Sorbus
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The genus Sorbus is a genus of about 100-200 species of trees and shrubs in the subfamily Maloideae of the Rose family Rosaceae. The exact number of species is disputed considerably between different authorities, due to the number of apomictic microspecies, treated by some as distinct species, by others grouped in a much smaller number of variable species. Sorbus species are commonly known as Whitebeams, Rowans, and Service Trees, and in North America, as Mountain Ash.
The genus is divided into two main and three or four small subgenera:
- Sorbus subgenus Sorbus, the rowans, with compound leaves usually hairless or thinly hairy below; fruit carpels not fused; type species Sorbus aucuparia (European Rowan). Distribution: cool-temperate Northern Hemisphere.
- Sorbus subgenus Aria, the whitebeams, with simple leaves usually strongly white-hairy below (hence the name, from German Weissbaum, 'white tree'); fruit carpels not fused; type species Sorbus aria (Common Whitebeam). Distribution: temperate Europe & Asia.
- *Sorbus subgenus Micromeles, an indistinct group of a few east Asian species (e.g. Sorbus alnifolia, Korean Whitebeam) with narrow leaves; doubtfully distinct from and often included in subgenus Aria. Distribution: temperate northeast Asia.
- Sorbus subgenus Cormus, with compound leaves similar to subgenus Sorbus, but with distinct fused carpels in the fruit; just one species, Sorbus domestica (Service Tree). Distribution: warm-temperate Europe.
- Sorbus subgenus Torminaria, with rather maple-like lobed leaves with pointed lobes; fruit carpels not fused; just one species, Sorbus torminalis (Wild Service Tree). Distribution: temperate Europe.
- Sorbus subgenus Chamaemespilus, a single shrubby species Sorbus chamaemespilus (False Medlar) 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 (self-fertile without pollination), so able to reproduce clonally from seed without any variation. This has led to a very large number of microspecies, particularly in western Europe (including Britain) and parts of China.
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