Sp. Pl. 2: 982. 175.
Gen. Pl. ed. 5, 433. 1754.
Trees or shrubs, to 30 m; trunks often several, branching excurrent, becoming deliquescent. Bark of trunks and branches dark brown to chalky white, smooth, often exfoliating; lenticels dark, prominent, sometimes horizontally expanded. Wood nearly white to reddish brown, light and soft to moderately heavy and hard, texture fine. Branches, branchlets, and twigs nearly 2-ranked; young twigs differentiated into long and short shoots, sometimes with taste and odor of wintergreen. Winter buds sessile, slender, terete, apex acute; scales several, imbricate, smooth. Leaves mostly on short shoots, nearly 2-ranked. Leaf blade ovate to deltate, elliptic, or nearly orbiculate, 0.5–10(–14) × 0.5–8 cm, thin, margins doubly serrate or serrate (or crenate to shallowly round-lobed in dwarf northern species); surfaces glabrous to tomentose, sometimes abaxially resinous-glandular. Inflorescences: staminate catkins mostly terminal on branchlets, solitary or in small racemose clusters, formed previous growing season and often exposed during winter, expanding with leaves; pistillate catkins proximal to staminate catkins, mostly solitary, erect, ovoid to cylindric, firm; scales and flowers crowded, enclosed within buds during winter, expanding with leaves. Staminate flowers in catkins 3 per scale; stamens (1–)2–3(–4), filaments divided below anthers, nearly to base. Pistillate flowers (1–)3 per scale. Infructescences erect or pendulous; scales usually deciduous with release of fruits (although persisting into winter in a few species), (1–)3-lobed, thickened or leathery but not woody. Fruits samaras, lateral wings 2, moderately wide to broad, membranaceous. x = 14.
Throughout n temperate, boreal, and arctic zones of the Northern Hemisphere, North America, Asia.
Species ca. 35 (18 species in the flora).
Birches, like alders, are common trees and shrubs of northern temperate and boreal zones of the Northern Hemisphere. Like Alnus, the group is highly diversified, especially in the Old World. The species are well known for their free hybridization, and specimens are therefore frequently difficult to identify. Birches occupy habitats in cool, moist regions, including peatlands, stream banks, and lakeshores, cool, damp woods, and moist slopes in cool coves. The wood of species that grow to a large size (including especially B. alleghaniensis) has many uses, including the manufacture of doors and windows, flooring, cabinetry, interior molding, wood paneling, furniture, and plywood.
Betula sect. Costatae (Regel) Koehne consists of large, mesophytic trees, often with dark, close or exfoliating bark, large thin leaves, infructescence scales with long narrow lobes, and fruits with narrow wings. North American representatives of this group include Betula alleghaniensis, B. lenta, and B. nigra. The mostly circumboreal Betula sect. Betula consists of small to medium trees with rather large thin leaves and fruits with wide wings (wider than the fruit body). A characteristic feature of trees in this group is their white bark that often peels apart in sheets. These include the familiar paper birch, B. papyrifera, and its European counterpart, B. pubescens, as well as the common eastern B. neoalaskana. A third line, Betula sect. Humiles W. D. Koch, consists of dwarf shrubby species of the cold circumpolar region. In North America this section is represented by B. glandulosa, B. pumila, and B. nana.
Birches are a difficult group taxonomically because of their high vegetative variability and frequent hybridization. Many morphologic and cytologic studies have attempted to deal with variation within the genus or its subgroups. Species of Betula form a polyploid series, with chromosome numbers of 2n = 28, 56, 70, 84, and 112, plus additional numbers in some hybrids. This and other research in the genus has been reviewed by J. J. Furlow (1990).
Brittain, W. H. and W. F. Grant. 1967b. Observations on Canadian birch (Betula) collections at the Morgan Arboretum. V. B. papyrifera and B. cordifolia from eastern Canada. Canad. Field-Naturalist 81: 251--262.
Brittain, W. H. and W. F. Grant. 1969. Observations on Canadian birch (Betula) collections at the Morgan Arboretum. VII. B. papyrifera and B. resinifera from northwestern Canada. Canad. Field-Naturalist 83: 185--202.
Brittain, W. H. and W. F. Grant. 1969b. Observations on Canadian birch (Betula) collections at the Morgan Arboretum. VIII. Betula from Grand Manon Island, New Brunswick. Canad. Field-Naturalist 83: 361--383.
