Alnus glutinosa

The common Alder and black Alder are all species of Betulaceae trees. They are native to most parts of Europe, Southwest Asia, and northern Africa. It thrives in wet locations where its association with the bacterium Frankia alni enables it to grow in poor quality soils. It is a short-lived, medium-sized tree that can reach a height up to 30 m (100 ft). It has short-stalked rounded leaves and separate male and female flowers in the form of catkins. Cone-shaped, the small, round fruits are cone-shaped and the seeds can be dispersed by wind or water.

The common alder provides food and shelter for wildlife, with a number of insects, lichens and fungi being completely dependent on the tree. It is a pioneer species, colonising vacant land and forming mixed forests as other trees appear in its wake. Common alder eventually dies from woodlands, as seedlings require more light than what is available on the forest floor. Its more usual habitat is forest edges, swamps and riverside corridors. It has been used for making paper and fibreboard, as well as for creating underwater foundations. Products of the tree have been used in ethnobotany, providing folk remedies for various ailments, and research has shown that extracts of the seeds are active against pathogenic bacteria.

Description

Alnus glutinosa is a tree that thrives in moist soils, and grows under favourable circumstances to a height of 20 to 30 metres (66 to 98 ft) and exceptionally up to 37 metres (121 ft). Young trees have an upright habit of growth with a main axial stem but older trees develop an arched crown with crooked branches. The base of the trunk produces adventitious roots which grow down to the soil and may appear to be propping the trunk up. Young trees have a smooth, shiny bark that is greenish-brown and glossy. Older trees have a dark grey, fissured bark. The branches are smooth and somewhat sticky, being scattered with resinous warts. The buds have short stalks and are purple-brown. Both male and female catkins form in the autumn and remain dormant during the winter.

The leaves of the common alder are short-stalked, rounded, up to 10 cm (4 in) long with a slightly wedge-shaped base and a wavy, serrated margin. The common alder has a glossy, dark green upper and paler underside. There are rusty-brown hairs at the angles of the veins. As with some other trees growing near water, the common alder keeps its leaves longer than do trees in drier situations, and the leaves remain green late into the autumn. As the Latin name glutinosa implies, the buds and young leaves are sticky with a resinous gum.

The species is monoecious and the flowers are wind-pollinated; the slender cylindrical male catkins are pendulous, reddish in colour and 5 to 10 cm (2 to 4 in) long; the female flowers are upright, broad and green, with short stalks. During the autumn they become dark brown to black in colour, hard, somewhat woody, and superficially similar to small conifer cones. They last through the winter and the small winged seeds are mostly scattered the following spring. They are reddish brown, flattened nuts with webbing that is filled with air pockets. This enables them to float for about a month which allows the seed to disperse widely.

Common alders don’t produce shade leaves, unlike other tree species. Shaded leaves have a lower rate of assimilation but the same respiration rate as well-lit foliage. This means that as a tree in woodland grows taller, the lower branches die and soon decay, leaving a small crown and unbranched trunk.

Taxonomy

Alnus glutinosa was first described by Carl Linnaeus in 1753, as one of two varieties of alder (the other being A. incana), which he regarded as a single species Betula alnus. In 1785, Jean-Baptiste Lamarck treated it as a full species under the name Betula glutinosa. Its present scientific name is due to Joseph Gaertner, who in 1791 accepted the separation of alders from birches, and transferred the species to Alnus. The epithet glutinosa means “sticky”, referring particularly to the young shoots.

Within the genus Alnus, the common alder is placed in subgenus Alnus as part of a closely related group of species including the grey alder, Alnus incana, with which it hybridizes to form the hybrid A. x hybrida.