From deepest Hertfordshire along a chalk stream called the Mimram comes a plea for appreciation of the alder tree (Alnus glutinosa) from the conservationist Rev Tom Gladwin. The felling of these trees along the riverbank has destroyed the habitat of many creatures. The alder, a lover of damp places, particularly bogs, river and lake banks, was one of the first trees to colonise Britain after the last ice age. In Hertfordshire its soft wood makes it a favourite for hole-nesting birds, such as woodpeckers, nuthatches and treecreepers.
The tree has an important relationship with a nitrogen-fixing bacterium, Frankia alni, that allows it to grow in nutrient-poor soils. In return for the nitrogen stored in nodules for the use of the tree, the bacteria get sugars from their partner. Alder wood is soft and porous so is not much good as a hardwood but it does not rot under water and is used for sluices and underwater piles. Historically its charcoal was used for gunpowder and a green dye favoured by Robin Hood. Alder roots dangling into the river provide cover for small fish and caddisflies, while otters often use the root system as an ideal nesting site. Groups of alders planted in damp upstream areas are a natural way of holding back flood water and increasingly important.
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