Alder (Alnus glutinosa)

20/10/14

  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
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