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 that 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 Alnus 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. In fact, all Alnus species are classed as actinorhizal, meaning flowering plants that work symbiotically with the Frankia bacterium (F. alni in this case), leading to the creation of root nodules. Alders are associated with 47 species of mycorrhizal fungi in total.

Given their propensity to rapidly germinate and grow, alders 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, which is when budburst takes place – dark green, rounded leaves grow from purple-grey buds, shedding the sticky hairs that cover them when young (hence ‘glutinosa’).

Black alder is the only cone-growing deciduous species native to the British Isles. Pollinated by wind, these cones disperse their seeds by wind and water.

Usage-wise, alders are all about their wood and its resistance to rotting in water which, counter-intuitively, hardens it when immersed. This makes the wood ideal for building boats and canal lock gates. Venice and Amsterdam are prime examples, built on alder wood piles.

If you’d like to see a black alder, head to the Alder Collection in St. Brelade, where you’ll find all sorts of alder species including bearded (A. barbata), red (A. rubra) and Nepalese (A. nepalensis).

Images: bud © Ricarda Bergen (iNaturalist); leaves © A Emmerson (iNaturalist); male flowers (catkins) and female fruit by Nennieinzweidrei (Pixabay)

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