In 2005-6 I discovered three new tree species – in Britain. They even overlap, in Cheddar Gorge in Somerset. The largest of the twelve individuals of one species (Gough’s Rock Whitebeam Sorbus rupicoloides) had a girth of 31cm: my hands encircled it. We cannot be sure that a larger, older ancestor never existed, but this tree, the oldest present, may well have been the first. If so, an entirely new species evolved here within the last – 30 years?
I’m not a scientist by original training and fell accidentally into this specialist area of botany.
Because I’m quite at home on steep slopes and cliffs, with or without ropes, I’ve become an expert on cliff plants – in particular, limestone cliffs, and Sorbus trees. Too small to compete in level woodland, Sorbuses are well adapted to life in steep places, growing out diagonally or horizontally from edges, ledges or rock-face cracks. The meanness of their environment may stunt or slow their growth, so that what at first appears a half-metre twig may be mature and fruiting; a waist-high whip may be 20 years old.
And when the main stem of a larger tree fails, sucker growth from the base can simply carry on.
In Britain we have three main normal sexual species of the genus (and one much rarer, outside this story), Rowan Sorbus aucuparia probably the best known – that lovely scarlet-berried upland tree with its feathery, tooth-edged, divided leaves, turned to for protection against evil. Less widespread, the Wild Service Tree Sorbus. torminalis is a taller, more woodland species, its sevenpointed leaves like badly-drawn stars, its brown fruit taken for colic. And then Common Whitebeam Sorbus aria, named for the white-felted undersides to its leaves which mark the tree out pale on spring hillsides (‘beam’ from OE beam = tree), less common than a bee orchid in the wild, but a favourite street tree, leaves variously oval, flowers and fruit like Rowan’s, itself never huge.
I think there is no record of Rowan and Wild Service hybridising. But Common Whitebeam can hybridise with either, a key factor in the continuing evolution of Sorbus species. Alongside Whitebeam, Rowan and the Wild Service Tree Poet, rock climber and botanist Libby Houston discovers three new tree species in Somerset.
Sofia 99 March/April 2011 our sexual species are forty apomictic species, i.e. that produce fertile fruit asexually. They need male pollen from another tree to trigger the process, but use none of its genetic material.
Apomictic children take after their mother, exactly; they are virtual clones – and may be very localised. Thirty-two of our apomictic Sorbuses are endemic to the British Isles – some, like Sorbus rupicoloides, to a very small corner – and variously rare. Apomixis occurs in other plants, producing for instance 232 known British species of Dandelion Taraxacum, or 334 Brambles Rubus. What for me differentiates apomictic Sorbuses is their beauty.
Recent research has been analysing the DNA of the rare Sorbuses to find their origins and relationships. In the field you can recognise inherited characters that apomixis has fixed. Species with Wild Service and Common Whitebeam in their origin, for instance, show toned-down variations of the star-pointed leaflobes, long leaf-stalks and brown berries of their ‘mother’ Sorbus torminalis, and can grow relatively tall.
But more than that: carrying out wild Sorbus surveys in Cheddar or Bristol’s Avon Gorge, looking at hundreds of trees, I began to notice that beauty might be an actual characteristic.
Where a species may ultimately have derived from Common Whitebeam alone, the process sharpens and simplifies the original characters. Common Whitebeam’s leaves are ovalish, roundish, floppy, flouncy, hard to flatten, overcrowded, quilted, crimped, misshapen, top- or bottom-heavy, teeth slightly blunted, crowded veins slightly curved and black against the light. These apomicts’ leaves tend to be flat above and glossy, the base often cuneate (streamlined), teeth sharp, often porrect or pointing straight outwards from the end of the vein, veins fewer and straighter, often translucent against the light. Kite-shaped, diamond-shaped, fan-shaped, paddle-shaped, paw-like, according to species, they all show bilateral symmetry. Never looking ‘bottom-heavy’, very often they are widest above the middle – perhaps at the golden section.
Both of which characteristics lead into aesthetic theory. In the field I could note a hundred trees, and suddenly, really, feel my heart lift – at the sharp-toothed rounded leaves of Sorbus eminens; or some repeated character, a certain green, or neat lobe, that made me think: this is a species in its own right. The leaves are held in sprays of 4-6, from generously open (Sorbus eminens) to upright – as if to say Pick a card, any card – on horizontal outspread branches (Sorbus leighensis), or upright on upward-reaching branches (Sorbus wilmottiana). Artists often make a straight flowerstalk wavy; these trees are typically the opposite, with sinuously graceful or arching stems.
The old plant identification characters were, and still are, studied dried and flattened on herbarium sheets in the botanical collections.
With digital colour photography, macro photos are easily sent, and we can now record all kinds of back-up features of colour and form. I clamber among the trees wordlessly, making myself remember to record necessary data: girth, height-estimate, basic form and GPS; and taking photographs.
Colour, shape, proportion: the theories are still outside my knowledge. It’s as wordless as dancing.
Much more could be said. This is what I have myself observed.
References:
Rich, T.C.G., Houston, L., Robertson, A. and Proctor, M.C.
F. (2010). Whitebeams, Rowans and Service Trees of Britain and Ireland. A monograph of British and Irish Sorbus L. B.S.B.I. Handbook No.14. Botanical Society of the British Isles. London.
Robertson, A., Rich, T.C.G., Allen, A.M., Houston, L., Roberts, C., Bridle, J.R.,Harris, S., Hiscock, S.J., (2010). ‘Hybridisation and polyploidy as drivers of continuing evolution and speciation in Sorbus’, MolecularEcology 19: 1675-1690.Stace, C.A. (2010). New Flora of the British Isles, 3rd ed. Cambridge University Press, Cambridge.
Libby Houston’s Cover of Darkness: Selected Poems, 1961-98 was published by Slow Dancer Press in 1999.
She lives in Bristol.