Simon Hiscock

The Avon Gorge has been declared a Site of Special Scientific Interest (SSSI) and an EU-designated Special Area of Conservation (SAC) as it supports 27 nationally rare and scarce plants. Many of these rare plants thrive in grassland ecosystems, but decades without management have meant that scrub, secondary forest and invasive plants have encroached upon the grasslands. In 1999, the Avon Gorge and Downs Wildlife Project was established to take conservation action and begin to restore the grasslands and the rare species they harbour.

Some of the rare and endemic plants in the Gorge belong to the Sorbus genus, which includes whitebeams, rowans and wild service trees; trees typically found in the intermediate scrub that exists between forests and grasslands. Two species, Bristol whitebeam and Wilmott’s whitebeam, are known nowhere else in the world and must have evolved in the Gorge.

Simon Hiscock, Professor of Botany at the University of Bristol’s School of Biological Sciences, has been investigating speciation among Avon Gorge Sorbus trees since 2004, with the support of the Leverhulme Trust and NERC together with colleagues at the National Museum Wales and the University of Oxford. His DNA-based methods have not only helped identify new Sorbus taxa within the Gorge, they have also revealed the chain of hybridisation events that have generated the Sorbus diversity seen in the Gorge today. Genetic markers have determined which species were involved in generating new taxa and, more specifically, who the parents were.

This research has exposed a very delicate reproductive system and ongoing evolution among the rare Sorbus species. Rare Sorbus species are triploid hybrids that reproduce primarily through an asexual process known as apomixis. This means that the plant produces an embryo that is genetically identical – a clone – but pollen is still needed to trigger seed production. Though sperm is not used to fertilize the clonal embryo, it does fertilise the cell that becomes the endosperm, which provides nutrition for the developing plant. This is a common reproductive strategy among the rose family, to which Sorbus belongs.

“That’s all well and good if you can use your own pollen,” said Hiscock. “However, these rare Sorbus species can’t because they’ve inherited a self-incompatibility system from the original sexual diploid parents. They recognise and reject their own pollen and also pollen from other con-specific individuals because they are genetically identical. Therefore, they need to receive pollen from another Sorbus species.”

PhD student Ms Shanna Ludwig, who is supported by a NERC studentship, is investigating the reproductive biology of these rare Sorbus species further. Her work has indicated that one of the most common species, S. aria, is an essential source of pollen for these rare apomictic Sorbus species.

As a result of this research, the conservation management plan in the Avon Gorge not only protects the rare Sorbus species, it also preserves the reproductive and evolutionary processes at play in the Gorge. Rare whitebeams are fenced to protect them from goats that have been placed in the gorge to graze scrub and restore the grasslands and exclusion areas have also been established to allow for the growth of new whitebeams.

“The involvement of the University of Bristol within the Avon Gorge and Downs Wildlife Project group is highly beneficial,” said Chris Westcott of Natural England. “The research opportunities, experience and technical understanding they bring regarding some of the key rarities, particularly the endemic Sorbus, is important to their long term management and the future development of the Avon Gorge as one of the key sites for rare species in the country.”

In a broader conservation context, the work at Bristol is adding to the global body of knowledge on complex systems where rare taxa may be transitional species within a broader evolutionary process. It is further evidence to support conservation actions that protect ecosystem and evolutionary processes rather than just rare individuals; a concept of particular relevance in terms of climate change as these evolutionary processes are critical in generating taxa that may potentially be adapted to changing environments.