The Alder
The Wood Guide · The forests
Tree portraitThe alder
An often underrated tree that thrives on wet ground where others give up — and leaves the soil better than it found it.
- Alnus glutinosa
- 60 to 120 years
- Fixes nitrogen
The alder in brief
A pioneer species: one of the first trees to settle on new or disturbed ground, where it prepares the soil for what comes after it.
Where most trees drown on waterlogged ground — their roots get no oxygen — the alder has tiny air channels in its roots that solve the problem. So it can stand where nothing else can: along streams, in marshes, on the bank.
And it does more than survive. Its roots host bacteria that fix nitrogen from the air and make it available in the soil. An alder wood therefore makes the ground richer, and that is exactly why other species can follow the alder.
Above ground alder is not very durable, but permanently under water it lasts for centuries — wood without oxygen is not attacked. Venice stands on timber piles that have been there for centuries, alder and oak among them, and the same principle lies beneath countless old Dutch and Flemish buildings.
- Botanical
- Alnus glutinosa (common alder, black alder)
- Lifespan
- 60 to 120 years
- Height
- 20 to 30 metres
- Leaves
- round to oval, often with a notched tip
- Distinctive
- nitrogen-fixing root nodules
- Site
- wet ground, banks, marsh
To be fair: above ground, alder is not durable. It is soft and light and lasts a few years at most outdoors. The exception — permanently under water — is exactly that: an exception that does not apply in an ordinary garden.
In pictures · Adaptations to the wet
Schematic drawing.
Adaptations to the wet
Air channels in the roots
Special tissues carry oxygen from above down to the roots, so they can breathe in oxygen-poor, waterlogged soil.
Nitrogen-fixing nodules
Bacteria in the roots convert nitrogen from the air into a form plants can take up — the alder fertilises itself and its neighbours.
Pioneer species
One of the first trees on new or disturbed ground. It prepares the soil for the species that come after it.
Bank stabiliser
The dense root system holds stream and riverbanks together and prevents erosion — one of the reasons alders are deliberately planted along watercourses.
How to recognise an alder
| Characteristic | Description |
|---|---|
| Leaf | Round to oval, often with a notched or blunt tip — as if someone had taken a bite out of it |
| Fruit | Small woody cones that stay on the tree all winter |
| Catkins | Long male catkins in late winter, before the leaves |
| Bark | Dark grey to almost black, deeply fissured on older trees |
| Site | Almost always wet: stream bank, marsh, waterlogged woodland |
| Wood, freshly sawn | Turns from white to orange-red within minutes |
Saw through an alder and the pale wood turns orange-red within a few minutes, as substances in the sap oxidise. In the past this was seen as "bleeding", which gave the alder an ominous reputation in folklore.
What the alder gives
Soft and light
Easy to work, even in structure and with little contrast between the rings — suitable for carving and turning.
Indestructible under water
As a foundation pile in oxygen-free ground, alder lasts for centuries. Above ground it does not.
Clogs and small everyday objects
Traditional clog wood in parts of the Low Countries: light, soft and slow to split.
Veneer and sheet materials
Alder veneer turns up in furniture making and in the core of some types of plywood.
Alder is not durable above ground. For garden furniture, look at acacia, teak or pressure-treated spruce.
Frequently asked questions about the alder
Why does the alder grow in water?
Because of tiny air channels in its roots that carry oxygen from above down to them. That lets the roots breathe in waterlogged, oxygen-poor soil where other trees suffocate.
Is it true that alder does not rot under water?
Yes. Wood-rotting fungi cannot live without oxygen. That is why alder and oak piles have carried old buildings in Venice and the Low Countries for centuries.
Why does alder turn orange after sawing?
Because substances in the sap oxidise. Within a few minutes the pale wood takes on an orange-red tint — once seen as "bleeding".
What does nitrogen fixation mean?
Bacteria in the alder's root nodules convert nitrogen from the air into a form plants can take up. So an alder wood enriches the soil instead of exhausting it.
The alder in perspective
The tree that leaves the ground better than it found it.
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Photo via Wikimedia Commons: Alnus glutinosa — Willow, CC BY 2.5