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Alnus incana (L.) Moench ssp. incana – European alder
Alnus incana ssp. rugosa (Du Roi) Clausen – speckled alder
Alnus incana ssp. tenuifolia (Nutt.) Breitung – mountain alder
General: Birch Family (Betulaceae). Native shrubs and (less commonly) small trees growing to 10 m tall, thicket-forming, with open crowns. The bark is gray, reddish, or brown, thin and smooth, becoming broken into irregular plates, often with conspicuous whitish lenticels (spongy openings for gas exchange). Leaves are elliptic to ovate, 4-11 cm long, 3-8 cm wide, broadest near or below middle, doubly and irregularly toothed, with 9-12 nearly straight, parallel veins on each side, with a ladder-like network of depressed veins, dull dark green above. Male (pollen, staminate) and female (seed, pistillate) flowers are in catkins, borne separately, but on the same tree (the species monoecious). The seed catkins are cone-like, cylindric to ovoid, 1-2 cm long, erect, sessile or on a short, stout stalk, generally remaining intact after release of fruits in spring. The pollen catkins are elongate, 2-7 cm long, in hanging clusters from near the shoot tip. The common name (speckled) is in reference to the numerous lenticels covering the bark.
Variation within the species: Speckled alder is similar in growth form and habitat to mountain alder (Alnus tenuifolia), which ranges from Alaska, Yukon, and Mackenzie southward to New Mexico. In mountain alder, the leaf blades are thin and papery, with rounded or blunt teeth, compared to the thick leaf blades with sharp teeth in speckled alder; mountain alder also is more treelike than speckled alder. They intergrade where their distributions overlap in Saskatchewan westward. Eurasian or European alder (Alnus incana) also is similar to these North American natives. When these three are ‘lumped’ (considered to represent only a single species, using the oldest name), the nomenclature is summarized as below. Many still consider them to represent three separate species.
synonym: Alnus rugosa (Du Roi) Spreng.
synonym: Alnus incana var. americana Regel
synonym: Alnus rugosa var. americana (Regel) Fern.
synonym: Alnus tenuifolia Nutt.
synonym: Alnus incana var. occidentalis (Dippel) C.L. Hitchc.
Brookside alder (Alnus serrulata), a species of the eastern and southeastern USA, has sometimes been included as a subspecies of A. incana but is now generally regarded as a separate species. Hybrids occur where brookside alder and speckled alder grow together.