatous or hypostomatous and blades glaucous abaxially or not) are usually geographically correlated, but there are exceptions. G. W. Argus (1986b) proposed, based on principal components analysis of morphological data, to treat them as a single species consisting of three subspecies. The geographic overlap of the northeastern S. lucida and the western S. lasiandra is a relatively small area in central Saskatchewan. Evidence of intergradation was based on cultivation of a plant that, in the wild, had leaves that were not glaucous abaxially but were glaucous in cultivation. It seems best to treat them as two species, S. lucida and S. lasiandra, the latter with two varieties, var. lasiandra and var. caudata.

Hybrids:

Salix lucida forms natural hybrids with S. alba and S. nigra. Hybrids with S. serissima have been reported (M. L. Fernald 1950) but no convincing specimens have been seen. Attempts to hybridize S. lucida with members of subg. Protitea (S. amygdaloides), subg. Longifoliae (S. interior), and subg. Vetrix (S. discolor, S. eriocephala, and S. petiolaris) were unsuccessful (A. Mosseler 1990).

Salix lucida × S. nigra (S. ×schneideri B. Boivin) seems to be a rare intersubgeneric hybrid between tetraploid S. lucida and diploid S. nigra. It is known only from the type specimen, an infertile plant, growing with both parents. It resembles S. lucida in bud-scale margins connate, in petiolar glands stalked or foliaceous, and in leaf shape, and S. nigra in stipules rudimentary on proximal leaves and sometimes even on early leaves, stipule apex acute, pistillate catkins relatively long and slender, and styles relatively short.

(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.)