Alder facts

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. This was probably during colonial times, and it 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. It 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 type of wet woodland.

Alder is a member of the birch family of trees, Betulaceae. It can reach 30 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. It can live to around 150 years old. The annual rate of growth can be up to 90 cm. a year when the tree is young. 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 greenish 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. This gives rise to the second part of its botanical name, glutinosa , which means sticky. 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 they have a notch at the apex, and they taper 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. Instead they darken and begin to wither before the tree sheds them in late October or November.

Alder is monoecious, meaning 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 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 can germinate on the surface of water. They then root 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 can give shelter to fish and provide a safe refuge from predators. Alder foliage provides shade which is beneficial to fish, including salmon and brown trout. Its leaves decompose fairly quickly in water. They provide nutrients for invertebrates such as the larvae of caddisflies, stoneflies and water beetles. These in turn form part of the aquatic food web. Larger organisms, including fish like to eat these invertebrates. Alder roots can also provide sheltered spots for otters to rest and breed.

Alder has an important symbiotic relationship with a bacterium called Frankia alni. Frankia forms nodules on the tree’s roots and live within the nodules. This nitrogen-fixing bacterium absorbs nitrogen from the air and makes it available to the tree. Alder, in turn, provides the bacterium with carbon, which it produces through photosynthesis. This relationship allows alder to improve the fertility of the soils where it grows. As a pioneer species, it helps provide nutrients for the successional species which follow.

A mature alder supports a variety of moss and lichen species on its bark and branches. Moisture-loving lichens such as tree lungwort are relatively common on alder. This is because it grows by rivers and streams, where there is often a higher humidity in the air due to spray.

Forty-seven species of mycorrhizal fungi grow with alder. In fact many trees and fungi form mycorrhizal relationships. Both the partners benefit through an exchange of nutrients, which each organism cannot access directly itself. Some species of fungi only form these partnerships with alder. The brown roll-rim is one example. The fruits of the brown cup fungus appear in spring, on alder catkins after they have fallen to the ground.

A rare fungus called Taphrina amentorum grows in the flowers and seeds of young alder catkins. They then produce galls, which harden as the cones ripen.

Galls are also induced on the leaves of alder by a mite called Eriophyes. These galls take the form of raised pustules on the upper surface of the leaves. They 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 plant-eating insects have been recorded on alder. These include the striped alder sawfly. There are few moths in Scotland which feed only on alder, but the May highflyer is specific to this tree. Its larva lives in a shelter which is formed by two leaves that have been sewn together by silk. In England a wider range of moths are associated with alder. These include the alder kitten moth but that species is absent from Scotland.

As with most tree species in Scotland, red deer eat alder. This prevents the natural regeneration of the tree in many parts of the Highlands. Domestic sheep have a similar effect throughout the country.  A more serious threat to the survival of alder is the algae-like Phytophthora. This causes a disease that could potentially kill alder trees on a large scale, if left unchecked.

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. Its survival and expansion is essential to the health of the land and rivers alike.

 

> Content contributors

In This Section