Whitebeam taxa (Sorbus spp.) are beautiful trees characterized by white-backed leaves, in the spring they produce white blossom and bright red/orange berries in the autumn. They can grow as trees and shrubs and are often found on rocky outcrops, open woodland and scree habitats. Unfortunately these habitats are becoming increasingly rare and almost all whitebeam taxa endemic to England are threatened. Some of the last populations of these endangered taxa can be found at the Avon Gorge, on the Devon and Somerset Coast and Morecombe Bay.
The Avon Gorge in South-west England is a Site of Special Scientific Interest (SSSI) and the most diverse Sorbus site in Britain with numerous endemic taxa being discovered there so far. Two species, Bristol whitebeam (S. bristoliensis) and Wilmott’s whitebeam (S. wilmottiana) are endemic to the gorge and evolved within it. Both species are threatened with extinction.
The Bristol whitebeam is classified as Vulnerable on the IUCN Red List with only 100 trees estimated to exist in the gorge. The Wilmott’s whitebeam is one of the rarest Sorbus species in the UK and is categorized as Critically Endangered on the IUCN Red List, with only 42 trees known to be growing in the gorge. S. emines and S.anglica are two other endangered whitebeams growing in the Avon Gorge, they are both categorized as Vulnerable on the IUCN Red List and also have very small populations of around 250 and 600 known individuals, respectively.
In 2009, a new species (S.leighensis) and four new hybrids of Sorbus were discovered in the Avon Gorge. Leigh Woods Whitebeam (S.leighensis), categorized as Vulnerable on the IUCN Red List, consists of 100 known individuals and is found growing in Leigh Woods on the Somerset side of the Avon Gorge. Three of the new hybrid taxa (S x houstoniae, S x proctoris and S x robertsonii) are each only represented by one known individual so far. S x avonensis was also found in the Avon Gorge but is expected to occur locally in the South-west of England.
The Devon and Somerset coastline are also home to endangered species of whitebeam. S.subcuneata is categorized as Vulnerable on the IUCN Red List and has a restricted range along the Devon and Somerset coast. S.vexans is also categorized as Vulnerable; its total population only consisting of around 100 trees. S. margaretae occurs only along the cliffs in the North coast of Devon and Somerset and only 120 individuals are known.
The main threat to whitebeam taxa is habitat loss, the rocky outcrops and scree habitats they are often found in are becoming rarer in the UK, restricting the amount of suitable habitat available for these trees to grow in. Scrub clearance is also a major threat to Sorbus in the UK, accidental felling of these endangered trees occurs during clearance for uses such as pathways or quarrying, including two reported cases of whitebeams being cleared during habitat management. Grazing from livestock and other herbivores is also a problem for these taxa. In the Avon Gorge, whitebeams have to be protected by fences from grazing by local goats. Invasion of non-native plants such as Rhododendrons and evergreen oaks in coastal regions threaten to out-compete whitebeams.
Whitebeams reproduce asexually producing seeds genetically identical to the parent tree. This means that offspring trees are clones; which can reduce the population’s genetic variation. This may make them more vulnerable to environmental changes and other threats compared to species with wider genetic variation.
Many locations of endangered whitebeams occur within Sites of Special Scientific Interest including the Avon Gorge, Watersmeet and West Exmoor Coast, and ten endemic taxa of whitebeam are listed as ‘priority species’ in the UK biodiversity action plan, supporting their survival. A lot of research is being carried out on these exceptional trees including research into their interesting reproductive strategy.