The Alder tree is one of our most common and widespread native trees.
Along with willows it can be found along all our river banks and it is frequent in damp woodlands where it can form pure stands in groves and copses. It has the ability to grow in very wet places where few other trees can grow and is often a pioneer tree in disturbed ground. One of the reasons for its success is that it has teamed up with a nitrogen fixing bacterium, which forms light brown nodules as large a human fist on the Alder roots.
The bacterium absorbs nitrogen from the atmosphere and makes it available to the tree, much in the same way as many plants in the pea family. This relationship allows the Alder to thrive in wet soils which may be deficient in nitrogen.
Alders are related to Birch trees and there are about 30 species. The Alder flowers are in early spring before the leaves emerge, they have monoecious, unisexual flowers, that is, they have separate male and female flowers on the same tree. The male flower is a catkin which sheds pollen into the atmosphere, where by chance it comes into contact with the small female flowers, which turn into little woody cones after pollination. The cones release their seeds into the air for wind dispersal during the following winter months. In the winter it is always worth looking up into the crown of Alder trees for flocks of feeding birds, as those seeds are a favourite food source for both Siskin and Redpoll.
The leaves of Alder are dull green with a serrated margin. In the autumn they turn a dark brown or black and drop off. The leaves are a favourite food source for the caterpillars of many moths which in turn provide food for lots of small insect eating birds and their young.
Alder branches often develop holes which are ideal for nesting birds such as tits and Redstarts and the rough flaky bark on the trunk makes crevices which are an ideal nest site for Treecreepers. The rough bark and the damp places where the tree grows makes the trunks and branches a good place for mosses to grow and thrive, they in turn provides shelter for many small insects which are then food for small birds. The Alder tree is a good larder.
Alder trees and their dead branches are hosts for a range of fungi. The Alder Bracket forms tiers of small “brackets” on the trunks of both living and dead standing trunks and the Cinnamon Porecrust forms really attractive suede like mats on fallen dead branches.
One strange fungi is Alder Tongue, this lives on the seeds inside the fruiting cone and in mid-summer a small pinkish tongue grows out of the cone. This was first found in Cornwall in the 1940s and has since spread and is now common on our trees. It is a good idea not to be to tidy minded in woodland management and leave fallen limbs for the harmless wood rotting fungi and insect larvae to break down naturally.