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 asymmetrical at the bottom. "S. aucuparia" foliage grows in May and turns yellow in autumn or a dark red in dry locations. Buds of "S. aucuparia" are often longer than 1 cm and have flossy to felted hairs. These hairs, which disappear over time, cover dark brown to black bud scales. The terminal buds are oval and pointed and larger than axillary buds, which are narrow, oval and pointed, close to the twig, and often curved towards it. "S. aucuparia" is monoecious. It reaches maturity at age 10 and carries ample fruit almost every year. The plant flowers from May