The devastating ash dieback fungus, which has ravaged British woodlands since arriving in 2012, may have met its match – not in a laboratory, but through the ancient power of evolution itself. Researchers have made a significant discovery: a new generation of ash trees growing naturally in UK woodlands is exhibiting greater resistance to the disease than their predecessors. This finding offers profound hope for the future of the iconic British ash landscape and provides compelling, real-world evidence of Charles Darwin’s theory of natural selection in action.
The Devastation of Ash Dieback
Caused by the fungus Hymenoscyphus fraxineus, ash dieback spread rapidly across the UK after its initial detection, prompting emergency government meetings. The disease blocks the tree’s water transport system, leading to withered leaves, lesions, dieback of the crown, and often, death. It has already killed millions of ash trees, dramatically altering the British countryside. Initial predictions were grim, estimating that up to 85% of ash trees could be lost, with projected costs running into billions of pounds (estimates range from £7.6bn to over £15bn). The potential loss was seen as a potential repeat of the Dutch elm disease tragedy.
Ash trees are vital to the UK ecosystem, being the third most common woodland tree and supporting over 900 species of birds, mammals, invertebrates, fungi, and plants. Their widespread loss would have a severe impact on biodiversity and the character of the landscape.
Evolution in Action: The Scientific Breakthrough
A groundbreaking study, published in the journal Science by scientists at the Royal Botanic Gardens, Kew, and Queen Mary University of London (QMUL), focused on ash tree populations in Marden Park wood in Surrey. Researchers compared the DNA of ash trees that became established before the arrival of the fungus with the DNA of younger trees that grew after the disease took hold.
They discovered subtle but significant shifts in the frequencies of DNA variants associated with tree health across thousands of locations within the ash tree genome. These shifts indicate that the younger generation possesses a naturally higher level of resistance.
Professor Richard Nichols, Professor of Evolutionary Genetics at QMUL, described this period as “A tragedy for the trees has been a revelation for scientists.” The study demonstrated that thousands of genes collectively contribute to the ash trees’ ability to fight back against the fungus.
This research provides unprecedented genetic proof for a long-standing prediction: that significant evolutionary change can occur not just through one or two major genetic alterations, but through the cumulative effect of many small genetic changes driven by natural selection – a phenomenon known as polygenic adaptation. The rapid pace of this evolution, occurring within a single generation under the intense pressure of a novel epidemic, is particularly remarkable.
Why Ash Can Fight Back (Unlike Elm)
A key factor enabling this rapid evolution is the ash tree’s prolific reproduction. A single mature ash tree can produce up to 10,000 genetically distinct seeds in a single season. This results in a vast number of saplings upon which natural selection can effectively act. The high mortality rate among young, susceptible trees (with some estimates showing around 30% killed by the fungus in affected environments) provides a powerful driving force for selection, ensuring that “only the fittest survive,” as Professor Nichols noted.
This contrasts sharply with the elm tree, which struggled to evolve resistance to Dutch elm disease. The abundance of ash seedlings provides a much larger genetic pool and faster turnover of generations upon which selection can operate.
Genetic Resistance: More Than Just One Gene
Previous research by the QMUL and Kew teams had already begun to identify the genetic basis of resistance. They found that resistance to ash dieback is not controlled by a single gene, but by multiple genes working together (a polygenic trait), similar to how human height is determined by many genes. Their work identified over 3,000 genetic loci associated with resistance, many of which are similar to disease-response genes found in other plant species. This detailed genetic understanding allows scientists to better predict which individual trees are more likely to survive.
Hope for the Future, But Challenges Remain
While the discovery of natural resistance is incredibly promising, offering hope that ash may not face the near-extinction initially feared, researchers caution that natural selection alone may not be enough to fully secure the species’ future. Dr. Carey Metheringham, whose PhD research contributed to the study, noted that the existing genetic variation within the ash population might be too low, or the rate of selection could slow as susceptible trees become scarcer.
Therefore, human intervention remains crucial to accelerate evolutionary change and support the recovery of ash trees.
Supporting Nature’s Fightback
Experts emphasize several strategies to bolster the ash’s chances:
- Selective Breeding: Identifying and breeding from the most resistant trees found in the wild is a vital long-term strategy. This research provides the genetic markers needed to accelerate such programs.
- Protection of Young Trees: Protecting young trees, particularly from threats like deer grazing, gives them a better chance to survive and contribute to the gene pool.
- Supporting Natural Regeneration: Allowing healthy ash trees to remain in woodlands and encouraging their natural regeneration is critical. As Rebecca Gosling of the Woodland Trust highlighted, this research underscores how vital it is to support natural processes, as it is among these naturally established seedlings that selection is driving resistance.
- Avoiding Preemptive Felling: Unless trees pose a safety risk, avoiding the felling and destruction of healthy or even mildly infected trees is important. These individuals, even if not fully resistant, carry crucial genetic variability that could contribute to future resistance.
Defra’s Chief Plant Health Officer, Professor Nicola Spence, acknowledged the significance of this research, noting that Defra has invested over £9 million in ash dieback research since 2012. The study confirms that tolerance is heritable, reinforcing the strategy of combining breeding programs with natural regeneration.
This research provides compelling evidence that nature is beginning to fight back against ash dieback. By understanding and supporting the natural evolutionary process, combined with targeted human intervention, there is renewed hope that the British ash tree can adapt and survive for future generations.
