A genus of about 25-30 species, trees (rarely shrubs), of temperate and boreal regions of the Northern Hemisphere (most diverse in c. and n. Asia). Subgeneric classification shown in the key follows Whittemore et al. (2021).
Ref: Kurz & Godfrey (1962); Sherman-Broyles (1997) In Flora of North America Editorial Committee (1997); Todzia In Kubitzki, Rohwer, & Bittrich (1993); Whittemore & Olsen (2011); Whittemore et al. (2021); Wiegrefe, Sytsma, & Guries (1994). Show full citations.