The common alder (Alnus glutinosa) grows in wetlands. It favours marshy depressions in spruce-dominated woodlands as well as the rocky shores of open sea islands, where it grows individually or forms narrow zones or rows.

Extensive, alder-dominated forests only form in places where the boundary between water and dry land is undefined and wide. In Helsinki, these types of alder forests can be found in the coastal swamps of Vanhankaupunginlahti. Just a hundred years ago, most of these swamps were open meadows used as pastures.

Common alders grow into large, beautiful trees, the tops of which curve into round shapes. In coastal swamps, they produce plenty of soft, decaying wood, and in Vanhankaupunginlahti they are often used as nesting trees by lesser spotted woodpeckers.