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Alnus glutinosa

(Black Alder)

Overview

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Interesting Facts

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Common Names

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Click on the language to view common names.

Common Names in Afrikaans:

Swartels

Common Names in English:

Black Alder, Common Alder, European Alder, European Black Alder

Description

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Family Betulaceae

Trees or shrubs deciduous, monoecious. Stipules present, free , often deciduous, rarely persistent . Leaves alternate, simple , petiolate , usually doubly serrate, rarely simply serrate, lobulate , or entire; veins pinnate. Flowers unisexual . Male inflorescence precocious , elongate , pendulous, with numerous overlapping bracts; each bract usually subtending a small dichasium with 1-3 male flowers; stamens as many as and opposite sepals or, if sepals obsolete , then stamens of inflorescence to 20; filaments very short, connate or nearly so; anthers 2-loculed, thecae connate or separate, opening by longitudinal slits. Female inflorescence pendulous or erect , with numerous overlapping bracts; each bract subtending a small dichasium with 2 or 3 flowers; calyx with 1-6 scalelike lobes , or obsolete; petals absent; ovary inferior, 2-loculed; styles 2, free; ovules 2, or 1 by abortion , pendulous from near apex of each locule. Fruit a nut or nutlet , winged or not. Seed 1, with straight embryo and flat or thickened cotyledons, without endosperm.

Six genera and 150-200 species: mainly in Asia, Europe, and North and South America; six genera (one endemic) and 89 species (56 endemic) in China.

Because of evolutionary divergence within the Betulaceae, the family has often been divided into tribes (i.e. , Betuleae, Carpineae, and Coryleae) or more recently into subfamilies (J. J. Furlow, J. Arnold Arbor . 71: 1-67. 1990) .[1]

Genus Alnus

Trees or shrubs , to 35 m ; trunks usually several, branching excurrent to deliquescent. Bark of trunks and branches light gray to dark brown, thin, smooth , close; lenticels often present, pale , prominent , sometimes horizontally expanded. Wood nearly white, turning reddish upon exposure to air , moderately light and soft, texture fine. Branches, branchlets , and twigs nearly 2-ranked to diffuse ; young twigs uniform or ( Alnus subg. Alnobetula ) differentiated into long and short shoots . Winter buds stipitate (nearly sessile in Alnus subg. Alnobetula ), narrowly to broadly ovoid or ellipsoid , terete , apex acute to rounded ; scales 2--3, valvate , or ( Alnus subg. Alnobetula ) several, imbricate, smooth, or ( Alnus subg. Clethropsis ) sometimes none. Leaves borne on long or short shoots, 3-ranked to nearly 2-ranked. Leaf blade ovate to elliptic or obovate , thin to leathery, base variable, cuneate to rounded, margins doubly serrate, serrate, serrulate , or nearly entire, apex variable, acute to obtuse or acuminate to rounded; surfaces glabrous to tomentose , abaxially sometimes resinous-glandular. Inflorescences: staminate catkins lateral , in racemose clusters or ( Alnus subg. Clethropsis ) solitary, formed ( Alnus subg. Alnus and Clethropsis ) during previous growing season and exposed or enclosed in buds during winter, or ( Alnus subg. Clethropsis ) formed and expanding during same growing season, expanding before or with leaves; pistillate catkins proximal to staminate catkins, solitary or in relatively small racemose clusters, erect to nearly pendulous, ovoid to ellipsoid, firm; scales and flowers crowded, developing and maturing at same time as staminate catkins. Staminate flowers in catkins, 3 per scale; stamens (3--) 4(--6) ; anthers and filaments undivided. Pistillate flowers usually 2 per scale. Infructescences erect or pendulous; scales persistent long after release of fruits, with 5 lobes , greatly thickened, woody. Fruits tiny samaras, lateral wings 2, leathery or membranaceous , reduced or essentially absent in some species. x = 7.

Species ca. 25 (8 sp: forested temperate and boreal Northern Hemisphere; North America; Asia.

