Alder, a genus of plants (Alnus) belonging to the order Betulaceae, the best-known of which is the common alder (A. glutinosa.) The genus comprises a few species of shrubs or trees, seldom reaching a large size, distributed through the North Temperate zone, and in the New World passing along the Andes southwards to Chile. The British species A. glutinosa is confined to the Old World.
This tree thrives best in moist soils, has a shrubby appearance, and grows under favourable circumstances to a height of 40 or 50 ft. It is characterized by its short-stalked roundish leaves, becoming wedge-shaped at the base and with a slightly toothed margin. When young they are somewhat glutinous, whence the specific name, becoming later a dark olive green. As with other plants growing near water it keeps its leaves longer than do trees in drier situations, and the glossy green foliage lasting after other trees have put on the red or brown of autumn renders it valuable for landscape effect.
The stout cylindrical male catkins are pendulous, reddish in colour and 2 to 4 in. long; the female are smaller, less than an inch in length and reddish-brown in colour, suggesting young fir-cones. When the small winged fruits have been scattered the ripe, woody, blackish cones remain, often lasting through the winter. The alder is readily propagated by seeds, but throws up root-suckers abundantly.
It is important as coppice-wood on marshy ground. The wood is soft, white when first cut and turning to pale red; the knots are beautifully mottled. Under water the wood is very durable, and it is therefore used for piles. The supports of the Rialto at Venice, and many buildings at Amsterdam, are of alder-wood. Furniture is sometimes made from the wood, and it supplies excellent charcoal for gunpowder. The bark is astringent; it is used for tanning and dyeing.
Frequently, such as in Brythonic and Norse mythology, the alder is a symbol of resurrection, possibly because the wood turns from white to reddish-purple when cut, similar to human blood. The first humans in Norse mythology were made from ash and alder trees. In Ireland, reverence for the alder tree was so great that cutting one down was a criminal offense. In other places, such as Newfoundland, the alder's medicinal effects were prized; it has been used to treat burns, rheumatism and itching.