Trees or shrubs, to 35 m; trunks usually several, branching excurrent to deliquescent. Bark of trunks and branches light gray to dark brown, thin, smooth, close; lenticels often present, pale, prominent, sometimes horizontally expanded. Wood nearly white, turning reddish upon exposure to air, moderately light and soft, texture fine. Branches, branchlets, and twigs nearly 2-ranked to diffuse; young twigs uniform or (Alnus subg. Alnobetula) differentiated into long and short shoots. Winter buds stipitate (nearly sessile in Alnus subg. Alnobetula), narrowly to broadly ovoid or ellipsoid, terete, apex acute to rounded; scales 2–3, valvate, or (Alnus subg. Alnobetula) several, imbricate, smooth, or (Alnus subg. Clethropsis) sometimes none. Leaves borne on long or short shoots, 3-ranked to nearly 2-ranked. Leaf blade ovate to elliptic or obovate, thin to leathery, base variable, cuneate to rounded, margins doubly serrate, serrate, serrulate, or nearly entire, apex variable, acute to obtuse or acuminate to rounded; surfaces glabrous to tomentose, abaxially sometimes resinous-glandular. Inflorescences: staminate catkins lateral, in racemose clusters or (Alnus subg. Clethropsis) solitary, formed (Alnus subg. Alnus and Clethropsis) during previous growing season and exposed or enclosed in buds during winter, or (Alnus subg. Clethropsis) formed and expanding during same growing season, expanding before or with leaves; pistillate catkins proximal to staminate catkins, solitary or in relatively small racemose clusters, erect to nearly pendulous, ovoid to ellipsoid, firm; scales and flowers crowded, developing and maturing at same time as staminate catkins. Staminate flowers in catkins, 3 per scale; stamens (3–)4(–6); anthers and filaments undivided. Pistillate flowers usually 2 per scale. Infructescences erect or pendulous; scales persistent long after release of fruits, with 5 lobes, greatly thickened, woody. Fruits tiny samaras, lateral wings 2, leathery or membranaceous, reduced or essentially absent in some species. x = 7.
Forested temperate and boreal Northern Hemisphere, North America, Asia.
Species ca. 25 (8 species in the flora).
Alders resemble birches but are easily distinguished from them by the infructescences, which consist of persistent, 5-lobed, woody scales (versus deciduous, 3-lobed, thin scales). Except in members of Alnus subg. Alnobetula Petermann (which have nearly sessile buds with several imbricate scales), alders are also distinctive in their stipitate buds bearing two stipular scales. The fruits, borne two to a scale, are laterally winged, although the wings are sometimes reduced or absent.
The genus is diverse, including several very distinct lines of specialization. The shrubby or arborescent Alnus subg. Alnus is characterized by winter buds with long stalks and two valvate scales, inflorescences borne in racemose clusters, and development of both pistillate and staminate inflorescences during the growing season prior to anthesis, with these fully exposed during winter. It includes the common A. rubra, A. incana, A. oblongifolia, and A. serrulata. Alnus subg. Alnobetula (represented in North America by three subspecies of A. viridis) consists of shrubby species of cold-climate regions. In this group, the buds are nearly sessile and covered by several imbricate scales. Both staminate and pistillate catkins are formed the season before anthesis, but only the staminate ones are exposed during winter. The predominantly Asian Alnus subg. Clethropsis (Spach) Regel is represented in America by a single species, A. maritima, a small tree or large shrub of stream banks, marshes, and the shores of shallow lakes. Members of this group are unique in that they bloom in autumn rather than spring. They also differ from other native species in Alnus in having essentially naked buds, leaves with semicraspedodromous venation (i.e., with the secondary veins branching and anastomosing with each other near the margin before reaching the teeth), and solitary pistillate inflorescences borne in the axils of foliage leaves. All of the alders associate symbiotically with species of the actinomycete Frankia, leading to the formation of nodules on the roots of the plants and the fixation of atmospheric nitrogen.
|1||Winter buds nearly sessile (stalks usually not over 1 mm), covered by 4–6 unequal, imbricate scales; staminate inflorescences formed late in growing season before blooming, exposed during winter; pistillate inflorescences enclosed within buds during winter, exposed with first new growth in spring (subg. Alnobetula).||Alnus viridis|
|1||Winter buds distinctly stalked, covered, sometimes incompletely, by 2–3 nearly equal, valvate scales; staminate and pistillate inflorescences both formed mid to late in growing season, not with first new growth in spring.||> 2|
|2||Pistillate inflorescences and infructescences solitary in leaf axils along main stems; flowering near end of growing season (subg. Clethropsis).||Alnus maritima|
|2||Pistillate inflorescences (and later infructescences) on short branchlets in racemose clusters; flowering at beginning of growing season (subg. Alnus).||> 3|
|3||Leaf blade margins serrulate or finely serrate, without noticeably larger secondary teeth (although sometimes slightly lobulate).||> 4|
|3||Leaf blade margins doubly serrate or crenate, with distinctly larger secondary teeth, or coarsely serrate or serrate-dentate.||> 5|
|4||Leaf blade broadly elliptic to obovate, apex obtuse to rounded; staminate flowers with 4 stamens; large shrubs of e North America.||Alnus serrulata|
|4||Leaf blade narrowly elliptic to rhombic, apex acute or obtuse, usually not rounded; stamens 2, or 4 with 2 reduced in size; trees of mountainous w United States.||Alnus rhombifolia|
|5||Leaf blade margins strongly revolute; large trees of nw North America.||Alnus rubra|
|5||Leaf blade margins flat or only slightly revolute; trees and shrubs.||> 6|
|6||Leaf blade narrowly ovate or lanceolate to narrowly elliptic; major teeth sharp, acuminate; trees of mountainous s Arizona and New Mexico, adjacent nw Mexico.||Alnus oblongifolia|
|6||Leaf blade ovate, elliptic, obovate, or nearly orbiculate; major teeth acute to obtuse or rounded.||> 7|
|7||Leaf blade obovate to ±orbiculate, apex rounded to retuse or obcordate; moderately large introduced trees naturalized in ne United States, adjacent Canada.||Alnus glutinosa|
|7||Leaf blade ovate to elliptic, apex acute to obtuse; native shrubs or shrubby trees.||Alnus incana|