Alder Action

Male catkins of tag alder in late winter

Alders are shrubs or small trees and their distribution includes most of the northern hemisphere.  They are classified in the genus Alnus and are related to birches, hazels and hornbeams. In the southeastern United States, the most common alder is Alnus serrulata, the tag or hazel alder.  

The tag alders grow in swamps, along streams and the edges of lakes.  Tag alders begin to flower in late winter before any leaves open.  Alders have an interesting reproductive system.  They make different male and female flowers, thus they are monoecious with both sexes on the same plant. This is to distinguish them from dioecious plants like willows that have separate male and female plants.  Alnus serrulata produces male flowers in a hanging array called catkins.  Each catkin has dozens of small, yellowish-brown male flowers that have stamens that release pollen.  The flowers of the catkins are dull because alder is wind pollinated and do not have to attract pollinators like insects or birds.  The female flowers are also borne on catkins but these are reddish brown in color and resemble small cones.  The leaves of tag alder emerge in early April and are bright green with serrations along the margin.  Small, winged fruits develop in the female cones and fly through the air to establish new alders in late summer. 

Female catkins (cones) of tag alder in early spring

Female cones from the previous year

Tag alder leaves in early spring

Alders play a role in preventing erosion and stabilize stream banks.  They also have another ecological role in enriching the soil where they grow.  Alders have a symbiotic relationship with a nitrogen-fixing bacterium in the genus Frankia. Like the better-known symbiosis between legumes and their nitrogen-fixing bacteria, alders host colonies of their bacterium in root nodules.  The plant provides sugars and other organic molecules to the bacterium and the bacterium in turn converts nitrogen gas from the atmosphere into ammonia that is usable by the alder.

Look for alders as you walk along the edge of a pond or riverbank.  They have a lot going on.