Arran whitebeams complex fashion, which involved the common whitebeam ("Sorbus aria") giving rise to the tetraploid rock whitebeam ("Sorbus rupicola") which is still found on Holy Isle. This species is able to survive at higher altitudes and therefore occupies a less competitive niche with fewer tree species able to tolerate the harsher conditions. The rock whitebeam interbred with the rowan / mountain ash ("Sorbus aucuparia") to produce the hybrid, a fertile separate species the Scottish whitebeam ("Sorbus arranesis") which grows well in this zone of reduced competitive growth at higher altitudes. The bastard mountain ash ("Sorbus pseudofennica") arose from a further cross
Sorbus aucuparia at elevations of up to 2000 m. "S. aucuparia" appears north of the boreal forest at the arctic tree line; in Norway, it is found up to the 71st parallel north. It has naturalized in America from Washington to Alaska and eastward in Canada and the northeast of the US very successfully. "Sorbus aucuparia" is an undemanding species and can withstand shade. It is frost hardy and can tolerate winter dryness and a brief growing season. The plant is also resistant to air pollution, wind, and snow pressure. "S. aucuparia" mostly grows on soil that is moderately dry to moderately