Alder (Alnus glutinosa) is the most common tree in wetland forests, and it improves soil fertility through its ability to fix nitrogen from the air.

Global distribution

Common or black alder occurs throughout most of Europe and across Russia to Siberia. Its range also includes the Caucasus, Iran, Turkey and North Africa, where it is native to Tunisia, Algeria and Morocco. It has been introduced to North America, probably during colonial times, and has become naturalised in eastern Canada and the northeast of the USA.

Distribution in Scotland

Alder is found throughout Scotland, although its presence in Shetland is due to planting. Today it is rare in the Outer Hebrides and the far northwest of the mainland, but this is the result of deforestation, rather than a limit in the tree’s range. Alder occurs at elevations of up to 500 metres and typically grows alongside streams and rivers, or on wet ground. As a result it is often found in narrow bands along watercourses, although in the right conditions it can form alder carr, which is a dense thicket or stand of alder, usually small in height, growing on wet swampy soils.

Alder is a member of the birch family of trees, Betulaceae, and can reach 25 metres in height, although in Scotland it is rarely more than 20 metres tall. Like the birches, it is a pioneer species which grows quickly and is relatively short-lived, with the maximum age typically being 150 years. The annual rate of growth can be up to 90 cm. a year when the tree is young, and after the death of the original trunk, new shoots can sprout from the base, forming a multi-stemmed clump of new growth.

Alder bark is dark grey and fissured, while the young branches are smooth and slightly green in colour. Branches are ascending in their growth form, and trees with two or three main trunks are common. The twigs are sometimes sticky to the touch (which gives rise to the second part of its binomial name, glutinosa , which means sticky), and the buds are 7 mm. in length. Alder is deciduous, and the new leaves open out in April. They are up to 10 cm. in size with a waved margin and are almost circular in shape, sometimes with a notch at the apex, and tapering to a slight point on the leaf stem. The leaves have shallow irregular teeth and remain on the tree quite late in autumn. However, they do not change to any dramatic colours, but instead just darken and begin to wither before being shed in late October or November.

Alder is monoecious, so each tree bears both male and female flowers. Male catkins are dark yellow-brown in colour, and are up to 5 cm. long when they are fully open. At 6 mm. in length, the female flowers are much smaller in size, and are red, erect and cone-like in shape. The flowers appear before the new leaves, in March (or early April in the Highlands), and pollination is by the wind. Pollinated female flowers grow into ovoid fruits about 1.5 cm. in length, which are green in colour and grow in clusters of up to 4 at the end of twigs. These ripen and turn woody by October, and release a number of small flat red-brown seeds, each weighing about 0.004 gm. The seeds have small 'wings', which are air-filled membranes that enable them to float on water, and dispersal is by both wind and water. Seeds have been recorded as germinating on the surface of water, and then rooting successfully when they are washed up on land. The empty cones can persist on the tree until the following spring and are a distinctive feature of the alder tree in winter.

Alder is the most common tree species in riparian forests, and it plays a crucial role in these stream- and loch-edge woodlands. As a deep-rooted species, it helps to maintain the soil in river banks and reduces the effects of erosion. Alder root systems which are exposed in the water give shelter to fish during times of high water flow, and provide a safe refuge from predators. Alder foliage provides shade which is beneficial to fish, including salmon (Salmo salar) and brown trout (Salmo trutta), and its leaves, which are relatively quick to decompose in water, provide nutrients for invertebrates such as the larvae of caddisflies, stoneflies and water beetles. These in turn form part of the aquatic food web, and are eaten by larger organisms, including salmonid fish.

Alder is particularly noted for its important symbiotic relationship with a bacterium (Frankia alni), which forms nodules on the tree's roots. This nitrogen-fixing bacterium absorbs nitrogen from the air and makes it available to the tree, with the rate of fixation estimated at up to 125 kg. of nitrogen per hectare per year. Alder, in turn, provides the bacterium with carbon, which it produces through photosynthesis. As a result of this mutually-beneficial relationship, alder improves the fertility of the soils where it grows, and as a pioneer species, it helps provide additional nutrients for the successional species which follow.

A mature alder supports a variety of moss and lichen species on its bark and branches. Because it grows by rivers and streams, where there is often a higher humidity in the air due to spray, moisture-loving lichens such as tree lungwort (Lobaria pulmonaria) are relatively common on alder. At least one lichen species (Stenocybe pullatula) only occurs on alder, while another (Menegazzia terebrata) is more common on alder than on any other tree species.

47 species of mycorrhizal fungi have been recorded as growing with alder, and in the symbiotic relationships formed by these fungi and the tree, both the partners benefit through an exchange of nutrients, which each organism cannot access directly itself. Notable species forming this relationship with alder include a rare Russula (Russula pumila), two milk caps (Lactarius obscuratus and L. cyathula) and the brown roll-rim (Paxillus filamentosus), all of which are restricted to alder, and several species in the genus Naucoria , which are mainly restricted to alder. The fruits of the brown cup fungus (Ciboria amentacea) appear in spring, on alder catkins after they have fallen to the ground. In recent years, a rare fungus (Taphrina amentorum) which produces curved, tongue-like galls on alder cones, has been discovered in Britain. Previously known only from mainland Europe, this species grows in the flowers and seeds of young alder catkins, before producing the galls, which harden as the cones ripen.

Galls are also induced on the leaves of alder by a mite ( Eriophyes laevis inangulis ), and these take the form of raised pustules on the upper surface of the leaves. The galls vary in colour from pale yellow-green to deep red, and the mite itself lives on sap, which it sucks from the cell tissue of the tree.

Over 140 phytophagous (ie plant eating) insects have been recorded on alder. These include the striped alder sawfly ( Hemichroa crocea ) and another sawfly ( Cimbex connatus ), although the latter is rare in the UK. In comparison to birches ( Betula spp. ), there are relatively few moths in Scotland which feed exclusively on alder, but the May highflyer ( Hydriomena impluviata ) is specific to alder, and its larva lives in a shelter which is formed by two leaves that have been sewn together by silk. Another moth whose larva is a specialist feeder on alder is the dingy shell ( Euchoeca nebulata ), while 4 species of micromoths in the genus Phyllonorycter make blister mines on alder leaves. In England a wider range of moths are associated with alder, including the alder kitten moth ( Furcula bicuspis ), but that species is absent from Scotland.

As with most tree species in Scotland, alder is browsed upon by red deer ( Cervus elaphus ), and this prevents the natural regeneration of the tree in many parts of the Highlands, while domestic sheep have a similar effect throughout the country. However, other potentially more serious threats to the survival of alder have recently been noted in the UK. One is a fungus ( Phytophthora sp. ) which grows upwards from the bottom of the tree, killing the roots and bark. This has become a widespread problem in England and Wales, where over 10% of riparian alders are either dead or infected with the fungus, but there have only been a few confirmed reports of this in Scotland so far.

The other problem affecting alders is crown dieback, which results in the tree dying from the top downwards. This condition was first noted in the northwest of Scotland in the 1980s and has subsequently spread throughout the Highlands: in some riparian areas of Glen Affric, for example, most of the alders are affected. The cause of crown dieback is still unknown, and research into the problem is ongoing.

Because it is the most common tree in riparian woodlands, which often form the ecological linkages between different forest patches, alder is an important species in the Caledonian Forest, and its survival and expansion is essential to the health of the land and rivers alike.