Common Alder Alnus glutinosa is a tree that likes to have its roots in water. There can be few rivers in the land which do not have have stands of it somewhere along their length. In Dulwich, alder grows on both sides of the water in Belair Park and its presence there in such numbers, behaving exactly as it does in the wild, strongly supports the view that the present stretch of water has its origins in a natural stream, whether the Effra itself or a tributary to it. William Gilpin, the 18th century proponent of the picturesque, considered alder “the most picturesque of the aquatic trees, except the weeping willow. He who would see the alder in perfection must follow the banks of the Mole, in Surrey, through the sweet vale of Dorking and into the groves of Esher”. The Mole, indeed, is far from being a beautiful river; but what beauty it has it owes greatly to the alder, which fringes its meadows and in many places forms very pleasing scenes”. The ‘sweet vale’ of Dorking and the ‘groves’ of Esher have changed beyond recognition since Gilpin’s day but Belair is still a good place to enjoy ‘pleasing scenes’ and to get to know the alder at first hand.

Alder is a member of the birch family and, like the other members of that family, it is wind-pollinated. The male catkins are usually the first to shed pollen in spring, when they turn a striking red-purple colour. They are carried on the tips of the twigs and drop after they have released their pollen. The female catkins are at first hard, green, and somewhat fleshy; after their flowers are pollinated, the female catkins become dry, brown and woody. At maturity, these woody cone-like structures (technically known as strobili) open to release tiny nuts, just as the cones of coniferous trees do. These ‘cones’ can be seen at any time of the year but are particularly striking in winter, when they attract birds such as siskin to Belair Park. Alder’s distinctively stalked leaf buds have a purple waxy bloom which can be very beautiful in the low sunlight of a winter’s day.

Alder has dark green leaves which are covered with a sticky resin in early summer, as are its young shoots. Hence the glutinosa of the scientific name. They are roundish, serrated, blunt (never pointed) and often indented at the end. Alder is particularly noted for its symbiotic relationship with a nitrogen-fixing bacterium called Frankia alni, which forms nodules on the roots of alder trees. The bacterium absorbs nitrogen from the air which it makes directly available to the tree, the bacterium receiving sugars from the tree in return. Alder wood is easily worked and was much used for the making of clogs. It is very durable if kept submerged or buried in wet earth, and so was extensively used for foundations for bridges, water-pipes etc. As a widespread native tree, alder frequently appears in English place names. The Welsh word for alders is gwern and the Scots Gaelic is fearn. Forms of both words are similarly common in place names, particularly farm names, in those countries.

Common alder has various cultivars, but I have not yet come across any in Dulwich. Two introduced species of alder, Grey alder (A. incana) and Italian alder (A. cordata), have been frequently planted as street trees in central London in recent years, as both are more tolerant of dry conditions and urban pollution than the native tree. The only mature example of either I know in Dulwich is an Italian alder in Turney Road (on the southern pavement towards the railway bridge), though young specimens of both species have recently been planted in Dulwich Park.