Alders resemble birches but are easily distinguished from them by the infructescences, which consist of persistent , 5-lobed, woody scales (versus deciduous, 3-lobed, thin scales). Except in members of Alnus subg. Alnobetula Petermann (which have nearly sessile buds with several imbricate scales), alders are also distinctive in their stipitate buds bearing two stipular scales. The fruits, borne two to a scale, are laterally winged , although the wings are sometimes reduced or absent.

The genus is diverse , including several very distinct lines of specialization. The shrubby or arborescent Alnus subg. Alnus is characterized by winter buds with long stalks and two valvate scales, inflorescences borne in racemose clusters, and development of both pistillate and staminate inflorescences during the growing season prior to anthesis , with these fully exposed during winter. It includes the common A. rubra, A. incana, A. oblongifolia, and A. serrulata. Alnus subg. Alnobetula (represented in North America by three subspecies of A. viridis ) consists of shrubby species of cold-climate regions. In this group, the buds are nearly sessile and covered by several imbricate scales. Both staminate and pistillate catkins are formed the season before anthesis, but only the staminate ones are exposed during winter. The predominantly Asian Alnus subg. Clethropsis (Spach) Regel is represented in America by a single species, A. maritima, a small tree or large shrub of stream banks, marshes, and the shores of shallow lakes . Members of this group are unique in that they bloom in autumn rather than spring . They also differ from other native species in Alnus in having essentially naked buds, leaves with semicraspedodromous venation (i.e. , with the secondary veins branching and anastomosing with each other near the margin before reaching the teeth), and solitary pistillate inflorescences borne in the axils of foliage leaves. All of the alders associate symbiotically with species of the actinomycete Frankia, leading to the formation of nodules on the roots of the plants and the fixation of atmospheric nitrogen.[2]

Physical Description

Species Alnus glutinosa

Trees , to 20 m ; trunks often several, crowns narrow. Bark dark brown, smooth , becoming darker and breaking into shallow fissures in age; lenticels pale , horizontal. 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. Leaf 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. 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. Flowering before new growth in spring . Infructescences ovoid to nearly globose , 1.2--2.5 × 1--1.5 cm; peduncles 1--10(--20) mm. Samaras obovate, wings reduced to narrow, thickened ridges . 2 n = 28. Flowering early spring. [source]

Alnus glutinosa has been called A. vulgaris Hill in some older literature; that name was not validly published. [source]

ID Features: Stalked buds, 0.25" to 0.5" long. Buds are purplish and valvate. A deciduous tree with a narrow pyramidal or columnar shape. Crenate leaf margin. Alternate leaf arrangement.

Habit: A deciduous tree with a narrow pyramidal or columnar shape . Multi-stemmed.

Flowers: 2" to 4" long catkins. Egg-shaped strobili. Monoecious. Not ornamentally important.

Seeds: Fruit: Egg-shaped nutlet . Maturing in October. Not ornamentally important.

Foliage: Summer foliage: Deciduous, simple leaves. Alternate leaf arrangement . Leaves are 2" to 4" long and 3" to 4" wide. Rounded leaf shape. Crenate leaf margin. Dark green leaf color. • Fall foliage: No fall color. Leaves fall off tree either green or brown.

Size/Age/Growth

Growth Rate: Fast. • Size: 40' to 60' tall, 20' to 40' wide.

Landscaping

Landscape Uses: Difficult sites. Street tree . • Liabilities: Powdery mildew , alder aphids, lacebug, flea beetle, leaf miner.

Habitat

Stream banks, moist flood plains , damp depressions , borders of wetlands; 0--200 m [3].

Typically found at an altitude of 0 to 5,013 meters (0 to 16,447 feet).[4]

Ecology: List of Habitats :

[more info]

Biology

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Growth

Culture: Easily transplanted. Prefers moist soil. Full sun to partial shade. Prune in late winter.

Soil: Minimum pH: 5.1 • Maximum pH: 7.8

Sunlight: Sun Exposure: Full Sun .

Temperature: Cold Hardiness: 3a, 3b, 4a, 4b, 5a, 5b, 6a, 6b, 7a, 7b. (map)

Taxonomy

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Last Revised: 2012-05-09