A slow death for the ash tree?

Remember ash dieback, the devastating tree disease infecting our forests? It may have gone quiet, but hasn't gone away, as Andrew Griffiths reports.

Yorkshire Wildlife Trust officer Jim Horsfall at Potteric Carr Nature Reserve near Doncaster. (Scott Merrylees).

Cast your mind back a few years and you may remember an explosion of headlines about a disease that had just arrived on our shores and was set to lay waste to the nation’s ash trees.

Ash dieback it was called, a mystery fungal infection that had evaded our biosecurity measures and was set to rampage through the land. Words such as ‘ash armageddon’ were used. Since then, well not a lot seems to have happened, really.

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Geoff Garrett is the senior trees and woodlands officer for the Yorkshire Dales National Park, and admits to being one of those who had been predicting the worst.

Tree plight: A diseased leaf. (Steve Collin).

“There was an armageddon scenario, very much so, and that was followed up in interviews I gave, on what might happen,” says Garrett. “And while it hasn’t really happened so far, we are pretty certain it will.”

Ash dieback is a fungal infection that causes leaves to shrivel and die and attacks the bark of the tree before killing it completely. It is spread by spores on the wind, which are released by the fungi’s fruiting body which grows from the previous year’s leaf litter on the woodland floor.

Infected trees have been found across Europe since ash was first reported as dying in large numbers in Poland in 1992. It was first recorded in the UK in February 2012 in a consignment of infected trees sent from a nursery in the Netherlands to one in Buckinghamshire.

By October 2012, a small number of cases were confirmed in Norfolk and Suffolk on woodland ash, which did not have any association with recently supplied nursery plants. Further finds on trees in the wider environment have since been confirmed in most regions of the UK and Ireland, and it was first confirmed on the Isle of Man in 2017. The Woodland Trust noted its increased presence last year in its Yorkshire managed woodlands, and is waiting to see what this year’s summer growth brings.

Geoff Garrett says that it is endemic in the Yorkshire Dales National Park. “You will always find it if you go and look for it,” he says, “So it has got to the point where we don’t look for it because we know it is going to be there.”

So perhaps the ‘armageddon’ is happening after all, it is just a very slow one. Perhaps it is our expectation of what a catastrophic event looks like, in terms of the lives of trees, that is wrong.

The ash is found throughout Yorkshire but prefers base soils, so is found in greatest numbers in limestone and chalk areas of Yorkshire such as the Wolds. Trees cover roughly four per cent of the national park (before recent boundary changes) and of that just less than half is conifers. So the broadleaf cover is about 2.5 per cent roughly. Of that, around 80 per cent of it is ash-based woodland. So that is a hefty proportion of ash trees making up total broadleafed woodland cover, and goes a long way towards explaining the fears when an unknown fungal disease such as ash dieback first hit.

But given the prevalence of the disease already, we haven’t yet noticed any big changes on a landscape scale because the death of a tree can be a drawn-out affair. Once the disease infects a woodland, it is the small, young trees which are worst affected, but of course we don’t notice those. The big, mature trees are taking much longer to die, because of the disease process.

“What we see is a lot of very young trees dying, and they die quite quickly, within two or three years,” explains Garrett. “The tree dies by lesions on the bark ultimately, and with bigger trees, it takes longer for those lesions to join up and kill the tree. So with a bigger diameter tree, that tree can ‘stagger on’.”

Jim Horsfall, Yorkshire Wildlife Trust’s Reserve Officer for South Yorkshire, explains that after Dutch Elm Disease in the 1970s, there were lots of gaps left by elm trees and they tended to be filled by ash. They have similar characteristics and perform similar roles in a woodland.

“Ash trees, like elms, come into leaf later than beech and oak, and have less shade, so there is more light reaching the ground,” says Horsfall, “So you get more flowers growing under elm and ash than you do under oak. And under beech you get almost nothing growing because the shade cast is so dark.”

There are almost 1,000 different species associated with the ash tree, and around 100 of these will either be lost with the ash or find it difficult to survive without its presence. These species include unusual fungi, lichens and insects.

While there is no getting round the fact that our countryside will be the poorer for the lack of ash, it is not quite all bad news. There is some hope on the horizon in the form of breeding a strain of the tree that is genetically resistant to ash dieback – trials are ongoing at the moment. And while it is impossible to generalise about ash because of the different environmental conditions in different regions, in many places ash has been allowed to achieve a dominance that with modern conservation management, it probably wouldn’t have been allowed to achieve.

Ash is known as a ‘pioneer species’, in that it is often the first to colonise bare ground. Hence it moved into the spaces following the Dutch Elm outbreak. It is only relatively recently that woodland is being actively managed for conservation and biodiversity. Prior to this, woodland that was not managed for commercial purposes was largely left to its own devices. Now, a greater variety of native trees are being planted. This will also increase a woodland’s resilience to future diseases. Monocultures always tend to be most at risk.

“So we are planting small leaved lime, or aspen, or where there was ash maybe putting in oak,” says Horsfall. “But we are diversifying. On the horizon there are so many other tree diseases that you just have to hedge your bets a bit.”

One positive that may emerge from Brexit is that it will provide an opportunity to introduce stricter biosecurity measures on imports from remaining member states, so providing better protection against these serious threats. But it is impossible in the modern world to be 100 per cent secure.

It is probably best to accept that as yet unspecified tree disease outbreaks are going to happen, and be as ready for them and equipped to deal with them as possible. But for now we are still in the middle of that very quiet armageddon, ash dieback. But Geoff Garrett of YDNP thinks it might yet have to reach its crescendo.

“It might not come across as a crisis, but my feeling is that over the next 20 years, we are going to see ash as a species become relatively minor in the landscape, because any young stuff that comes up is killed quite quickly by the disease, but the old stuff will take quite a long time to die,” says Garrett. “Over time I think there will be a measurable change in the types of species you find in woodland. It happened with elm, and it is happening with ash now.”

Lessons from elm outbreak

The death of a sizable tree is not just a drawn-out affair, it can be, paradoxically, a productive one too. Ash dieback will produce a lot of deadwood which provides a valuable habitat in woodland.

That is why the Forestry Commission guidance now is to leave infected ash trees where possible. This is in part because of experience gained during that last, great tree disease outbreak, Dutch Elm disease in the 1970s. Then the policy was to fell infected trees as soon as they were discovered. This strategy didn’t work at all, partly because once a disease is actually obvious in a woodland, it is too late to stop the spread. This is particularly true with a fungal infection such as ash dieback, where millions of spores are released on the wind.