There is a useful article from the Royal Botanic Garden, Edinburgh by Max Coleman (18 December 2014), on the three Sorbus species endemic to the Isle of Arran in the Firth of Clyde, western Scotland:
http://stories.rbge.org.uk/archives/13616
The trees concerned are the Arran whitebeam , Sorbus arranensis, the Arran service-tree, S. pseudofennica and the Catacol whitebeam, S. pseudomeinichii.
