The alder, Alnus glutinosa, is a water-loving tree of riverbanks, marshes and wet ground, and it carries a striking natural feature that shaped its lore: when the pale wood is cut it slowly turns a deep orange-red, as though the tree were bleeding, which gave it an uncanny and sometimes ominous reputation. Set against that darkness is a genuine practical virtue, alder wood, worthless in the dry, is almost imperishable underwater, so that it was the timber of choice for piles, sluices and foundations sunk in water, and the city of Venice is said to stand on alder.
Alder has a genuine and ecologically important botanical trait behind its pioneering reputation: its roots host colonies of Frankia bacteria in small nodules, fixing nitrogen from the air and enriching poor, waterlogged or newly exposed soil, much as legumes do, which is why alder is often among the first trees to colonise riverbanks, spoil heaps and other disturbed ground and why foresters still plant it deliberately today to improve soil ahead of other species. This real capacity to improve barren ground sits alongside, and perhaps quietly explains, the tree's old reputation as a plant of transformation and of the boundary between waste and fertile land.