The common alder – Alnus glutinosa – is a small tree that grows up to about 25 metres. It is native in the UK and most of Europe, everywhere except the extremes of hot and cold.
The leaves are quite distinctive, a broad oval shape with a notch at the end and in the middle. The leaves have good length stalks.
Alders have distinctive reproductive cones that change colour from green in summer to brown. They survive through the winter, making the tree easy to identify long after the leaves have fallen. You might also spot the crimson male catkins.
Alders are colonising trees, which means they are opportunists, growing well from seed on bare ground.
For natural navigators, alders are extremely useful because they are a strong sign. They nearly always indicate wet ground. Alder trees do best in the cool damp soil found near the freshwater of rivers, streams, ponds and marshes.
A long line of alder trees is a very strong sign of a river or stream. A dense clump of them is more likely a pond or boggy area. A wide expanse of alders indicates marshland or damp woodland.
Waterlogged soil kills many trees , but the alder has an affinity with water and a couple of evolutionary advantages in wet environments:
Alder seeds float on water allowing them to find a fresh riverbank or other open wet spot to colonise.
Alder wood is resistant to rotting in water.
The trees do not like ground that is regularly disturbed or trodden, but the wet soil deters most disturbance anyway.
The wood is pale when first cut, but turns a vibrant orange-brown colour soon after coming into contact with air.
There is a bacteria on the roots of alder trees – Frankia alni – that fixes nitrogen from the air, allowing the trees to survive in soils that are too nutrient-poor for most trees. The nodules where this bacteria lives grow up to the size of an adult fist and can be seen where the roots are exposed.
The young twigs have a sticky feel, which is where the Latin name ‘glutinosa’ comes from, they are glutinous.
It is sometimes coppiced but even in its natural form it often has many stems, which can make it appear coppiced, even when it is not.
Historically, its wood had a reputation for making the best charcoal for use in making gunpowder.
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