Sorbus is a genus of about 100–200 species of trees and shrubs in the rose family, Rosaceae. Species of Sorbus 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 subgenusSorbus, raising several of the other subgenera to generic rank. Sorbus is not closely related 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 :
Sorbus subgenus Sorbus, commonly known as the rowan or mountain-ash, with compound leaves usually hairless or thinly hairy below; fruit carpels not fused; the type is Sorbus aucuparia. Distribution: cool-temperate Northern Hemisphere.
Sorbus subgenus Aria, the whitebeam, with simple leaves usually strongly white-hairy below ; fruit carpels not fused; the type is Sorbus aria. Distribution: temperate Europe & Asia.
Sorbus subgenus Cormus, with compound leaves similar to subgenus Sorbus, but with distinct fused carpels in the fruit; just one species, Sorbus domestica. Distribution: North Africa, warm-temperate Europe, West Asia.
Sorbus subgenus Torminaria, with rather maple-like lobed leaves with pointedlobes; fruit carpels not fused; just one species, Sorbus torminalis. Distribution: temperate Europe, south to the mountains of North Africa and east to the Caucasus ranges.
Sorbus subgenus Chamaemespilus, a single shrubby species Sorbus chamaemespilus with simple, glabrous leaves and pink flowers with erect sepals and petals. Distribution: mountains of southern Europe.
Hybrids are common in the genus, including many between the subgenera; very often these hybrids are apomictic, so able to reproduce clonally from seed without any variation. This has led to a very large number of microspecies, particularly in western Europe and parts of China.
Sorbus species are used as food plants by the larvae of some moth species—see list ofLepidoptera that feed on Sorbus. Sorbus domestica is used to flavour some apple wines, see Apfelwein. Sorbus species are cultivated as ornamental trees for parks and gardens, and have given rise to several cultivars. The following, of mixed or uncertain parentage, have gained the Royal Horticultural Society’s Award of Garden Merit:
