Characteristics
The Mountain Ash has a leaf made up of a number of leaflets, whilst the Whitebeam leaf is entire and doesn't even have lobes. The result of crossing the two is that the hybrids begin to merge or mix characteristics, so that S. arranensis has lobes but no leaflets, while S. pseudofennica, having an extra cross with the 'leafleted' Mountain Ash, has a variable number of true leaflets and lobes. These characteristics are not always definitive and sometimes the actual species cannot be ascertained with certainty, possibly due to hybridisation between the species in question.
Some differences in the flower and seed characteristics are also noted.
Unlike other endemic British species, they do not seem to grow on base-rich soils.
Read more about this topic: Arran Whitebeams