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. 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 2 n = 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).
PLANT: Trees or shrubs to 30 m tall; trunks often several; twigs nearly 2-ranked, differentiated into long and short shoots; winter buds sessile, with several imbricate scales; bark of trunks and branches dark brown or reddish to white, often exfoliating; lenticels conspicuous. INFLORESCENCE: staminate catkins solitary or in racemose clusters, formed the previous growing season, expanding with the leaves; pistillate catkins mostly solitary, proximal to staminate catkins, erect or sometimes becoming pendulous in fruit, the scales and flowers crowded and enclosed within the buds during winter, deciduous in fruit. LEAVES: mostly on short shoots, nearly 2-ranked; blades ovate to deltoid, elliptic, or nearly orbiculate; margins serrate, doubly serrate, or crenate; surfaces glabrous to tomentose, sometimes abaxially resinous. STAMINATE FLOWERS: 3 per scale; stamens (1-)2-3(-4), the filaments divided below the anthers, the 2 anther sacs separate. PISTILLATE FLOWERS: 1-3 per scale. FRUITS: tiny leathery samaras; wings 2, lateral, membranaceous. x = 14. NOTES: Ca. 50 spp.; circumboreal. (Latin betula = birch). Species of Betula yield important wildlife food, wood for interior finishing, and popular ornamentals. NOTES: Brasher, Jeffrey W. 2001. Betulaceae. J. Ariz. - Nev. Acad. Sci. Volume 33(1)
Staminate catkins elongate, the scales ovate to rotund, each subtending 3 fls; cal minute, 4-parted or usually 2-parted, one lobe much exceeding the other; stamens 2, each short filament divided near the summit and the anther-sacs separate; pistillate catkins ovoid to cylindric, the scales closely imbricate, generally 3-lobed, relatively thin, at maturity herbaceous or coriaceous, deciduous with or soon after the frs, the catkin disintegrating; fr ordinarily a small samara, the elliptic body with 2 membranous lateral wings and terminated by the short, persistent styles, or sometimes the wings virtually obsolete; trees or shrubs, the bark often separable into thin layers; the catkins appear in the fall and come into anthesis early in the spring (midsummer in B. michauxii). 50, N. Hemisphere.
The taxonomy of Betula is complicated by extensive hybridization and introgression, yet the species maintain their identity over large areas of sympatry. Many of the hybrids have been given names. B. آorealis Spach is B. papyrifera נpumila; B. أaerulea Blanch. and B.أaerulea-grandis Blanch. are B. papyrifera var. cordifolia נpopulifolia; B. حinor (Tuck.) Fernald is B. glandulosa var. glandulosa נa dwarf, alpine form of B. papyrifera, perhaps locally alloploid; B. ذurpusii C. K. Schneid. is B. alleghaniensis נpumila; B. حurrayana B. V. Barnes & Dancik is an octoploid back-cross of B. ذurpusii with B. alleghaniensis; and B. سandbergii Britton is B. glandulosa var. glandulifera נpapyrifera.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.