Once considered a single species across its range in North America and eastern Asia, Adiantum pedatum is considered to be a complex of at least four vicariant species ( Adiantum pedatum and Adiantum aleuticum occur in North America) whilst two other very closely related species can be distinguished - one in China and at least one in Japan. Adiantum pedatum in the strict sense is restricted to deciduous woodlands in eastern North America[]. See also Jin-Mei Lu et al; 'Biogeographic disjunction between eastern Asia and North America in the Adiantum pedatum complex (Pteridaceae)' American Journal of Botany Vol 98 No 10 pp 1680 - 1693.
We are treating Adiantum aleuticum as distinct here but, lacking validly published names at present(2016) for the Chinese and Japanese forms, have retained them under Adiantum pedatum[].

Common Name: Northern Maidenhair