Sorbus is a genus of about 100–200 species of trees and shrubs in the rose family, Rosaceae. Species of Sorbus (s.l.) are commonly known as whitebeam, rowan, service tree, and mountain -ash. The exact number of species is disputed depending on the circumscription of the genus, and also due to the number of apomictic microspecies, which some treat as distinct species, but others group in a smaller number of variable species. Recent treatments treat Sorbus in a narrower sense to include only the pinnate leaved species of subgenus Sorbus, raising several of the other subgenera to generic rank.
Sorbus is unrelated to the true ash trees which belong to the genus Fraxinus, although the leaves are superficially similar.
As treated in its broad sense, the genus is divided into two main and three or four small subgenera (with more recent generic assignments in parentheses):
Sorbus subgenus Sorbus (genus Sorbus s.s.), commonly known as the rowan (primarily in the UK) or mountain -ash (in both North America and the UK), with compound leaves usually hairless or thinly hairy below; fruit carpels not fused; the type is Sorbus aucuparia (European rowan ). Distribution: cool-temperate Northern Hemisphere. (Genus Sorbus s.s.)
Sorbus subgenus Aria (genus Aria ), the whitebeam, with simple leaves usually strongly white -hairy below (hence the name, from German Weissbaum, ' white tree '); fruit carpels not fused; the type is Sorbus aria (common whitebeam). Distribution: temperate Europe & Asia.
Sorbus subgenus Micromeles (genus Aria ), an indistinct group of a few east Asian species (e.g.
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