Alders belong to the scientific genus Alnus which live throughout the cooler regions of the Northern Hemisphere, southwards over the high mountains of central America and down to the Andes. Depending on whose book you read, there are some 17 species and five of those are really shrubs or dwarf trees. Alders seldom grow very tall. Alders possess special nodules on their roots which contain nitrogen fixing bacteria which improve the soil.
Most alders are pioneer species.
They invade and thrive in gaps and clearings in forests, often on the poorest soils. Many tolerate a high water-table and periodic flooding. Five species of alder are commonly found in the UK - one native and four imported. Most alders coppice well. Introduced to Britain in 1780, it resembles the native Black Alder in its lifestyle and site requirements.