2 posts tagged with

Arran whitebeams (Sorbus arranensis,
Sorbus pseudofennica & Sorbus pseudomeinichii)

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Royal Botanic Garden Edinburgh (27th July 2008) Part 1: Sorbus arranensis and Sorbus pseudofennica

Well, I was down at Edinburgh's Royal Botanic Gardens on Sunday (third time that week) and it was a glorious day. I had myself a good time walking around in the sun admiring trees and taking several hundred photographs. My favourite bit of the Gardens is the Highlands area just on the left when you come in at the East Gate. Not because it is particularly well laid out, or that there are any especially interesting specimens in there, but because it is planted with the things that I am most familiar with and fond of: Scots pines, silver and downy birches, gorse, heather, bilberries (blaeberries), rowans, brooms, alders, foxgloves… These kinds of flora are familiar to me from trips to the Highlands, but mainly from living on the edge of the Peak District, where all of the above are abundant.

Anyway, three small trees in the Botanics' Highland bit specially caught hold of my attention after writing this recent post on Arran whitebeams. Two of the trees were Sorbus arranensis (a.k.a. Arran whitebeam); the third was a Sorbus pseudofennica (a.k.a. Arran cut-leaved whitebeam a.k.a. Arran service tree a.k.a. bastard mountain ash). Neither of the two species are found anywhere in the world except the Isle of Arran. Says the Forestry Commission:

Only a few hundred trees of each species exist, clinging perilously to the steep rocky slopes of two remote glens at the north of the island.

The Arran whitebeam was first recorded in 1897...

The other rare hybrid, the Arran cut-leaved whitebeam, was first noted in 1952. [...] Both species were more abundant in the past, but have been forced to retreat to their restricted enclaves as the island was progressively improved for agriculture.

small Sorbus arranensis

This is the smaller of the two S. arranensis trees. As you can see, it is only a young 'un.

leaves on the small Sorbus arranensis

And these are some of the leaves of the small S. arranensis.

larger Sorbus arranensis

This is the larger S. arranensis. A poor photo, but it gives an idea of the form.

 Sorbus arranensis leaves

Leaves from the top...

 Sorbus arranensis leaves

...and some from the bottom.

S. pseudofennica

The S. pseudofennica, squeezed in between a rowan and a birch.

S. pseudofennica leaves

S. pseudofennica leaves...

S. pseudofennica leaves

...more leaves...

S. pseudofennica leaves (undersides)

...and the silvery undersides of even more leaves.

I found a paper by Robertson, Newton and Ennos (my dissertation supervisor and one of my old lecturers) - Multiple hybrid origins, genetic diversity and population genetic structure of two endemic Sorbus taxa on the Isle of Arran, Scotland (Molecular Ecology (2004) 13, 123–134) – that clears up the origins of both of these Arran whitebeams. From the abstract:

In this study, we use an array of genetic markers in a population analysis to elucidate the hybrid origins of the Arran whitebeams Sorbus arranensis and S. pseudofennica, two woody plant taxa endemic to the Isle of Arran, Scotland. It has been proposed that S. arranensis was derived by hybridization between S. aucuparia [rowan] and S. rupicola [rock or cliff whitebeam], and that subsequent hybridization between S. arranensis and S. aucuparia gave rise to S. pseudofennica. Analyses of species-specific isozyme, nuclear intron and chloroplast DNA markers confirm the proposed origin of S. arranensis, and indicate that S. aucuparia was the female parent in the hybridization. Analysis of microsatellite markers suggests that there have been at least three origins of S. arranensis on Arran. Microsatellite markers also support the proposed hypothesis for the origin of S. pseudofennica, and indicate at least five hybrid origins of this taxon.

And from the Introduction, an explanation of the origins of S. rupicola, the subject of the last treeblog post:

...S. rupicola... (rock whitebeam), itself an autotetraploid derivative of S. aria [common whitebeam]. It is most likely that S. aucuparia was the female parent involved in this cross because S. rupicola produces seed apomictically, but has significant pollen fertility of 20%.

Being the kind and generous human being that I am, I have attempted to sum up all of that information in the following simplified graphic:

Arran whitebeams family tree (haha!)

Now, I think I'll end the post here. I have more material for a future post about whitebeams in Holyrood Park (again), but I think regular readers must be sick of Sorbi by now so I'll lay off 'em for awhile!

Bonus picture!


When I was photographing the S. pseudofennica this fly accosted my hand. So I took a photograph and was pleased to see what a big fly looks like from so close.

Posted in Miscellany

A brief introduction to the Arran whitebeams

Many Sorbus species, including the common and well-known rowan [a.k.a. mountain ash] (Sorbus aucuparia) and the less common and less well-known common whitebeam (Sorbus aria), have a habit of forming hybrids (I have a friend that calls trees that readily hybridise like this, such as the European and Japanese larches, ‘sexy trees’). This has led to a bewildering range of species and microspecies throughout the British Isles, often very rare and/or unique to particular locales. Perhaps the best well-known examples of this phenomenon are the ‘Arran whitebeams’. According to the Forestry Commission:

The Isle of Arran is home to two species of tree which do not occur anywhere else in the world, the Arran whitebeam (Sorbus arranensis) and the Arran cut-leaved whitebeam [a.k.a. bastard mountain ash] (Sorbus pseudofennica).

They are [...] officially classed as dangerously close to extinction by the WWF.

Only a few hundred trees of each species exist, clinging perilously to the steep rocky slopes of two remote glens at the north of the island.

The fun on Arran doesn’t stop there. A third speicies, the Catacol whitebeam (Sorbus pseudomeinichii), is believed to be the rarest tree in the UK with only two specimens found growing wild on Arran.

This page from Scottish Natural Heritage (SNH) elaborates:

[...] “It has long been recognised that one of the botanical highlights of Arran are the endemic whitebeam trees. These are unique trees which are native to Arran and not found anywhere else in the world. But the recent investigations into the genetics of the trees with the University of Bristol have shown that the population is much more diverse than previously thought.

“We knew about the Arran whitebeam and the cut-leaved Arran whitebeam, [...] but it has been really exciting to discover a completely new species. It is very complex picture but we think that the Arran whitebeams are gradually evolving towards a new type of tree which will probably look very similar to a rowan.

“So far our surveys have only been able to find two individual specimens of the Catacol whitebeam, so it is extremely rare and pretty vulnerable. Most of the Arran whitebeams, including the new species, are found in the mountains around Glen Catacol and Glen Diomhan at the north end of the island.”

SNH goes on to report that deer fences are being erected to encourage seedlings to grow into mature trees. At present specimens are restricted to cliff faces where they are safe from grazers. Furthermore:

It was previously thought that the Arran whitebeam was a simple hybrid between the rock whitebeam [(Sorbus rupicola)] and the native rowan, and that the cut-leaved Arran whitebeam was a back-cross between the Arran whitebeam and rowan.

However the recent research has shown that these crosses have not been one-off events and that further back-crossing has also taken place. The view now is that the Arran population is best thought of as a complex of hybrids. These seem to be gradually evolving towards a new type of tree which will probably look very similar to a rowan.

The Catacol whitebeam is thought to be one of these hybrids. As it was clearly different from both the parent species it has been recognised as a species in its own right.

According to this article from the Scotsman [15 June 2007] a Catacol whitebeam sapling has been raised by the Royal Botanic Garden Edinburgh, so that the species can be propagated and preserved.

Posted in Notable trees

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