Alder is a native tree in the same family as Birch and Hazel. Although it will grow in dry conditions, it prefers damp places, where given enough room it can grow up to 25 metres high with a broad, roughly conical crown. Planted along rivers, its roots help to keep the banks stable, and it has the unusual ability to improve the soil by producing nitrogen. The leaves are a distinctive shape, broader at the tip than at the stalk, with a flattened end.
Like all its family, it produces wind-pollinated male and female flowers on the same tree. The pollen-producing male catkins are attractive but less conspicuously decorative than those of Hazel (see March Spotlight). The much smaller female catkins tend to form in little groups just behind the prominent males.
The easiest way to identify Alder is to look for the ripe fruits, which look like little dark cones hanging on the tree for months after the last seeds have gone. At this time of year, a mature tree is likely to show evidence of three years’ reproduction. Last year’s empty fruit catkins are still hanging there. The leaves are among the last to fall, and the new fruits are ripening (rather slowly and late this year!).
Meanwhile next year’s catkins are already fully formed, ready to flower between February and March, well before the new leaves open.
Alder apparently has fewer therapeutic properties than many native species, but its bark and leaves are used by herbalists as astringents and in the treatment of rheumatism. The bark has been used to produce red dyes, and the wood is particularly useful for making items that need to withstand wet conditions, from spoons and clogs to canal lock gates. Like most native species, it plays a part in Celtic legends, particularly those from Ireland, but the stories are hardly known in England, and no superstitions about it have been strong enough to last to the present day. However our Park Rangers have noticed a feature that did once cause a certain amount of anxiety: immediately after pruning, Alder wood is quite pale-coloured, but the wound rapidly turns a striking reddish-orange.
At St Nicks, Alders are easiest to see on the North side of Osbaldwick Beck, on the South of Tang Hall Beck and along the Bund path. In winter, Birdwatchers like these areas: Siskins (January Spotlight) love the seeds, and can often be seen in small flocks hanging from the clusters of fruit or picking fallen seeds from the paths below. Goldfinches will also feed on them, and it’s always worth watching out for little groups of Lesser Redpolls which occasionally tag along with the feeding flocks and might just include a rare Mealy Redpoll.
Closely related Grey Alder, Alnus incana, is also found on the reserve. This is an introduced species, distinguishable by its smoother bark and pointed leaves.
Background sources: Deni Brown The RHS Encyclopedia of Herbs, London, Dorling-Kindersley
Keith Rushforth: The Mitchell Beazley pocket guide to trees, London, Mitchell Beazley