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 but wide and the plant often grows multiple trunks. A trunk is slender and cylindrical and reaches up to 40 cm in diameter, and the branches stick out and are slanted upwards. The bark of a young "S. aucuparia" is yellowish gray and gleaming and becomes gray-black with lengthwise cracks in advanced age; it descales in small flakes. Lenticels in the bark are elongated and colored a bright ocher. The plant does not often grow older than 80 years and is one of the shortest-lived trees in temperate climate. Wood of "S. aucuparia" has a wide reddish white sapwood and