Worldwide there are approximately 30 species of alder (Alnus).1 There are seven indigenous alders as well as the introduced, well established black alder (A. glutinosa) in North America. Alders (especially speckled alder, A. incana rugosa) are abundant from the Pacific Northwest up into Alaska, across the Northern forest of Canada to the Maritime Provinces and south into the northern states. Mountain alder (A. incana tenuifolia) is prevalent in the west, extending southerly along the Rocky Mountain range at altitudes of 4,500 to 10,000 feet. It is found along stream banks, floodplains, and moist meadows, although not liking stagnant water. The bark is rich in tannin and has been used as a red dye. Deer, rabbits, and beavers eat the bark. Alder leaves, alfalfa, fennel, and endive can be mixed for a spring tonic.2 A. glutinosa and A. incana are in central and northern Europe a relevant cause of early spring hay fever and are considered serious inducers of pollinosis in the Pacific Northwest and California.
Alders are shrubs or small trees, although mountain alder may grow up to 10 m high. They are frequently multitrunked, with several crooked stems arising from a root collar. Clone formation with sprouting from the root collar frequently forms dense thickets along creeks. Leaves of mountain alder are oblong and doubly serrated, with main teeth having smaller serrations. Male and female flowers are separate, but occur on the same tree. Pollen anthers are found on long dangling catkins at the tip of the shoots. Pollen production per catkin can be up to 4.5 x 106 for A. glutinosa. The female catkin is more proximal on the shoots, is shorter and upright, and develops into a seed-bearing body like a small pinecone. These will persist on the branches into the next year. In central Europe, alders are early pollinators, beginning in February or even late January, and extending through April into early May. In the United States, alders will pollinate from March through May, occasionally earlier. In California, white alder, A. rhombifolia, will pollinate December and January. There are summer and fall pollinating species also. In the northwest, alder will account for >50% of the pollen captured in March.
The birch family, Betulaceae, includes birch (Betula), alder, hazel (Corylus), hornbeam (Carpinus), and hophornbeam (Ostryis). The closely related beech family (Fagaceae) includes beech (Fagus), oak (Quercus), and chestnut (Castanea).
Adapted from Richard W. Weber, MD, Annals of Allergy, April 2002
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