Throughout the year, Alnus glutinosa is one of Britain's most distinctive small trees. In the summer, it has discolorous, rounded, dark-green leaves with wavy, serrated margins. On the lower surface the leaves have rusty-brown hairs in the vein axils and tiny, shield-like glands that make the young leaves sticky - giving the species its epithet. Alder has separate clusters of male and female flowers. These are particularly obvious on the leafless tree during the winter, when maturing male catkins and mature cone-like fruits are found on the branch tips. Even the winter buds are distinctive; they are stalked.
Alder is found across the whole of Europe and extends as far as Central Asia and North Africa. The distribution of genetic variation across alder's geographic range shows that during the last glaciation it retreated to a few refugia around the Mediterranean. During the Ice Age, trees in these refugia started to diverge from each other and become genetically distinct. Once the ice sheet across Europe started to melt ten millennia ago, alder began to recolonise the north. The signature of these colonization events remains in the DNA of today's alder populations.
Ecologically, alder is associated with permanently or periodically flooded soil, e.g., the margins of rivers, marshes and lakes. Indeed, the word 'carr' is used to denote woodland on waterlogged soils, which is frequently dominated by alder. One of the reasons alder is so successful under conditions where few other trees can survive is because of its symbiotic association with the nitrogen-fixing actinomycete bacterium Frankia alni. Nodules on alder roots are inhabited by Frankia, which absorb atmospheric nitrogen and convert it into forms of nitrogen the tree can use. Not only does alder have a source of ready nitrogen, the fertility of the soil surrounding the tree is improved making alder an important pioneer species in ecological successions. Alder seedlings need high amounts of light to germinate and are shaded out by other tree species; hence it is a species of woodland margins.
Alder is a multipurpose tree that provides indirect ecological services through habitat stabilisation, and direct commercial value. Alder timber is flexible and easily worked, and has traditionally been used where buildings are likely to have waterlogged foundations. The bark was an important source of tannic acid for tanning and a yellow dye. Alder is readily coppiced and produces a high-quality charcoal that has traditionally been preferred for gunpowder manufacture.
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Polme S et al. 2014. Global biogeography of Alnus-associated Frankia actinobacteria. New Phytologist 204: 979-988.