|European Rowan (Sorbus aucuparia) with fruit|
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 Cormus (genus Sorbus), with compound leaves similar to subgenus Sorbus, but with distinct fused carpels in the fruit; just one species, Sorbus domestica (True Service Tree). Distribution: North Africa, warm-temperate Europe, West Asia.
- Sorbus subgenus Torminaria (genus Torminalis), with rather maple-like lobed leaves with pointed lobes; fruit carpels not fused; just one species, Sorbus torminalis (Wild Service Tree). Distribution: temperate Europe, south to the mountains of North Africa and east to the Caucasus ranges.
- Sorbus subgenus Chamaemespilus (genus 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.
- D. Potter, T. Eriksson, R. C. Evans, S. Oh, J. E. E. Smedmark, D. R. Morgan, M. Kerr, K. R. Robertson, M. Arsenault, T. A. Dickinson & C. S. Campbell (2007). "Phylogeny and classification of Rosaceae" (PDF). Plant Systematics and Evolution. 266 (1–2): 5–43. doi:10.1007/s00606-007-0539-9. Note that this publication pre-dates the 2011 International Botanical Congress which mandates that the combined subfamily referred to in the paper as Spiraeoideae must be called Amygdaloideae.
- Robertson, K. R., J. B. Phipps, J. R. Rohrer, and P. G. Smith. 1991. A Synopsis of Genera in Maloideae (Rosaceae). Systematic Botany 16: 376–394.
- McAllister, H. 2005. The Genus Sorbus: Mountain Ash and Other Rowans. Richmond, Surrey, UK: Royal Botanic Gardens, Kew.
- Potter, D., T. Eriksson, R. C. Evans, S.-H. Oh, J. E. E. Smedmark, D.R. Morgan, M. S. Kerr, and C. S. Campbell. (2007). Phylogeny and classification of Rosaceae. Plant Systematics and Evolution. 266(1–2): 5–43.
- Campbell C. S., R. C. Evans, D. R. Morgan, T. A. Dickinson, and M. P. Arsenault. 2007. Phylogeny of subtribe Pyrinae (formerly the Maloideae, Rosaceae): Limited resolution of a complex evolutionary history. Pl. Syst. Evol. 266: 119–145.
- Price, D.T. 2007. One-way introgressive hybridisation between Sorbus aria and S. torminalis (Rosaceae) in southern Britain. Watsonia. 26: 419–431.
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