Belfast Botanic Gardens Tree Archive

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This is one of a series of short articles about trees in the Belfast Botanic Gardens compiled by the Friends group. The tree tag number relates to the small aluminium discs, usually fixed on the tree trunk, 2 to 3m above ground level.
Alnus rubra
Red Alder

Tag -
54deg 34.943N
5deg 55.823W
The red alder is a native of a coastal strip from South Alaska to California; it is fairly rare in the UK. It is fast growing and will reach 25m but is said to be short lived.

There are a number of alder species, but this one may be distinguished by the toothed leaves and a fine but distinctively rolled leaf margin. You need to look carefully to see this but the red alder is the only species to have it (see the photo below). The flowers are produced early in the year with long and attractive catkins appearing in January/February long before the leaves appear. The female flowers are tiny, cylindrical and reddish. The female flowers develop into green cones which eventually turn brown in Autumn and shed their seeds. The dry cones persist on the tree for a number of years.

Alders have a symbiotic relationship with a bacterium Frankia alni (previously Schinzia alni) which fixes atmospheric nitrogen. This supplies the roots with nitrogen compounds and helps the alders to thrive in soils poor in nitrogen - particularly waterlogged soils in swamps and river banks.

Alnus rubra in Belfast Botanic Gardens Alnus rubra leaves
Alnus rubra in Belfast Botanic Gardens Leaves of Alnus rubra
rolled leaf margin of Alnus rubra persistent dead cones of red alder
Characteristic rolled leaf margin of Alnus rubra Persistent old cones of Alnus rubra
catkins of Alnus rubra root nodules on Alnus glutinosa
Catkins of Alnus rubra in January Root nodules on Alnus glutinosa
Photos taken in Belfast Botanic Gardens in 2009 and 2010. Copyright: Friends of Belfast Botanic Gardens.

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