|1||Larger leaf blades generally more than 5 cm, with 5–18 pairs of lateral veins; medium to large trees, 10–30 m.||> 2|
|1||Larger leaf blades mostly less than 5 cm, with 2–6 pairs of lateral veins; shrubs and small to medium trees, 0.5–12 m (B.pubescens to 20 m).||> 10|
|2||Samaras with wings narrower than body; bark of mature trunks and branches mostly dark to medium red or reddish or yellowish brown, sometimes partly (but not completely) creamy or grayish white, with or without horizontally expanded lenticels; infructescences erect, conic or nearly globose, scales often persistent into early winter.||> 3|
|2||Samaras with wings as broad as or broader than body; bark of mature trunks and branches bright chalky to creamy, reddish, pinkish, or brownish white to reddish tan or bronze, often with greatly expanded, dark, horizontal lenticels; infructescences pendulous (erect to nearly pendulous in B.populifolia and B.pendula), elongate, cylindric, readily shattering when mature.||> 6|
|3||Leaf blade rhombic-ovate, base broadly cuneate to truncate, apex acute; scales of infructescences lobed distal to middle, with 3 narrow, ascending lobes, ± equal to somewhat unequal in length and breadth; twigs without wintergreen taste or odor.||Betula nigra|
|3||Leaf blade ovate to elliptic or oblong, base rounded, cordate, or nearly cordate, apex acute to acuminate; scales of infructescences lobed at or proximal to middle, lobes dissimilar in size and shape; twigs with distinctive wintergreen taste and odor.||> 4|
|4||Apex of leaf blade acute or only slightly acuminate; lateral veins 7–10 pairs; swampy habitats.||Betula murrayana|
|4||Apex of leaf blade distinctly acuminate; lateral veins 12–18 pairs; mesic, often streamside habitats.||> 5|
|5||Margins of leaf blade with coarse, rather irregular teeth; bark of mature trunks and branches yellowish, irregularly exfoliating or sometimes darkening and remaining close; scales of infructescences sparsely to moderately pubescent.||Betula alleghaniensis|
|5||Margins of leaf blade with fine, sharp teeth; bark of mature trunks and branches light grayish brown, close, not freely exfoliating; scales of infructescences mostly glabrous.||Betula lenta|
|6||Leaf blade ovate or rhombic-ovate to elliptic or oblong, abaxially pubescent at least along veins, apex acute or short-acuminate; central lobe of infructescence scales equal to or longer than basal and lateral lobes; bark of mature trunks exfoliating.||> 7|
|6||Leaf blade broadly ovate to ± rhombic, abaxially glabrous to somewhat pubescent on veins, apex acuminate to long-acuminate; central lobe of infructescence scales shorter than basal and lateral lobes; bark of mature trunks close or exfoliating.||> 8|
|7||Base of larger leaf blade cuneate, rounded, or truncate; lateral veins 9 or fewer pairs; lateral lobes of infructescence scales held nearly at right angles to axis; mature bark creamy to chalky white or pale to (infrequently) dark brown.||Betula papyrifera|
|7||Base of larger leaf blade mostly cordate (rarely rounded); lateral veins 9–12 pairs; lateral lobes of fruiting catkin scales turned toward apex; mature bark pinkish or brownish white to reddish tan or bronze.||Betula cordifolia|
|8||Apex of leaf blade acuminate, but not extended into long tapering tip; central lobe of infructescence scales nearly as long as lateral lobes; twigs densely covered with large resinous glands.||Betula neoalaskana|
|8||Apex of leaf blade drawn out into long tapering tip; central lobe of infructescence scales much shorter than lateral lobes; twigs sometimes glandular, glands small, inconspicuous.||> 9|
|9||Mature bark grayish white; leaf apex long-caudate; infructescence scales densely pubescent on adaxial surface; native trees of ne North America.||Betula populifolia|
|9||Mature bark creamy to silvery white, exfoliating as long strands; leaf apex acuminate; infructescence scales sparsely pubescent on adaxial surface; domesticated trees adventive or persisting after cultivation.||Betula pendula|
|10||Leaf blade 0.5–2 cm, orbiculate, orbiculate-obovate, or reniform, margins simple-toothed; depressed or low upright shrubs.||> 11|
|10||Leaf blade 2.5–5(–7) cm, ovate to nearly orbiculate, margins distinctly to obscurely doubly serrate, dentate, or crenate; trees and small to large shrubs.||> 12|
|11||Scales of infructescences 3-lobed; samaras with narrow but definite lateral wings.||Betula nana|
|11||Scales of infructescences unlobed; samaras with wings reduced to narrow ridges or without apparent wings.||Betula michauxii|
|12||Leaf blade nearly orbiculate to broadly elliptic, base rounded to cordate or truncate, apex broadly obtuse to rounded; twigs with taste and odor of wintergreen.||Betula uber|
|12||Leaf blade ovate to elliptic, base rounded to cuneate or truncate, apex acute to obtuse or rounded; twigs without taste and odor of wintergreen.||> 13|
|13||Margins of leaf blade crenate to blunt-dentate; shrubs with close bark.||> 14|
|13||Leaf blade margins simply or doubly serrate to dentate, teeth obtuse to relatively sharp; trees and shrubs with close or exfoliating bark.||> 15|
|14||Leaf blade 2.5–5(–7) cm, broadly ovate, obovate, or elliptic to nearly orbiculate, base cuneate, apex broadly acute to obtuse or rounded, surfaces abaxially glabrous to tomentose; twigs pubescent or glabrous, sometimes inconspicuously small-glandular, without large resinous glands.||Betula pumila|
|14||Leaf blade 1–2(–4) cm, mostly obovate to nearly orbiculate, base cuneate to rounded, apex obtuse to rounded, surfaces abaxially mostly glabrous to moderately pubescent; twigs essentially glabrous and warty with large resinous glands.||Betula glandulosa|
|15||Young twigs covered with scattered short, stiff, erect hairs; infructescences 1–6 cm; bark light brownish or tannish white, not readily exfoliating; native shrubby trees of sw Greenland, introduced trees adventive or persisting after cultivation in United States and Canada.||Betula pubescens|
|15||Young twigs glabrous to pubescent, without short, stiff, erect hairs; infructescences mostly (1–)2–3(–4) cm; bark light to dark, close or exfoliating; native trees and shrubs.||> 16|
|16||Central lobe of infructescence scales much shorter than lateral lobes; bark dark reddish brown to bronze, not readily exfoliating; large shrubs and small trees of Rocky Mountains, nw Great Plains, w,c northern Canada, and Alaska.||Betula occidentalis|
|16||Central lobe of infructescence scales equal in length to longer than lateral lobes; bark dark brown to grayish white, exfoliating in thin sheets or close; large shrubs or small trees, nw, boreal, and subalpine ne North America.||> 17|
|17||Bark brown to pinkish or grayish white, exfoliating in thin sheets; small trees of nw North America.||Betula kenaica|
|17||Bark dark reddish brown, not readily exfoliating; shrubs of boreal and subalpine ne North America.||Betula minor|