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False Rowan (Sorbus pseudomeinichii Ashley Robertson) Robert Blackhall-Miles
Discovery of a second extant plant of Arran’s False Rowan (Sorbus pseudomeinichii Ashley Robertson)
ROBERT BLACKHALL-MILES
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Arecent work visit to Scotland took my partner (Ben Ram) and I to the Isle of Arran (v.c100) to visit the collection held at National Trust for Scotland’s Brodick Castle and Goatfell Nature reserve. Two aims of our visit were to see revegetation work being done by the ranger team in Glen Rosa and to see all three of Arran’s endemic Sorbus L. microspecies in Glen Catacol, in the north-west of the island.
On 15 September 2020 we visited Glen Catacol with the sole intention of observing the whitebeams and rowans growing in the valley. At the Glen Catacol trail head there is a small enclosure planted with the five taxa involved in the complex sequence of hybridisation in the evolution of the three endemic Arran Sorbus. This enclosure presented a perfect opportunity to familiarise ourselves with the taxa found in Glen Catacol: S. aucuparia L. (Rowan), S.arranensis Hedl. (Arran Whitebeam), S. pseudofennica E.F. Warb. (Arran Service-tree) and S. pseudomeinichii Ashley Robertson (the Catacol Whitebeam or False Rowan) in their pristine state.
The three unique Arran Sorbus are the result of crosses between S. aucuparia and S.rupicola (Rock Whitebeam), with S. arranensis being an apomictic hybrid between S. aucuparia and S. rupicola; S.pseudofennica being a backcross (also apomictic) between S. arranensis and S. aucuparia, and S.pseudomeinichii being a further backcross (also proven to reproduce apomictically) between S.pseudofennica and S. aucuparia (Coleman, 2014).
Sorbus pseudomeinichii was published as a new endemic tree from Arran in 2006 and has leaf morphology intermediate between S. pseudofennica and S. aucuparia (Robertson & Sydes, 2006). Originally three trees were found, two mature and one sapling, however subsequently flooding has destroyed one tree and the sapling is presumed to have been killed by deer
A leaf of the newly discovered specimen of Sorbus pseudomeinichii. Photographs by the author.
grazing. This has left a single mature individual on the streamside of the Catacol Burn, making it arguably one of the rarest trees in the world (Coleman, 2014; Rich et al., 2010).
The Catacol Burn has a number of tributaries, the banks of one of these, the Diomhan Burn in Gleann Diomhan, holds the majority of plants of S.arranensis and S. pseudofennica, whilst the upper reaches of the main Catacol Burn, and a tributary of it, holds further plants of S. arranensis and S.pseudofennica as well as the surviving plant of S.pseudomeinichii.
On our visit to Glen Catacol we chose to follow the Diomhan Burn to its head to view trees along the entire length of the gorge and then walk over the nearby 527m hill of Bienn Tarsuinn to Loch Tanna in order to follow the Catacol Burn from its headwaters down so as we could take in all of the trees that Glen holds.
To protect the trees from grazing the majority in Glen Catacol and Gleann Diomhan are now growing within deer-proof enclosures with only
those trees in the steepest, and as such unreachable, sections of each gorge left unprotected (as is the case for the single tree of S. pseudomeinichii).
A significant way before reaching the majority of the trees on the Catacol Burn we observed two lone, unprotected and heavily grazed Sorbus on the side of the path just below the first of two waterfalls on the stream. We noticed that whilst one of the trees was clearly a Rowan; the other had larger terminal leaflets, some of which had lobes, resembling fused leaflets and just 4–5 (sometimes 6) pairs of lateral leaflets across the majority of its remaining autumnal foliage. With the plant having piqued our interest and having taken a number of photographs, an accurate 10-digit grid reference and collecting both the largest terminal lobed leaf and the smallest terminal lobed leaf, we moved on to visit the lone plant of S. pseudomeinichii in its gorge.
On seeing the large tree of S. pseudomeinichii, with its multiple vertical trunks, and observing its leaves and individual leaflets in their weather battered state, we realised that the tree we had seen higher up the valley may very well also be a plant of S. pseudomeinichii. We continued out of the valley with many questions about the trees we had seen and our discovery higher up.
The Sorbus pseudomeinichii tree by the Catacol Burn, 15 September 2020.
The following day we discussed our find with Goatfell and Glen Rosa lead ranger, Kate Sampson, who took the leaves we had collected and showed them to local Sorbus grower, Henry Murdo. Mr Murdo thought the samples interesting but was unable to be conclusive in his identification. Later that day, however, photographs were shared with Dr Tim Rich and Dr Ashley Robertson both of whom were happy to give a positive identification of the plant as S. pseudomeinichii. Dr Robertson was also able to confirm this as a new location for the taxa rather than the rediscovery of one of the two previously known plants that had been lost.
All three Sorbus taxa endemic to Arran are considered endangered (Rivers et al., 2019), for the Critically Endangered Sorbus pseudomeinichii this discovery of a further plant is of great conservation significance. All of this evolutionary complex should be conserved and as such it is important that this second plant is investigated and protected from further grazing. The collected leaves from the plant have now been sent for analysis, and further investigation of the plant will be undertaken during spring 2021.
Acknowledgements
Dr Tim Rich, Dr Ashley Robertson, Kate Sampson (National Trust for Scotland), Henry Murdo, John Nicholson and Ben Ram.
References
Robertson A. & Sydes C. 2006. Sorbus pseudomeinichii, a new endemic Sorbus (Rosaceae) microspecies from Arran,
Scotland. Watsonia, 26: 9–14. Rich, T.C.G., Houston, L., Robertson, A. & Proctor, M.C.F. 2010. Whitebeams, Rowans and Service Trees of Britain and
Ireland. A Monograph of British and Irish Sorbus L. BSBI
Handbook No. 14. Botanical Society of the British Isles,
London. Coleman, M. 2014. Arran’s unique trees. The Royal Botanic
Garden, Edinburgh: stories.rbge.org.uk/archives/13616. Rivers et al., 2019. European Red List of Trees. IUCN, Cambridge,
UK and Brussels, Belgium. DOI: doi.org/10.2305/IUCN.
CH.2019.ERL.1.en.
