The Cheddar Gorge Whitebeams

Cheddar Gorge is a Carboniferous limestone cleft in the Mendip Hills in Somerset, England and it is notable for many rare plants. The gorge is of great importance for Sorbus taxa of the aria section (whitebeams) but has not been thoroughly surveyed for these trees until recently.

In the past the gorge was heavily grazed with trees and shrubs only surviving on the steeper cliffs. After World War II grazing declined and the grasslands were invaded by scrub, which included tree seedlings, and much of the area is well on its way to secondary woodland. Many whitebeams occurred in this spread of woody plants.

The common whitebeam (Sorbus aria) is abundant over much of the gorge and as a cross-pollinating sexually reproducing species is very variable in its leaf shape and other characteristics. However, it has been known for some time that there were other, apomictic microspecies (i.e. self-fertile without pollen) present. Of these the English whitebeam (Sorbus anglica), the grey-leaved whitebeam (S. porrigentiformis) and the round-leaved whitebeam (S. eminens) were known in the gorge before Libby Houston was commissioned to do Sorbus survey in 2005 and 2006.

As a result of this survey, and with the assistance of Tim Rich from National Museum of Wales and others, three new species were described in 2009, though botanists had long suspected that there were further whitebeam microspecies at Cheddar. The three, all small trees growing 7 metres or more tall, are the Cheddar whitebeam (Sorbus cheddarensis), the twin-cliffs whitebeam (Sorbus eminentoides) and Gough's rock whitebeam (Sorbus rupicoloides). The last of these is named after Gough's Cave in the gorge. (Richard Gough was a 19th C resident of Cheddar who made explorations into the cave which extends a considerable distance into the hillside.) Most of the Gough's rock whitebeams so far discovered grow near the mouth of this cave.

All the new species qualify for 'Critically Endangered' status according to IUCN guidelines. So far only 19 examples of S. cheddarensis have been found, only 15 of S. eminentoides and only 12 of S. rupicoloides though more may occur on the inaccessible parts of the gorge or areas that have not been surveyed.

Goats, among the grazing stock present in the gorge, create the greatest threat to these trees and many of the whitebeams have been damaged or killed by them. In the case of the Cheddar whitebeam, for example, out of 13 trees examined for goat damage, 6 (46%) had been affected. The Cheddar Gorge stock were introduced for conservation purposes and it is ironic that they should threaten critically endangered tree species, though I am sure this was not the original intent. Indeed, there could have been Sorbus microspecies present that are now globally extinct as a result of conservation grazing.

Cheddar Gorge is a popular destination for the general public and the whitebeams are an important landscape feature. The rarer whitebeams, however, grow mainly in places where access is difficult and where there is a danger of falling rock and people are strongly advised not to go into dangerous situations without prior permission and the appropriate safety arrangements.

REFERENCES

Houston, L. (2006) Cheddar Gorge Sorbus survey 2005. Unpublished report to the National Trust, March 2006.

Houston, L. (2009) Cheddar Gorge Sorbus survey 2006. Unpublished report to the Longleat Estate, 2009.

Houston, L., Robertson, A., Jones, K., Smith, S.C.C., & Hiscock, S.J. (2009) An account of the Whitebeams (Sorbus L., Rosaceae) of Cheddar Gorge, England, with description of three new species. Watsonia 27: 283-300.