Arran whitebeams

Whitebeam species

The Arran whitebeams are species of whitebeam endemic to the island of Arran, Ayrshire, Scotland.

These trees, sometimes called the Scottish or Arran whitebeam (Sorbus arranensis), the bastard mountain ash or cut leaved whitebeam (Sorbus pseudofennica) and the Catacol whitebeam (Sorbus pseudomeinichii) are, if rarity is measured by numbers alone, amongst the most endangered tree species in the world. They are protected in Glen Diomhan off Glen Catacol, which was formerly part of a National Nature Reserve; although this designation was removed in 2011 the area continues to form part of a designated Site of Special Scientific Interest (SSSI). Only 283 Arran whitebeam and 236 cut leaved whitebeam were recorded as mature trees in 1980, and it is thought that grazing pressures and insect damage are preventing regeneration of the woodland.

Sorbus arranensis: Abhainn Bheag (Uisge solus), Glen Diomhan (and tributary), Glen Catacol, Allt nan Calman, Allt Dubh, Gleann Easan Biorach and Glen Iorsa (Allt nan Champ). Sorbus pseudofennica: Abhainn Bheag (Uisge solus), Glen Diomhan and Allt nan Calma. Landsborough in 1875 noted the two kinds growing in Glen Diomhan and called them French rowan or whitebeams. The Scottish mountain ash, S. arranensis, evoked most collecting interest in 1870–1890 and 1920–1940, although older herbarium specimens exist.

The trees developed in a highly 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 between S. arranensis and the mountain ash (S. aucuparia). The Sorbus group are apomictic, producing viable seed without the need for pollination and fertilisation. Each time this hybrid cross occurs a new clone is effectively produced. Smart showed by using physical characteristics that the species were separate and not a result of random variation. Some overlap does however occur and this suggests that some hybridising may occur between the two species. A number of other Sorbus species have been produced in this way, such as the Devon whitebeam, the Bristol whitebeam, the Cheddar whitebeam, Irish whitebeam, Lancaster whitebeam, etc.

In 2007 it was announced that two specimens of the newly named Catacol whitebeam (Sorbus pseudomeinichii) had been discovered by researchers on Arran. The tree is again a cross between the native rowan and whitebeam, the discovery being made following work by Scottish Natural Heritage (SNH), Dougarie Estate and Royal Botanic Garden Edinburgh. Research into the genetics of whitebeam trees had shown that the population was much more diverse than previously thought and that the Arran whitebeams seem to be gradually evolving towards a new type of tree which will in all likelihood look very similar to a rowan. A team from the Royal Botanic Gardens collected seeds and cuttings to ensure the long term survival of the trees and steps were taken to protect the two known specimens. As of 2016, only one of the two specimens could be found.