There are a number of alder species, but this one may be distinguished by the toothed leaves and a fine but distinctively rolled leaf margin. You need to look carefully to see this but the red alder is the only species to have it (see the photo below). The flowers are produced early in the year with long and attractive catkins appearing in January/February long before the leaves appear. The female flowers are tiny, cylindrical and reddish. The female flowers develop into green cones which eventually turn brown in Autumn and shed their seeds. The dry cones persist on the tree for a number of years.
Alders have a symbiotic relationship with a bacterium Frankia alni (previously Schinzia alni) which fixes atmospheric nitrogen. This supplies the roots with nitrogen compounds and helps the alders to thrive in soils poor in nitrogen - particularly waterlogged soils in swamps and river banks.