By Meerabai Kings, Kingfisher Writer

Published January 11th, 2022

Ash dieback is transforming the British countryside, leaving extinction cascades and a £15 billion price tag in its wake. Have we learnt our lesson, or will history repeat itself?

If you are taking a stroll through the British countryside, you will most likely be enjoying the wildlife, the lush and verdant trees, and the fresh air. But would you enjoy a view of shrivelled leaves, decaying trees, depleted wildlife and air full of fungal spores? The future looks bleak for the British countryside, as our third most common tree is set to be erased.

European ash, or common ash, is widespread across the British landscape. Not only is the ash our most abundant hedgerow species, it also makes up 12% of our deciduous woodlands (that is, woodlands which lose their leaves in the autumn, rather than evergreen forests).

Ash is an ecologically and economically important tree—any woodsman will tell you that ash makes excellent firewood, hockey sticks, axe handles and furniture. Any biologist, on the other hand, will tell you that ash provides nitrogen rich soils and hosts a wealth of wildlife.

Over the next few decades, at least 80% of British ash trees will be killed by the ash dieback disease, with some estimates on losses reaching 98%. The culprit, Hymenoscyphus fraxineus, a fungus originally from East Asia, spreads its spores by hitching a lift with the wind, travelling for tens of miles before falling onto new ash trees.

In 1992, H. fraxineus was reported in Poland, the first time it had ever been seen in Europe. Twenty years later, the fungus reached the shores of Southeast Britain, and has since spread like wildfire—even reaching North Wales and Scotland in 2020.

But how could the fungus get from Eastern Asia to Eastern Europe, and then to Britain? Did it drift all that way? No, if it did it would not be considered an invasive species. We are the reason the fungus has exceeded its natural range.

Up until 2012, Brits were importing ash saplings from abroad, unwittingly importing the fungus with it. In 2012, the importation of ash sapling was banned in the UK in the hopes to stop ash dieback in its tracks, but this was too little, too late. Now that we have brought the fungus over here, it is infecting the leaves of ash trees, leading to millions of their deaths.

While the numbers may seem dramatic, the estimated figures are by no means an exaggeration. Take Poland, for example, where ash trees have effectively been wiped out in the last 30 years. Poland’s neighbour, Lithuania, has lost 99% of its European ash trees and further west, Denmark has lost 90%.

‘In Poland, ash trees have effectively been wiped out in the last 30 years.’

It is clear from our European neighbours that ash dieback is devastating European ash tree populations, but ash trees in East Asia haven not seen the same demise—why?

Chinese ash and Manchurian ash are both native to East Asia and have evolved alongside the H. fraxineus fungus, meaning they have had millions of years to come up with immune responses specific to combating it.

There is, and always will be, an evolutionary arms race between a pathogen and its host. Whilst the pathogen evolves mechanisms to infect its host, the host will adapt anti-mechanisms to boost its immunology, out-witting the fungus if you like. In response, the pathogen will adapt another way to overcome it, and so the cycle goes on, the two species evolve in relation to one another over thousands of generations.

This phenomenon is called the ‘Red Queen hypothesis’, derived from Lewis Carroll’s Through the Looking Glass, when the Red Queen tells Alice ‘it takes all the running you can do to stay in the same place’. For the Chinese ash, this means it is evolving defence mechanisms time and time again just to stay alive.

Each time a Chinese ash successfully ‘out-wits’ the fungus, that tree will produce offspring with the same genes for immunity. Eventually, after many generations of immune trees reproducing whilst vulnerable trees die, the whole population will possess these genes for immunity. Sound familiar? This is Darwin’s idea of natural selection.

European ash, by contrast, has had a measly 30 years to adapt against the fungus. Ash trees can live to be over 200 years old, so 30 years is no time at all for the population to evolve immunity—as immunity in a population evolves over hundreds, even thousands, of generations.

Our poor European ash has been thrown in the deep end here, up against a fungus which has evolved many ways to attack them, with no means of self-defence. So, how can the European ash keep pace with this sudden exposure to the H. fraxineus fungus?

Well, it cannot, or at least, most of them cannot. The majority of Britain’s European ash trees are, as we already know, not going to survive. The fungus infects an ash tree through its leaves, and then multiplies, filling the tree with more and more fungus. The fungus clogs up the ash tree’s xylem, its water transportation system, which in turn starves the tree of water and nutrients.

On your next summer stroll in the countryside, look out for dark patches on the leaves of ash trees, which will shrivel and wilt to black many months before they naturally should shed their leaves. You will see dark, diamond shaped lesions where branches meet the trunk, eventually weakening them so that they fall to the ground.

Even from a distance, the effects of ash dieback are obvious; where we would expect to see a luscious green carpet of woodland, we see empty spaces, dark branches and withered black bunches of leaves—an ash tree graveyard.

You may be thinking, why all the fuss? Why have I wasted my time reading about a tree? But you have not been reading about one tree, you have been reading about millions of trees, each home to thousands of mammals, insects, birds and lichens.

