Black alder

Alnus glutinosa (black, common or European alder) is a deciduous tree native to most of Europe. It’s part of the Betulaceae (birch) family which includes hazel and hornbeam but is the only alder species of 30 worldwide that’s native to the British Isles.

Typically growing 25-30m high, black alder can reach up to 37m. Foresters usually allow 60-70 years of growth before felling as this is when heart rot sets in. Left alone they can live to around 160.

Like its relatives, black alder germinates readily, grows rapidly, is highly water-tolerant and can thrive in poor quality soil, thanks in part to a symbiotic relationship with the nitrogen-fixing bacteria, Frankia alni (it’s also associated with 47 species of mycorrhizal fungi).

Alders therefore often colonise free land as pioneers but die out as species requiring less light and water prevail. Along the way their nitrogen-rich leaves improve soil quality for fussier competitors while their roots help prevent erosion.

All alders are monoecious, i.e. both female (oval, cone-like) and male (pendulous catkins) flowers share the same tree. Black alder females can be seen all year, males from February to April.

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Alnus glutinosa cover photo by trees.hillier.co.uk

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Black alder’s dark green, non-pointed leaves grow from purple-grey buds and when young are covered in sticky hairs (hence ‘glutinosa’).

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Black alders are the only cone-growing deciduous trees native to the British Isles. Pollinated by wind, they disperse their seeds by wind and water.

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When nitrogen-packed alder leaves fall to the forest floor they enrich the soil as they decompose, improving conditions for species that require better quality soil.

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Alders are actinorhizal, i.e. flowering plants that work symbiotically with the Frankia bacterium (F. alni in this case), leading to the creation of root nodules.

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Alder is resistant to rotting in water and hardens when immersed, making it ideal for building boats and canal lock gates. Venice and Amsterdam are both built on alder wood piles.

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Black alder can be found at the Alder Collection in St. Brelade with other fellow species including bearded (A. barbata), red (A. rubra) and Nepalese (A. nepalensis).

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