Alder (Alnus glutinosa)
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Alder is native to Britain and is also found throughout Europe as far as Siberia.
Common name: alder, common alder, black alder, European alder
Scientific name: Alnus glutinosa
UK provenance: native
Interesting fact: alder is particularly noted for its important symbiotic relationship with a nitrogen-fixing bacterium called Frankia alni. This bacterium is found in the root nodules. The bacterium absorbs nitrogen from the air and makes it available to the tree. Alder, in turn, provides the bacterium with sugars, which it produces through photosynthesis.
As a result of this mutually beneficial relationship, alder improves the fertility of the soil where it grows, and as a pioneer species, it helps provide additional nitrogen for the successional species which follow.
What does alder look like?
Overview: conical in shape, mature trees can reach a height of around 20m and live to around 60 years. The bark is dark and fissured and is often covered in lichen. Twigs have a light brown spotted stem which turns red towards the top. Young twigs are sticky to touch.
Leaves: the purple or grey leaf buds form on long stems and the 3–9cm long dark green leaves are racquet-shaped and leathery, with serrated edges. The leaf tip is never pointed and is often indented.
Flowers: are on catkins which 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.
Fruits: once pollinated by wind, the female catkins gradually become woody and appear as tiny, cone-like fruits in winter. They open up to release seeds, which are dispersed by wind and water.
Look out for: small brown cones, which are the female catkins, stay on the tree all year round.
Could be confused with: hazel (Corylus avellana). The rounded leaf shapes are similar however hazel leaves are softly hairy compared to the shiny ones of alder.
Identified in winter by: female catkins and purple twigs have orange markings (lenticels).
Where to find alder
Alder is native to almost the whole of continental Europe (except for both the extreme north and south) as well as the United Kingdom and Ireland. Its natural habitat is moist ground near rivers, ponds and lakes and it thrives in damp, cool areas such as marshes, wet woodland and streams where its roots help to prevent soil erosion.
It can also grow in drier locations and sometimes occurs in mixed woodland and on forest edges. It grows well from seed and will quickly colonise bare ground. Because of its association with the nitrogen-fixing bacterium Frankia alni, it can grow in nutrient-poor soils where few other trees thrive.
There are 20 to 30 species in the genus Alnus. They are distributed throughout the North Temperate zone and in North, Central and South America. A. glutinosa is the only species in the genus native to the UK.
Value to wildlife
Alder is the food plant for the caterpillars of several moths, including the alder kitten, pebble hook-tip, the autumnal and the blue bordered carpet moth. Catkins provide an early source of nectar and pollen for bees, and the seeds are eaten by the siskin, redpoll and goldfinch.
The wet conditions found in alder woodland are ideal for a number of mosses, lichens and fungi, along with the small pearl-bordered fritillary and chequered skipper butterflies, and some species of crane fly. Alder roots make the perfect nest sites for otters.
Mythology and symbolism
Wet and swampy, alder woods, or carrs, were thought to have a mysterious atmosphere. 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. When cut, the pale wood turns a deep orange, giving the impression of bleeding. As such, many people feared alder trees and the Irish thought it was unlucky to pass one on a journey.
How we use alder
Soft and porous, alder wood is only durable if kept wet, and its value to humans is down to its ability to withstand rot under water. Historically it has been used in the construction of boats, sluice gates and water pipes, and much of Venice is built on alder piles. These days alder wood is used to make timber veneers, pulp and plywood.
It is thought that the female woodworm lays eggs in alder in preference to other wood. Traditionally, alder branches were cut and placed in cupboards to deter woodworm from laying eggs in them.
Alder coppices well and the wood makes excellent charcoal and gunpowder. The roots have nitrogen-fixing nodules which make it an excellent soil conditioner. The trees are therefore used to improve soil fertility on former industrial wasteland and brownfield sites. They are also used in flood mitigation.
Alder used to be the preferred wood to make clogs, and it was said that a few alder leaves placed in the shoes before a long journey would cool the feet and prevent swelling.
Some alders in the UK have been infected by a type of fungus, Phytophthora. Diseases caused by Phytophthora are quite common on broadleaf tree species, but it was thought to be uncommon on alder until the discovery of a new hybrid strain, which causes root rot and stem lesions.
Sometimes known as alder dieback, diseased alders are conspicuous in summer because the leaves are abnormally small and yellow, and often fall prematurely. Infected trees have dead twigs and branches in the crown. They may also bear an unusually large number of cones - a sign of stress. Sometimes the trees die rapidly and other times they deteriorate gradually. Symptoms include bleeding from the bark, which resembles brown, rusty spots. When exposed, the reddish, mottled inner bark contrasts with the creamy colour of healthy bark.