The pandemic facing UK ash trees

The world is currently in the thrall of the COVID pandemic, which has claimed over a million lives so far. But trees aren’t immune to disease outbreaks either, and ash trees are facing their own pandemic. Ash dieback is a lethal fungal disease known as Hymenoscyphus fraxineus that originated in Asia. It first appeared in Europe 30 years ago. UK trees are highly susceptible and this year has been a particularly severe one. Luke Barley, from the National Trust, spoke to Adam Murphy about the disease...

Luke - It's a fungal tree disease. The spores are windborne, they're microscopic and they can travel many miles on the wind. They land on the leaves, and then the fungus establishes itself and grows down through the tree’s cells that we heard about earlier, particularly the hollow tubes at the outer edge of the tree that transport water and nutrients around the tree; and the body of the fungus blocks those cells essentially and kills the tree. In young trees it can kill it really quickly; in older trees it's more progressive, and it kills them slowly but surely from the outside in. The reason our ash are so susceptible is that they haven't co-evolved to deal with the fungus. It's from Asia and it's co-evolved with ash species in Asia; so over there it's more like a leaf blotch, it just discolours the leaves, it's quite a minor ailment because the trees are accustomed to it. But over here, our trees aren't accustomed to it at all, they have no defences against it, no resistance to it. And so it's likely to kill up to 95% of our European ash in Britain.

Adam - How does it spread, and how easily does it flit from one tree to the other?

Luke - It's highly infectious because those spores are windborne. It's been in England since... well, we've known about it since 2012; it was probably here before that. It certainly arrived that year on infected nursery trees from Europe, but it may have arrived on the wind prior to that. And then since then it's been making its way progressively across the UK, and it's now present... certainly here in the National Trust we manage land in England, Wales, and Northern Ireland, and everywhere we look after, ash dieback is now present.

Adam - And what about this year made it so bad? And just how bad was it?

Luke - It's been very bad. We've known it was coming obviously since 2012, so it wasn't a huge surprise in some respects; but like every other infectious disease there's a bell shaped curve to the shape of the infection, and a couple of our landscapes - mainly the chalk and limestone landscapes - the ash in those landscapes seem to be more susceptible. We're already into the exponential phase, but lots of other places weren't, and we seem to have hit the exponential phase everywhere this year. And then that's also been exacerbated by the fact that we had a very, very dry spring, and a very late, very heavy frost; both those things really stress trees out, and that lowers their ability to resist pathogens. So we've seen it really kick off this year absolutely everywhere that we look after. It's been very severe.

Adam - What kind of numbers are we talking about? How many trees?

Luke - On National Trust land it's very hard to quantify, because we don't have a really accurate census of our tree population anywhere. But we know that we're going to need to fell around 40,000 trees this year on National Trust land, just where they could affect public safety if they die - this is by roads or by buildings where people congregate - because as the ash trees die, they become unstable and compose a risk to people. But we think we've got anywhere up to two and a half million ash trees on National Trust land that we won't have to fell; they're just going to die of ash dieback. And it's hard to estimate how many of them are very severely affected this year, but it's quite a high proportion I'm afraid.

Adam - Let's say the worst happens and we lose all those trees; what kind of an impact is that going to have for British wildlife and ecology?

Luke - It's going to have a really big impact. It's going to be very, very sad, frankly. I suppose there's a couple of separate things to tease apart: there's ash in woodlands, and there are some places where it won't have such a catastrophic impact. If ash forms a relatively low proportion of a mixed wood with other species present, then the increased light in that wood as the ash die might actually benefit some species. So that's a thread of hope to hang onto. But where we've got ash dominating entire woodlands we often get a really special ground flora: lots of interesting wildflowers in ash woodlands, because it casts a particularly light shade and its leaves are cycled into the nutrient cycle much more rapidly, so the conditions for wildflowers are great; and that's going to change hugely as whole ash woods die, potentially. So that's going to be really sad. And then the other really sad thing is open grown trees, trees outside woods, places like the Lake District where we've got very significant ash pollards. Pollards are trees that have been caught continually through human history, for firewood and for fodder and for other uses. They're very culturally significant, but they're also home to lots of rare invertebrates that live in the decaying wood in their centre and lichens on their bark. And so we're going to lose all the special species related to open grown ash trees as well. That's before we even get onto ecosystem services. We think that the ash trees on National Trust land might be sequestering anything up to about 700,000 tons of CO2 equivalent; and they provide all kinds of other benefits obviously, whether it's shading or flood alleviation, all those great things that trees do. So the impact is going to be very, very significant.

Adam - What can we do about it? It seems like a massive problem that we should be on top of.

Luke - It's a massive problem. There are subtle things we can do to mitigate some of those impacts for nature in ash woods, by planting other appropriate species, or outside woods by making sure we plant lots of trees. And that really is the long and short of it. In terms of the climate crisis, the crisis in biodiversity, we really, really, really need to establish a lot more trees. And we also need to establish a lot more species, and really make a huge increase in tree cover and make it more resilient to future threats.