Alder trees lined up along a small brook - Riehen, Switzerland

Alder is the most common tree in riparian forests, and it improves soil fertility through the ability to fix nitrogen from the air due to a symbiotic friendship of alder with a nitrogen-fixing bacterium. This bacterium is found in root nodules, which may be as large as a human fist, with many lobes and light brown in appearance. The bacterium absorbs nitrogen from the air and makes it available for the tree. Alder, in turn, provides the bacterium with sugars. As a result of this mutually-beneficial friendship, alder improves the fertility of the soils where it grows.