Alnus glutinosa |
Betulaceae |
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black alder, European alder, European black alder |
birch family |
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Habit | Trees, to 20 m; trunks often several, crowns narrow. | Trees and shrubs, deciduous. | ||||||||||||||||
Bark | dark brown, smooth, becoming darker and breaking into shallow fissures in age; lenticels pale, horizontal. |
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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 sometimes lobed, pinnately veined, margins toothed, serrate to nearly entire; surfaces glabrous to tomentose, abaxially often with resinous glands. |
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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. |
unisexual; staminate catkins pendulous, elongate, cylindric, conspicuously bracteate, consisting of crowded, reduced, 1–3-flowered clusters; pistillate inflorescences either of erect to pendulous bracteate catkins, or of compact 2–3-flowered clusters subtended by leafy involucres; bracts often nearly foliaceous or woody in infructescences. |
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Staminate flowers | bracteate; stamens (1–)4–6; anthers 2-locular, dehiscing by longitudinal slits, pollen sacs often ± distinct; pistillode sometimes present. |
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Pistillate flowers | small, highly reduced; pistil 1, 2(–3)-carpellate; ovary inferior, usually 2-locular proximally, 1-locular distally; placentation axile; ovules 1–2 per locule, pendulous; styles 2, distinct or nearly so; stigmas dry; staminodes usually absent. |
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Infructescences | ovoid to nearly globose, 1.2–2.5 × 1–1.5 cm; peduncles 1–10(–20) mm. |
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Fruits | nuts, nutlets, or 2-winged samaras, 1-seeded, without multibracteate cupule, often subtended or enclosed by foliaceous hull developed from 2–3 bracts; seed coat membranous; endosperm present, thin at maturity; embryo straight, as long as seed; cotyledons flat or greatly thickened, oily. |
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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 or sessile, with either 2–3 valvate scales (stipules) or few to many imbricate scales (or occasionally naked); terminal bud absent. |
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Flowering | before new growth in spring. |
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Samaras | obovate, wings reduced to narrow, thickened ridges. |
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2n | = 28. |
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Alnus glutinosa |
Betulaceae |
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Phenology | Flowering early spring. | |||||||||||||||||
Habitat | Stream banks, moist flood plains, damp depressions, borders of wetlands | |||||||||||||||||
Elevation | 0–200 m (0–700 ft) | |||||||||||||||||
Distribution |
CT; IA; IL; IN; MA; MI; MN; NJ; NY; OH; PA; RI; WI; ON; Europe
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Primarily boreal and cool temperate zones of Northern Hemisphere |
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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.) |
Genera 6, species ca. 125 (5 genera, 33 species in the flora). Betulaceae belong to an ancient lineage, traceable in the fossil record to the upper Cretaceous. They are easily distinguished by their woody habit; simple, pinnately veined, usually ovate, sharp-toothed leaves; long staminate catkins that often develop the season before anthesis; and (except in Corylus and Ostryopsis) strobiluslike infructescences. The family is held together on the basis of many features, including habit, leaf structure and arrangement, trichome morphology, wood anatomy, inflorescence morphology, ovary and ovule structure, pollen morphology, embryo structure, and fertilization and germination patterns. Five of the six constituent genera inhabit the boreal and cool temperate zones of Eurasia, North America, and the mountains of Mexico and Central America, and two genera also grow in the Andes to northern Argentina in South America. The remaining genus, Ostryopsis Decaisne (most closely related to Corylus Linnaeus), consists of two species of shrubs restricted to northern and western China. The group is sometimes divided into two families, Betulaceae (Alnus and Betula) and Corylaceae (Carpinus, Ostrya, Corylus, and Ostryopsis), especially in Europe (e.g., H. K. Airy Shaw in J. C. Willis 1973; R. M. T. Dahlgren 1980; J. Hutchinson 1959). In America, this treatment has been followed by A. J. Rehder (1940), J. K. Small (1903), and a few others. Some of those writers have based their recognition of two families in part on the belief that a fundamental difference exists in the staminate inflorescences of the two groups. This view is no longer widely accepted, and most modern authors maintain the family as a single group composed of two subfamilies, Betuloideae and Coryloideae. Betulaceae may have been derived from hamamelidaceous stock (O. Tippo 1938); this idea has been advanced by others (e.g., J. A. Doyle 1969; P. K. Endress 1977; M. W. Chase et al. 1993). The ancestor of more advanced Hamamelidae (including Betulaceae) may have been a member of the Normapolles complex, a Cretaceous fossil pollen flora that includes grains similar in structure to those of the modern Betulaceae (J. A. Doyle 1969). The pollen of modern Betulaceae is much more specialized than that of modern Hamamelidales, however, and the relationships of the Normapolles complex to Juglandaceae, Fagaceae, and other amentiferous groups, as well as to Betulaceae, are not certain. Additional fossil information will be needed if botanists are to gain stronger insights into the phylogeny of these families (P. R. Crane 1989; K. C. Nixon 1989). The most obvious evolutionary trends within the family are those related to fruit dispersal (winged fruits in the Betuloideae, nutlets with accessory winglike bracts in Carpinus and Ostrya, and dispersal by small mammals in Corylus), and with apparent adaptations for survival in cold climates (including shrubby growth forms, small leaves, and the protection of pistillate catkins during winter by bud scales in boreal species). Some radiation has also occurred with respect to the pollination system (E. J. Jäger 1980). Please see P. R. Crane (1989) and J. J. Furlow (1990) for reviews of literature related to the origin and diversification of the family. Members of Betulaceae are economically important as timber trees, as the source of hazelnuts and filberts (Corylus), as ornamental trees and shrubs, and as aids in soil nitrification and stabilization (Alnus). Some are important as causes of pollen allergies in regions where they grow abundantly. (Discussion copyrighted by Flora of North America; reprinted with permission.) |
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Key |
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Source | FNA vol. 3. | FNA vol. 3, p. 507. | ||||||||||||||||
Parent taxa | Betulaceae > subfam. Betuloideae > Alnus | |||||||||||||||||
Sibling taxa | ||||||||||||||||||
Subordinate taxa | ||||||||||||||||||
Synonyms | Betula alnus var. (a) glutinosa | |||||||||||||||||
Name authority | (Linnaeus) Gaertner: Fruct. Sem. Pl. 2: 54. (1790) | Gray | ||||||||||||||||
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