The black alder or European alder (Alnus glutinosa) is a medium-to-large deciduous tree with a roughly conical or pyramidal crown. Its leaves are small (2-4 inches long) and egg-shaped, and stay green much later in the season than the leaves of most other trees.
Since alder trees are monoecious (meaning that they will have all male flowers or all female flowers), the best way to identify them is by their flowers, which are borne on structures called catkins.
Male catkins are about two inches long, green, and hang down from long stalks at the ends of twigs. Female catkins are inconspicuous, but mature in the fall into fruits called strobiles that resemble little pine cones. Since these stay on the trees for a long time, they can often be used to identify alders well after their season has passed.