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 damp, acidic, low on nutrients, sandy, and loose. It often grows in stony soil or clay soil, but also sandy soil or wet peat. The plant grows best on fresh, loose, and fertile soil, prefers average humidity, and does not tolerate saline soil or waterlogging. It can be found in light woodland of all kinds and as a pioneer species over fallen dead trees or in clearcuttings, and at the edge of forests or at the sides of roads. Seeds of "S. aucuparia" germinate easily, so the plant may appear on inaccessible rock, ruins, branch forks, or on hollow trees.