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Alnus
See list of 4 species in this genusHybridization is known to occur between some species in New England. Steele (1961) described introgressed plants he observed in New Hampshire. Reference: Furlow (1979).
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1a. Trees to 20 m tall; leaf blades obovate to suborbicular, rounded or retuse to obcordate at the apex
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1b. Shrubs or small trees to 10 m tall; leaf blades elliptic or narrow-ovate to broad-elliptic, broad-ovate, or obovate, rounded to acute at the apex
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2a. Winter buds sessile or on stalks up to 1 mm, with 3–6 scales of unequal sizes, acute to acuminate at the apex, glabrous and glutinous; samara with a thin wing; only the staminate aments emerging from naked buds; peduncles of carpellate infructescences often subtended by leaf-like bracts, 0.5–1.2 mm thick in fruit
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2b. Winter buds on stalks 2–4 mm long, with 2 or 3 scales of equal size, rounded to acute at the apex, scurfy-pubescent; samara merely thin-edged; staminate and carpellate aments emerging from naked buds; peduncles of carpellate infructescences usually without subtending bracts, 0.8–1.5 (–2) mm thick in fruit
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3a. Leaf blades coarsely double-serrate, narrow-ovate to elliptic, with conspicuous cross-veins on the abaxial surface, slightly or not at all resinous and frequently glaucous later in the season abaxially, broad-cuneate to rounded or subcordate at the base; bark with evident, white lenticels; branchlet supporting the staminate aments usually weakly to moderately diverging from the inflorescence axis [Fig. 481]
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3b. Leaf blades finely and regularly serrulate to somewhat double-serrate, obovate to broad-elliptic, with delicate or inconspicuous cross-veins on the abaxial surface, slightly to moderately resinous and without bloom abaxially, cuneate (rarely to rounded) at the base; bark with fewer, darker lenticels; branchlet supporting the staminate aments usually strongly diverging from the inflorescence axis [Fig. 482]
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Each photo represents one species in this genus.