Scientific Name
Alnus glutinosa ‘Black Alder’
Black alder is a medium-sized, short-lived tree native to most of Europe and parts of southwest Asia, extending far into Russia. It easily tolerates temperatures below minus 30 degrees Fahrenheit. The species has a symbiotic relationship with the nitrogen-fixing bacterium Frankia alni, which enables it to thrive in very infertile environments. It has a strong affinity for wet soil but can also grow in dry sites. Black alder is a pioneer species that readily colonizes disturbed areas, roadsides and similar environments. It is considered a highly invasive exotic species in many states, especially in the northeastern United States.
Black alder has rounded leaves with wavy, doubly serrated margins, and the tips of the leaf blades are blunt and sometimes notched. The buds and new leaves are sticky, which explains the species name glutinosa. Like many members of the birch family, black alder produces prominent male catkins up to 4 inches long that remain visible through winter and flower before the leaves emerge. The much smaller female catkins later develop into semi-woody brown cones about ½ inch long that contain the seeds. These fruits begin green, somewhat resembling mulberries, before turning brown as they ripen.
This tree can mature to an average height of 50 feet. The Regis specimen grows among a grove of even more invasive trees, European buckthorns, which likely spread from female buckthorn trees elsewhere on campus. The buckthorns appear to be the stronger competitors and are certainly more numerous. It is believed the alder was intentionally planted as part of the arboretum collection, though under current conditions it is unlikely to grow much larger than it is today.