On the hunt for some of the rarest trees in the world

Originally published in the Guardian’s Country Diary on August 6th, by Carey Davis. (Featured picture is of an Arran whitebeam in Glen Catacol).

The Isle of Arran, Firth of Clyde: These self-cloned Sorbus species are unique not only to Arran but to two particular glens.

Glen Catacol is hot, humid, slightly overcast, and nothing stirs the air. In other words, the midges are ferocious. I march resolutely up the path, movement keeping the tiny bastards at bay, but the sight of a red-breasted merganser and her eight chicks huddling closely around her in the waters of the Abhainn Mór briefly stops me in my tracks.

A little further along, I branch into Glen Diomhan, looking for some of the rarest trees in the world. The Arran whitebeam (Sorbus arranensis), the Arran service-tree (S pseudofennica) and the Catacol whitebeam (S pseudomeinichii) are unique not only to Arran but to these two neighbouring glens.

Leaves of Sorbus pseudofennica, the Arran service-tree. Photograph: Carey Davies

The first two types, both identified by the late 1950s, number in the hundreds each, but the third, only confirmed in 2007, now exists in the wild as just one tree. All three have been reproduced by Royal Botanic Garden Edinburgh scientists in “captivity” and can be seen in an enclosure at the foot of Glen Catacol, but having read about them while holidaying on Arran, a combination of fascination and bloody-mindedness has compelled me to try to locate the originals. Mostly huddled around steep gorges, naturally protected from the depredations of deer, they prove difficult “quarry”.

The trees are the result of a sequence of cross-fertilisations derived from an original hybrid of rowan (S aucuparia) and rock whitebeam (S rupicola). Their uniqueness stems from the ability of some Sorbus to reproduce asexually, resulting in stable but often intensely localised populations of self-cloned trees, typically labelled types of whitebeam. Despite their family ties, the differences in leaf shapes between the three Arran species are quite pronounced – all are visibly part-rowan, part-whitebeam, but noticeably distinct from either, and from each other.

A fetish for the scarce can obscure the importance of the commonplace, but in a world of declining biodiversity and increasing geographical monotony there is something compelling in the idea of an island of evolution, a lost world of local distinctiveness. My enthusiastic but amateurish searching turns up the Arran whitebeam and the service-tree, but that lonely Catacol whitebeam, clinging to the edge of the abyss somewhere, eludes me.