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European black alder
DESCRIPTION: Alnus glutinosa is a rapidly growing tree that can reach 20 m (65 ft.) in height. The tree often has a multi-stemmed trunk. The bark is smooth and dark brown. With age, the bark can show some shallow fissures. The leathery leaves are obovate to nearly orbicular in shape and are 3-9 cm (1-3.5 in.) long and 3-8 cm (1-3 in.) wide. The leaf bases are rounded to broadly cuneate and the margins of the leaves are coarsely or irregularly doubly serrate. The teeth can be acute to obtuse or nearly rounded. The upper leaf surface is glabrous, while the veins of the leaf on the lower surface are pubescent. The young leaves are coated in resin. The leaves of this plant persist late into the fall. Alnus glutinosa is monoecious. The catkins are glutinous, with the staminate catkins being in one or more clusters of 2-5, and are 4-13 cm (1.5-5 in.) long. The pistilate catkins are much smaller and they are also in one or more clusters of 2-5. The flowers appear on the tree before the new growth in the early spring. The hard fruits are ovoid to nearly globose and measure 1.2-2.5 cm (0.5-1 in.) long and 1-1.5 cm (0.5 in.) wide. The seeds are obovate, their wings are short and narrow. Page References Bailey 327, Crow & Hellquist 72, Fernald 539, Flora of North America 511, Gleason & Cronquist 91, Holmgren 84, Magee & Ahles 402. See reference section below for full citations.
SIMILAR SPECIES: Alnus incana (L.) Moench (speckled alder), A. serrulata (Ait.) Willd. (hazel alder), A. viridis (Vill.) Lam. & DC. (green alder) Alnus glutinosa has similar leaf, flower and fruit form to the native bush alder species of the U.S.; however, A. glutinosa is a tree and can grow much bigger than the natives. REPRODUCTIVE/DISPERSAL MECHANISMS: Alnus glutinosa reproduces by mechanically dispersed seeds. Dispersal by water may play a major role in its spread along waterways. Wind plays a minor role in its dispersal, most notably when the seeds are able to blow over the top of crusted snow. DISTRIBUTION: Alnus glutinosa is native to Europe, northern Africa and western Asia. In the United States it is found from Vermont to Minnesota and south from Tennessee to Nebraska. The USDA Plants Database reports that it is currently present in Vermont, Massachusetts and Connecticut. This plant is also known to be present in Rhode Island. It is locally naturalized within its New England distribution. The degree of its naturalization, as opposed to its horticultural presence, has not been fully established. HISTORY OF INTRODUCTION IN NEW ENGLAND: Alnus glutinosa has been in cultivation in the northeast since colonial times. Early records have it escaping cultivation on Long Island as early as the 1870s (J. of the Torrey Botanical Club 1876, Vol. 6: 115, 259). A 1900 record (Rhodora, Vol. 2: 157) has it "well established" in West Medford Massachusetts, though there was "no record of introduction." Fernald (1950) reported that it was "locally naturalized" from Newfoundland to Illinois and south from Delaware to Pennsylvania. HABITATS (IN NEW ENGLAND): Early Successional Forest, Edge, Floodplain Forest, Forest Wetland, Roadside, Shrub Wetland, Yard or Garden. It does best in low lying areas with moist, acidic soils. It is commonly found along stream banks and other waterways. THREATS: This plant's tendency to be disperse by water, and its ability to form monospecific stands, makes it a threat to native wetland species. Alnus glutinosa can fix nitrogen, giving it the ability to become established on very poor soils.
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