Alder (Alnus glutinosa)

Common alder
Alnus glutinosa

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Common alder belongs to the genus Alnus and is a member of the birch family (Betulaceae).

The common alder (Alnus glutinosa) is a deciduous tree growing to around 30m and flowering February to March.

Lifespan: a short-lived species, around 60 years.

 

Characteristics

The bark of the common alder is dark and deeply fissured and is often covered in lichen. The tree is frequently multi–stemmed and produces suckers.

Young twigs have a sticky feel (giving the name glutinosa) but older twigs are smooth and hairless. Leaf buds are blunt and purplish in colour.

Alder tree

The leaves are dark green, stalked and round in shape with no leaf point. They have a double toothed wavy edge and measure 3–9cm.

Alder is monoecious; so both male and female flowers are found on the same tree. Male catkins appear before the leaves and are yellow and pendulous 2–6cm. Female flowers are green and oval-shaped, three to eight on each stalk.

Alders are wind pollinated; the wind carries pollen from male to female flowers. In autumn the female catkins become woody resembling small pine cones. The scales open to release seeds which are dispersed via wind and water.

Alder leaves
Alder flowers

Distribution

A native species in the UK and throughout much of Europe.

Alder is usually found in moist or wet habitats such as marsh and fen or near rivers, streams or lakes within floodplain zones. The species is able to withstand flooding and tolerates poor soils and brackish waters.

Wet woodland dominated by alder is traditionally referred to as carr. Alders help to stabilise river banks and prevent soil erosion.

 

Human value

Alder wood is relatively soft and porous and is only durable if kept under water. Once it is seasoned the wood is pinkish-brown.

Modern uses for alder timber include veneers, pulp and plywood. Traditionally, it has been used for making sluice gates, water pipes, bridge piles and small boats.

Alder can be coppiced and is suitable for making charcoal. In the past its charcoal was favoured for gunpowder. Alder was also a traditional timber for turnery work and was used to make clogs and broom handles.

Alder roots contain bacteria nodules which enable them to ‘fix’ atmospheric nitrogen returning it to the soil and improving ground conditions. Alder can therefore be used to improve the soil in undeveloped areas such as brownfield sites. Its ability to stabilise soils in wet conditions also makes it an ideal tree to assist in flood mitigation.

In folklore the alder was feared as the timber is white when freshly cut but rapidly turns blood-orange; making the tree appear to bleed.

Alder leaf
Alder cones

Wildlife value

Alder is a popular food plant for many insects and moth larvae including the alder kitten, pebble hooktip, the autumnal and the blue bordered carpet moth.

Catkins provide an early source of nectar for bees and the seeds are enjoyed by finches such as siskins, redpolls and goldfinches.

The wet conditions found in alder woodland support a variety of mosses, lichens and fungi, butterflies such as the small pearl-bordered fritillary and chequered skipper, as well as specialist species of cranefly.

Damp conditions usually ensure an abundance of dead wood making alder woodland an ideal foraging site for the lesser spotted woodpecker.

The roots of alder provide cover and nesting sites for otter.

Management

Alder is a light–demanding pioneer species able to rapidly colonise bare open ground. Growing on elevations of up to 500m it is a hardy species able to tolerate poor soils and even coarse sands and gravels if moisture is adequate. It grows rapidly; improving conditions for other species to establish.

Seeds ripen from September–November and are best collected in October. Alders grow easily from seed or from hardwood cuttings.

Some alders in the UK have succumbed to variant fungi from the Phytophthora group. Some of the variants are very damaging and may pose a serious threat to alder and the stability of riparian habitats.

Alder wood

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Photographs © Forestry Commission