British woodlands appear to be evolving resistance to the invasive fungus causing the devastating tree disease ash dieback, scientists have said.
Researchers from the Royal Botanic Gardens, Kew, and Queen Mary University of London say they have discovered a new generation of young ash trees in the wild which are showing greater resistance to the fungus compared to older trees.
The findings from the study focused on Marden Park wood in Surrey, a semi-natural ancient woodland dominated by ash, which is a species that produces a large number of seedlings from each adult.
It could mean ash does not go the same way as elms, which have been largely lost from the British landscape as a result of Dutch elm disease, the scientists said – although breeding programmes may be needed to bolster the fightback.
Ash dieback, which was first seen in the UK in 2012, causes leaf loss and crown dieback and can lead to tree death, with fears it could wipe out up to 85% of Britain’s native ash trees, as they have not evolved natural defences against the fungus originally from Asia.
But now researchers suggest that natural selection in the wild is taking place on thousands of locations within ash tree DNA, driving increased resistance to the disease.
The study compared the DNA of ash trees established before and after the fungus arrived in Britain, and found shifts in variants associated with tree health in thousands of places across the overall genome of the trees.
This indicates the younger generation of trees has greater resistance to the fungus – as well as showing natural selection for a trait being influenced by multiple genes, which the researchers said was a phenomenon that was widely assumed but difficult to prove.
Their study in the journal Science said natural selection could be occurring partly due to reduced seed or pollen production by adult trees damaged by ash dieback, and partly through the rapid death of young trees infected by the fungus and so are not present in the wood.
But they cautioned that it was not yet known whether the rate of change would be enough to rescue the plants naturally, or whether there was enough genetic variation in ash trees to lead to a fully resistant tree.
The findings could be used to support breeding programmes that aim to make European ash trees resilient to the disease, they added.
Study author Dr Carey Metheringham, from Kew and Queen Mary, said: “Thanks to natural selection, future generations of ash should have a better chance of withstanding infection.
“However, natural selection alone may not be enough to produce fully resistant trees.
“The existing genetic variation in the ash population may be too low, and as the trees become scarcer, the rate of selection could slow.
“Human intervention, such as selective breeding and the protection of young trees from deer grazing, may be required to accelerate evolutionary change.”
Another of the study’s authors, Professor Richard Buggs, from Kew and Queen Mary, said: “We are so glad that these findings suggest that ash will not go the way of the elm in Britain.
“Elm trees have struggled to evolve to Dutch elm disease, but ash are showing a very different dynamic because they produce an abundance of seedlings upon which natural selection can act when they are still young.
“Through the death of millions of ash trees, a more resistant population of ash is appearing.”
Rebecca Gosling, from the Woodland Trust which owns and manages Marden Park wood, said: “Ash dieback demonstrates how devastating introduced pathogens can be for our trees and the species which rely upon them.
“This important research gives us hope for the future of our ash populations.”
She added: “The findings highlight how vital it is to support natural regeneration in woodlands, furthering our understanding of how to best manage our ash woodlands.”
The study was mainly funded by the Environment Department (Defra) whose chief plant health officer Professor Nicola Spence said it demonstrated that tolerance to ash dieback can be inherited, and breeding programmes and natural regeneration together could secure the future of native ash.
