Tree a Week: Alder

Characteristics and Interesting Information:

  • Alder is a deciduous tree in the beech family Betulaceae, native to most of Europe, southwest Asia and northern Africa.
  • One of only 10 plant species that can form a symbiotic relationships with the nitrogen fixing bacterium Frankia alni, allowing alder to not only grow in poor quality soils but fertilises the soil for other plants to follow in succession. There is evidence that suggests Incas planted alder in their terraced farm lands for this very purpose.
  • It is a medium sized, short-lived tree growing to an age of 150 and to a height of up to 30 metres (100 ft). 
  • The leaf buds and small twigs are sticky and resinous, hence the Latin name glutinosa.
  • Its natural habitat is in moist ground near rivers, ponds and lakes but it can also grow in drier locations and sometimes occurs in mixed woodland and on forest edges.
  • The green dye from the flowers was used to colour and camouflage the clothes of outlaws like Robin Hood, and was thought to also colour the clothes of fairies.
Alder European distribution

Ecological Function

The common alder is most noted for its symbiotic relationship with the bacterium Frankia alni, which forms nodules on the tree’s roots. This bacterium absorbs nitrogen from the air and fixes it in a form available to the tree. In return, the bacterium receives sugars produced by the tree through photosynthesis. This relationship, which improves the fertility of the soil, has established the common alder as an important pioneer species in ecological succession, colonising vacant land and forming mixed forests as other trees appear in its wake. Eventually common alder dies out of woodlands because the seedlings need more light than is available on the forest floor. Its more usual habitat is forest edges, swamps and riverside corridors.

Uses

  • The common alder is used 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.
  • The nitrogen-rich leaves falling to the ground and the annual die back of roots enrich the soil and increase the production of nearby trees and bushes. It is recommended to have alders strategically placed in orchards and other forest garden systems as a natural fertiliser.
  • 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
  • The wood is very durable underwater and is used for foundations of buildings in Venice and several medieval cathedrals. 
  • The bark of the common alder contains 16 to 20% tannic acid and long been used in tanning and dyeing. Various shades of brown, fawn, and yellowish-orange hues can be imparted to wool, cotton and silk. If you are looking for naturally dyed yarn, I highly recommend Renaissance Dying. They work in high quality small batch production and ship worldwide.
  • It is also the traditional wood that is burnt to produce smoked fish and high grade charcoal.
  • Recent clinical studies on red alder, used for centuries by Native Americans, showed that it contains betulin and lupeol – compounds shown to be effective against a variety of tumors.
  • The bark can be boiled into a tea to extract salicin (or more commonly known synthesised form – Aspirin) to be used as an anti-inflammatory and to treat skin irritations such as insect bites.

Identification

Bark: 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.

Leaf: 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.

Cones and flowers: One of the only deciduous trees in the UK to produce cones. The flowers on catkins appear between February and April. Alder is monoecious, which means that both male and female flowers are found on the same tree. Male catkins are yellow and pendulous, measuring 2–6cm. Female catkins are green and oval-shaped, and are grouped in numbers of three to eight on each stalk.

For Further Reading

I first came across Alder and it’s incredible nitrogen fixing symbiotic relationship with bacteria while at Martin Crawford’s forest garden. It was a scales off my eyes moment when I saw that planting alder strategically fertilised the surrounding environment and reduced human intervention. Check out his book here for more in depth information on how to plan a forest garden. You should also see Stephan Sobkowiak’s incredible permaculture orchard function beautifully with nitrogen fixer planted in between fruit trees and perennial gardens.

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