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
Fraxinus excelsior narrow crown. The bark is smooth and pale grey on young trees, becoming thick and vertically fissured on old trees. The shoots are stout, greenish-grey, with jet black buds (which distinguish it from most other ash species, which have grey or brown buds). The leaves are opposite, long, pinnately compound, with 7–13 leaflets with coarsely serrated margins, elliptic to narrowly elliptic, long and broad and sessile on the leaf rachis. There are no stipules. These features distinguish ash from mountain ash ("Sorbus aucuparia") in which the leaves are alternate with paired stipules. The leaves are often among the last to