2. Betula Linnaeus, Sp. Pl. 2: 982. 1753; Gen. Pl. ed. 5, 433, 1754.
Birch [Latin betula, birch]
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.
Species ca. 35 (18 species in the flora): throughout n temperate, boreal, and arctic zones of the Northern Hemisphere; North America, Asia.
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).
SELECTED REFERENCES
Alam, M. T. and W. F. Grant. 1972. Interspecific hybridization in birch (Betula). Naturaliste Canad. 99: 33--40. Brittain, W. H. and W. F. Grant. 1965. Observations on Canadian birch (Betula) collections at the Morgan Arboretum. I. B. papyrifera in eastern Canada. Canad. Field-Naturalist 79: 189--197. Brittain, W. H. and W. F. Grant. 1965b. Observations on Canadian birch (Betula) collections at the Morgan Arboretum. II. B. papyrifera var. cordifolia. Canad. Field-Naturalist 79: 253--257. Brittain, W. H. and W. F. Grant. 1966. Observations on Canadian birch (Betula) collections at the Morgan Arboretum. III. B. papyrifera from British Columbia. Canad. Field-Naturalist 80: 147--157. Brittain, W. H. and W. F. Grant. 1967. Observations on Canadian birch (Betula) collections at the Morgan Arboretum. IV. B. caerulea-grandis and hybrids. Canad. Field-Naturalist 81: 116--127. 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. 1968. Observations on Canadian birch (Betula) collections at the Morgan Arboretum. VI. B. papyrifera from the Rocky Mountains. Canad. Field-Naturalist 82: 44--48. 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. Dugle, J. R. 1966. A taxonomic study of western Canadian species in the genus Betula. Canad. J. Bot. 44: 929--1007. Fernald, M. L. 1902. The relationships of some American and Old World birches. Amer. J. Sci. 169: 167--194. Fernald, M. L. 1945. Some North American Corylaceae (Betulaceae). 1. Notes on Betula in eastern North America. Rhodora 47: 303--329. Fredskild, B. 1991. The genus Betula in Greenland---Holocene history, present distribution, and synecology. Nordic J. Bot. 11: 393--412. Grant, W. F. and B. K. Thompson. 1975. Observations on Canadian birches, Betula cordifolia, B. populifolia, B. papyrifera and B. ×caerulea. Canad. J. Bot. 53: 1478--1490. Johnsson, H. 1945. Interspecific hybridization within the genus Betula. Hereditas (Lund) 31: 163--176. Lepage, E. 1976. Les bouleaux arbustifs du Canada et de Alaska. Naturaliste Canad. 103: 215--233. Sulkinoja, M. 1990. Hybridization, introgression and taxonomy of the mountain birch in SW Greenland compared with related results from Iceland and Finnish Lapland. Meddel. Grfnland, Biosci. 33: 21--29.
