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A shrub or small tree to about 25 ft. Leaves 11⁄2 to 4 in. long, oval or ovate, rounded or pointed at the apex, broadly obtuse or rounded at the base, sharply and unevenly double-toothed, glabrous above, downy or glabrous and pale green beneath. Male catkins up to 4 in. long, appearing before the leaves. Female catkins upright, 2⁄5 to 3⁄5 in. long.
Native of N. America from Newfoundland to British Columbia, also ranging southward on the eastern side of the continent to W. Virginia, Ohio, and Minnesota. It is a hardy shrub of no particular merit, but useful in cold, wet situations. Its close relationship to A. incana, the grey alder of the Old World, is most evident in the following variety:
As remarked, this species is closely allied to A. incana. Some botanists include it in A. incana without differentiation, since there seems to be no constant character by which it can be distinguished, except that it is more shrubby. It has also been placed under A. incana as subsp. rugosa (Du Roi) R. T. Clausen, and this is the status accepted by Furlow. The var. americana is not worth recognising, since the leaves of the American race are usually glaucous beneath. The subspecies rugosa is confined to eastern North America, and British Columbia should be deleted from the distribution (see A. tenuifolia).
A. serrulata – This is recognised by Furlow as a distinct species. In addition to the differences mentioned, the buds are more globose and the bark paler, with less prominent lenticels.
Betula serrulata Ait
A. incana americana Reg.
A. incana var. glauca Loud., not Ait