Original: The largest species are Red Alder ( A. rubra ) on the west coast of North America and Black Alder ( A. glutinosa ) , native to most of Europe and widely introduced elsewhere , both reaching over 30 m. By contrast , the widespread Green Alder ( A. viridis ) is rarely more than a 5 m tall shrub .
Simplified: The best known species are the Black Alder , which can be seen throughout Europe . The largest is probably the Red Alder , native to North America .