Common Alder (Alnus glutinosa) - male catkins - November 2017

A very young Common alder in the Hill Barton industrial estate of Exeter, UK.

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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

It is conical in shape and 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.

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.

The 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.

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.

The small brown cones, which are the female catkins, stay on the tree all year round.

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.

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.

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.

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Scientific classification
Kingdom: Plantae
(unranked): Angiosperms
(unranked): Eudicots
(unranked): Rosids
Order: Fagales
Family: Betulaceae
Genus: Alnus
Subgenus: Alnus
Species: A. glutinosa