Flora of North America species comparison

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Alnus glutinosa

Alnus

black alder, European alder, European black alder

alder, aulne, aune
Habit Trees, to 20 m; trunks often several, crowns narrow. Trees or shrubs, to 35 m; trunks usually several, branching excurrent to deliquescent.
Bark

dark brown, smooth, becoming darker and breaking into shallow fissures in age;

lenticels pale, horizontal.

of trunks and branches light gray to dark brown, thin, smooth, close;

lenticels often present, pale, prominent, sometimes horizontally expanded.
Branches

, branchlets, and twigs nearly 2-ranked to diffuse;

young twigs uniform or (Alnus subg. Alnobetula) differentiated into long and short shoots.
Leaves

blade obovate to nearly orbiculate, 3–9 × 3–8 cm, leathery, base obtuse to broadly cuneate, margins flat, coarsely and often irregularly doubly serrate to nearly dentate, major teeth acute to obtuse or rounded, apex often retuse or obcordate, or occasionally rounded;

surfaces abaxially glabrous to sparsely pubescent, often more heavily on veins, both surfaces heavily resin-coated.

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

formed season before flowering and exposed during winter; staminate catkins in 1 or more clusters of 2–5, 4–13 cm; pistillate catkins in 1 or more clusters of 2–5.

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

ovoid to nearly globose, 1.2–2.5 × 1–1.5 cm;

peduncles 1–10(–20) mm.

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.
Winter

buds stipitate, ellipsoid to obovoid, 6–10 mm, apex obtuse;

stalks 2–5 mm;

scales 2–3, outer 2 equal, valvate, usually heavily resin-coated.

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.
Flowering

before new growth in spring.

Samaras

obovate, wings reduced to narrow, thickened ridges.

Wood

nearly white, turning reddish upon exposure to air, moderately light and soft, texture fine.
x

= 7.
2n

= 28.

Alnus glutinosa

Alnus
Phenology Flowering early spring.
Habitat Stream banks, moist flood plains, damp depressions, borders of wetlands
Elevation 0–200 m (0–700 ft)
Distribution
from FNA
CT; IA; IL; IN; MA; MI; MN; NJ; NY; OH; PA; RI; WI; ON; Europe
[WildflowerSearch map]
[BONAP county map]
from USDA
North America; Forested temperate and boreal Northern Hemisphere; Asia
[BONAP county map]
Discussion

Alnus glutinosa is cultivated as an ornamental tree throughout eastern North America and is available in a variety of cultivars, including cut-leafed and compact-branching forms. This species has also been used extensively to control erosion and improve the soil on recently cleared or unstable substrates, such as sand dunes and mine spoils. It has escaped and become widely naturalized throughout the temperate Northeast, occasionally becoming a weedy pest. In Europe the black alder has served for many centuries as an important source of hardwood for timbers and carved items, including wooden shoes.

Alnus glutinosa has been called A. vulgaris Hill in some older literature; that name was not validly published.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

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.

(Discussion copyrighted by Flora of North America; reprinted with permission.)
Key
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).
A. 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).
A. 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.
A. 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.
A. rhombifolia
5. Leaf blade margins strongly revolute; large trees of nw North America.
A. 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.
A. 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.
A. glutinosa
7. Leaf blade ovate to elliptic, apex acute to obtuse; native shrubs or shrubby trees.
A. incana
Source FNA vol. 3. FNA vol. 3.
Parent taxa Betulaceae > subfam. Betuloideae > Alnus Betulaceae > subfam. Betuloideae
Sibling taxa
A. incana, A. maritima, A. oblongifolia, A. rhombifolia, A. rubra, A. serrulata, A. viridis
Subordinate taxa
A. glutinosa, A. incana, A. maritima, A. oblongifolia, A. rhombifolia, A. rubra, A. serrulata, A. viridis
Synonyms Betula alnus var. (a) glutinosa
Name authority (Linnaeus) Gaertner: Fruct. Sem. Pl. 2: 54. (1790) Miller: Gard. Dict. Abr. ed. 4. (1754)
Web links 
Unless otherwise noted, all data comes from the online version of the Flora of North America, who have made most of their content available via a Creative Commons license (CC BY 4.0).



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