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 particularly birds in the winter and early spring. Sorbus decora Sorbus decora, commonly known as the northern mountain ash, showy mountain-ash, or dogberry, is a deciduous shrub or very small tree native to northeastern North America. It occurs throughout the Laurentian Mixed Forest Province, the New England-Acadian forest region, and the eastern Canadian boreal forests. Showy mountain-ash grows tall. Its leaves are odd-pinnately compound, with 11–17 leaflets. Each leaflet is long and wide. All parts are hairless to slightly hairy. Flowers are borne in 125- to more than 400-flowered panicles across. Each flower is across and has five white petals