Research from the James Hutton Institute in Aberdeen has shown that over 1000 species live in and on European ash trees. Of these, almost 50 are obligate species, meaning they solely live on the European ash, having spent millions of years evolving alongside it.

‘Look out for dark patches on the leaves of ash trees, which will shrivel and wilt to black.’

Such species include the ash-bark piercer moth, nationally scarce in Britain, and the centre-barred Sallow moth, with turmeric-yellow wings and a fluffy body. The brown hairstreak butterfly, of high Butterfly Conservation priority, does not lay its eggs on ash trees but does indirectly depend on ash for food, as it feeds on the honeydew made by obligate aphid species.

More species still are highly dependent on ash trees, and research at the Swedish University of Agricultural Sciences has shown that nine tree species would need to be planted in order to fill the ecological niche that ash trees will leave behind. In other words, the job that one ash tree does alone could only be done, collectively, by nine different trees. Beech, Norway maple, aspen and varieties of oak would, accumulatively, sustain 95% of ash-associated biodiversity.

This research emphasises the ash’s role as a keystone species, one which helps define an entire ecosystem. As such, poor management of ash woodlands can and will lead to a vast and sad loss of biodiversity, an ‘extinction-cascade’, as the Swedish researchers coin it.

Believe it or not, ash dieback is not the first epidemic that British trees have faced. Have you seen an elm tree recently? The chances are you have not, as Britain lost 90% of its elm trees to Dutch elm disease between the 1970s and the 2000s.

In the 1970s, Brits were importing huge amounts of elm timber from Canada and, with it, thousands of elm bark beetles—almost all of them carrying Ophiostoma fungi.

British elms put up a good fight against the Ophiostoma, plugging their own xylem to stop the fungus spreading further. However, this self-defence strategy had the opposite effect to what our elm trees had hoped, as they all effectively committed suicide by blocking their own systems. Dutch elm disease perfectly illustrates how a naïve host is ill-equipped to defend itself against an invasive species.

Disastrously, further research from Sweden has shown that ash and elm share many of their associated species. As such, many of the species which depend on ash do not have an alternative home to go to, because British elms have also disappeared.

We never learnt our lesson from Dutch elm disease, opening up the question—how are Brits going to save the ash, and what will we do when it has gone in order to avoid a cascade of extinction events?

A silver lining shines from the withered, black leaves of the ash, as one to five percent of British ash trees show tolerance to H. fraxineus and with it, a degree of immunity to ash dieback.

The intricacies behind ash dieback are complex and multifaceted; pathogen load and site factors (such age and soil pH) are as important as genetic traits when it comes to any given tree’s immunity, stressing that a ‘one size fits all’ approach to woodland management would likely be unsuccessful nation-wide.

Though treasured, this rare genetic tolerance is insufficient in defending trees from secondary pathogens. Honey fungus, for example, would be fatal to any tree fighting ash dieback, yet relatively harmless on its own.

It is currently unknown if these levels of tolerance are enough to generate a resistant population of European ash in, say, 50 years time.

‘A silver lining shines from the withered, black leaves of the ash, as one to five percent of British ash trees show tolerance to H. fraxineus.’

And as if the ash is not fighting enough, another threat could reach British shores in the next few decades. The emerald ash borer beetle, native to Asia, has already killed millions of ash trees in North America. If the beetle ever established a population in the UK, it could mean extinction for UK ash trees, and all the species which depend on it.

So far, the best strategy we have for fighting ash dieback is removing leaf litter, as the fungus overwinters on fallen leaves. This will decrease pathogen levels significantly, whilst leaving infected trees to stand gives many species a home.

The Woodland Trust have pledged to only plant UKSG (UK sourced and grown) trees. The Trust will also only fell infected trees when they pose a threat to health and safety, leaving as many trees standing as possible for ash-associated species.

The brittle branches of infected ash trees do, indeed, pose a threat to the British public, as branches are likely to fall onto footpaths and roads. In 2019, the financial cost of ash dieback in the UK was estimated at £15 billion.

Fifteen billion pounds. While this cost is spread over the next century, half (£7.6 billion) is expected within the next ten years.

So, when will we learn our lesson? The only way to sustain the natural world as we know and love it is to responsibly use resources within their natural distribution. For the ash trees, this means only using and planting native trees to the UK, rather than relying on imports.

By disregarding the effects of Dutch elm disease and continuing to import wood and saplings from overseas, British organisations have cost us valuable ecosystems, species and finance. And for what? Cheap access to wood?

The 2019 Current Biology article, which estimates the cost of ash dieback, claims that ‘In 2017, the annual value of trade in live plants to and from Britain was roughly £300 million, representing only two percent of our estimated cost of ash dieback.’

By cutting corners and money-grabbing, we continue to defy biology, and are reluctant to face the consequences for doing so.

If you spot a case of ash dieback in your area, you can report it to Forest Research, using their Tree Alert Tool. If you see an emerald ash borer beetle, you must report it straight away, as these have not been seen in the UK yet. You can also donate to The Woodland Trust which will help fight ash dieback.

Featured Image: Björn S | Flickr

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