The Commen Alder (Alnus glutinosa) is a deciduous tree that is native to Britain, it makes its home by streams, rivers and marshlands and is comfortable with its roots submerged in water. It provides a strong form of timber especially underwater. It is a member of the Birch family of trees and like birch grows relatively quickly and is short-lived.
The Alder is a tree that grows under favourable circumstances to a height of 20–30m but can grow taller in exceptional circumstances. It is characterized by its short-stalked rounded leaves, becoming wedge-shaped at the base and with a slightly toothed margin. The glossy green foliage lasts long after other trees have put on the red or brown of autumn, which renders it valuable for landscape effect. However, some ancient civilisations considered the tree to be a natural embodiment of evil because when the wood is cut it quickly turns a reddish colour as if bleeding. In Ireland the tree was so revered that cutting one down was a criminal offence, while in Norse mythology the first people were said to be made from ash and alder.
The Alder is a pioneer species in an ecological environment. Its symbiotic relationship with certain bacterium and ability to fix nitrogen from the atmosphere improves soil fertility, whilst it is also a host to a wide variety of moss and lichen.
It is important as coppice-wood on marshy ground. The wood is soft, white when first cut and turning to pale red; the knots are beautifully mottled. Under water the wood is very durable, and it is therefore used for piles. The supports of the Rialto at Venice, and many buildings at Amsterdam, are of Alder wood. Furniture is sometimes made from the wood, and it supplies excellent charcoal for gunpowder.
The biodiversity on alder is amazing. There are numerous species of mycorrhizal fungi grow with the alder which benefit both the fungi and the tree through the exchange of nutrients which neither the alder nor the fungi can access themselves. There are also well over a hundred separate plant eating insects that thrive on the alder. From an ecology point of view the alder is an important species.
There are however potential serious threats that endanger the alder like the fungus Phytophthora sp. which can grow upwards from the base of the tree killing the roots and bark. It has been apparently reported that 10% of the alder in England could be affected.
Alder is an important species and its survival and expansion is essential to the health of the land and rivers alike.