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 decora The fruits (pomes) are bright red to orange-red and across. Showy mountain-ash is very similar to the closely related American mountain-ash ("Sorbus americana"). Like the American mountain-ash, the showy mountain-ash has odd-pinnately compound leaves and often large clusters of flowers and fruits. Showy mountain-ash can be distinguished by its shiny, sticky buds, and its slightly larger flowers and fruit. It is said to bloom a week earlier. It is often cultivated as an ornamental plant for its cold-hardiness, its attractive flowers, and its large clusters of small red berry-like pomes. The fruits are an important source of food for wildlife,