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 a light brown to reddish brown heartwood. It is diffuse-porous, flexible, elastic, and tough, but not durable, with a density of 600 to in a dried state. The roots of "S. aucuparia" grow wide and deep, and the plant is capable of root sprouting and can regenerate after coppicing. The compound leaves are pinnate with 4 to 9 pairs of leaflets on either side of a terete central vein and with a terminal leaflet. There are paired leaf-like stipules at the base of the petiole. The leaves are up to 20 cm long, 8 to 12 cm wide, and arranged