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. The bark of young trees is smooth, glossy and greenish-brown while in older trees it is dark grey and fissured. The branches are smooth and somewhat sticky, being scattered with resinous warts. The buds are purplish-brown and have short stalks. 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. They have a glossy dark green upper surface and paler green underside with rusty-brown hairs in 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. The seeds are flattened reddish-brown nuts edged with webbing filled with pockets of air. This enables them to float for about a month which allows the seed to disperse widely.
Unlike some other species of tree, common alders do not produce shade leaves. The respiration rate of shaded foliage is the same as well-lit leaves but the rate of assimilation is lower. This means that as a tree in woodland grows taller, the lower branches die and soon decay, leaving a small crown and unbranched trunk.
Usage & Cultivation
The common alder is used as a pioneer species and to stabilise river banks, to assist in flood control, to purify water in waterlogged soils and to moderate the temperature and nutrient status of water bodies. It can be grown by itself or in mixed species plantations, and the nitrogen-rich leaves falling to the ground enrich the soil and increase the production of such trees as walnut, Douglas fir and poplar on poor quality soils. Although the tree can live for up to 160 years, it is best felled for timber at 60 to 70 years before heart rot sets in.
On marshy ground it is important as coppice-wood, being cut near the base to encourage the production of straight poles. It is capable of enduring clipping as well as marine climatic conditions and may be cultivated as a fast-growing windbreak. In woodland natural regeneration is not possible as the seeds need sufficient nutrients, water and light to germinate. Such conditions are rarely found at the forest floor and as the forest matures, the alder trees in it die out. The species is cultivated as a specimen tree in parks and gardens, and the cultivar ‘Imperialis’ has gained the Royal Horticultural Society’s Award of Garden Merit.