ALDER TREE, (ALNUS GLUTINOSA)

Alde

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(Alnus glutinosa)

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Alder is native to Britain and is also found throughout Europe as far as Siberia.

The conical-shaped alder tree can reach a height of 20 meters 

The leaves of the alder are racquet shaped and feel leathery to the touch Alder is monoecious, which means that both male and female flowers are found on the same tree 

The tips of the leaves are often indented and they all have serrated edges

Once pollinated the female catkins gradually become woody and then distribute seeds 

The twigs have a light brown spotted stem 

Alder bark is dark in color and fissured, it is also often covered in lichen 

An alder copse in the woods

Common name: alder, common alder, black alder, European alder

Scientific name: Alnus glutinosa

Family: Betulaceae

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.

A year in the life of an alder tree

 

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 that 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 colonize 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 color and camouflage the clothes of outlaws like Robin Hood and was thought to also color 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 underwater. 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 alders leave placed in the shoes before a long journey would cool the feet and prevent swelling.

Threats

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 color of healthy bark.

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