Birch Family (Betulaceae)
The birch family is a group of flowering plants of tree or shrub form that includes the birches (Betula), alders (Alnus), hornbeams (Carpinus), and hazels (Corylus). Members of the birch family have simple and alternate leaves that bear appendages (stipules) where they join the branch. The leaves are also deciduous, generally thin and often doubly toothed along the margin. The flowers are densely borne on elongate, spike-like structures called catkins. Each catkin bears flowers of only one sex but male and female catkins occur on the same plant. Female catkins are stiffer and fewer-flowered than male catkins. The flowers lack petals or sepals, although some species have small scale-like appendages that represent reduced perianth parts. Pollination occurs in the spring by the wind. The fruit is a one-seeded nut or nutlet which is often winged and enclosed or surrounded at the base by leaf-like appendages called bracts.
The family includes six genera and about 170 species worldwide. The Betulaceae occur throughout the temperate and boreal regions of the Northern Hemisphere, where they are prominent members of forest communities. Some alders and dwarf birches extend into the southern Arctic where they are common shrubs. Members of the family occur only in a few regions of the Southern Hemisphere, notably in the tropical mountains of the northern Andes of Columbia and in Argentina.
Ecologically, this an important family. Many members of the Betulaceae play pivotal roles in early successional stages following disturbance. In North America, for example, the paper or white birch (Betula papyrifera) is a transcontinental species that rapidly recolonizes areas disturbed by fire or logging, thus stabilizing the soil and providing suitable conditions for recolonization by other species. Paper birch is able to invade disturbed areas by dispersal of its winged seeds. In addition, birches typically resprout from the base when the main trunk is killed. Birches grow fast and die young; paper birch, for example, reaches maturity in 60-75 years and seldom lives longer than 140 years.
Alders are also important early successional species. In northern regions, cold soils limit the activity of nitrogen-producing
soil bacteria, thus, northern soils frequently lack adequate quantities of nitrogen for vigorous plant growth. Alders are among the few plants that form symbiotic relationships with nitrogen-fixing bacteria. In alders, filamentous bacteria known as actinomycetes form nodules on the roots. The nitrogen compounds produced by the actinomycetes are in a form directly usable by the host alder plant. In addition, some of the nitrogen compounds are leached from the nodules or released when nodule-bearing roots die. These compounds accumulate in the soil where they remain available for use by other plants of later successional stages that cannot produce their own nitrogenous compounds.
In the United States there are five genera and about 25 species of Betulaceae. Yellow birch (Betula alleghaniensis) and paper birch are common in the northern hardwood forests of northern New England. The American hornbeam, also called blue beech, is the only North American member of the genus Carpinus (C. caroliniana). It grows in moist woods as a tall shrub or small tree reaching heights of 39 ft (10 m) and with a smooth, gray, ridged trunk that is often described as fluted. American hornbeam has a curious range that covers much of eastern North America, parts of Texas, and then skips to southern Mexico, Guatemala, and Honduras. Eastern hop hornbeam (Ostrya virginiana) has a similar distribution to American hornbeam. However, eastern hop hornbeam is taller, reaching heights of 65 ft (20 m), and has scaly bark that breaks off in short strips. The wood of eastern hop hornbeam is extremely dense, making it difficult to drive a nail into it, hence its other common name of ironwood. A variety of alder species occur as shrubs in moist ground throughout North America. In the Pacific Northwest, however, red alder (A. rubra) is an abundant, fast-growing tree of disturbed areas that attains heights of 131 ft (40 m). Hazel (Corylus) is a widespread shrub (3-10 ft [1-3m] tall) in eastern North America and parks of the Midwest, with the beaked hazel (C. cornuta) reaching Oregon.
The birch family is economically valuable. In North America, yellow and sweet birch are important sources of wood for cabinet-making, furniture of various kinds, floors, and doors. Paper birch is excellent as firewood and is used in the making of plywood and boxes. The bark of paper birch was once used by indigenous North Americans for making canoes. In northern Europe and especially in Russia, birch switches are traditionally used to beat one's skin during sauna baths. The sap of birches is sweet and can be collected and condensed into syrup. Hazels produce edible nuts that are sometimes called filberts or cobnuts. Store-bought hazelnuts generally come from cultivated European species, most commonly Corylus avellana, C. colurna, and C. maxima. Turkey is the largest producer of hazelnuts at 300,000 metric tons annually, followed by Spain and Italy. In the United States, hazelnuts are produced mostly in Oregon with annual harvests of about 10,000 metric tons.
Most species of alder are shrubs of no commercial value, but red alder of the Pacific Northwest and a number of European species reach tree-size and are valuable sources of wood. Alder wood produces a superior charcoal that imparts a delicious flavor to meat. Charcoal made from red alder is preferred for smoking salmon on the west coast of North America.
Heywood, Vernon H., ed. Flowering Plants of the World. New York: Oxford University Press, 1993.
Raven, Peter, R.F. Evert, and Susan Eichhorn. Biology of Plants. 6th ed. New York: Worth Publishers Inc., 1998.
Les C. Cwynar