So quick? The unstoppable spread of ash dieback disease

It started with a chance conversation with a workmate who commutes to our offices in Bainbridge from Sedbergh.   Had I seen the young ash trees dying on the Garsdale road verges?  I confessed I hadn’t.

A week or so later I was driving that way and my eyes were opened.  It was true. All along the Garsdale road there were young ash trees in a bad state.  It was mid-summer, but their branches were bare. 

I stopped the car to take a closer look.  That’s when I saw lozenge-shaped scars at the base of dead branches.  That is the sure, dreaded sign of a fungal pathogen called Hymenoscyphus fraxineus that has blown in from Asia and wreaked carnage in European ashwoods.

Tree with ash dieback disease on the road just west of Garsdale Station

Like most people, I had heard of ash dieback, but I hadn’t heard much in a while.   The next day I spoke to Geoff Garrett, the Authority’s Senior Trees and Woodlands Officer.

He said ash dieback could be seen not only on the Garsdale road, but across the National Park; the disease had spread “phenomenally quickly” in the years since the first UK case was confirmed in 2012.

A date was arranged where he would show me the impact it was having on the biggest native woodland in the National Park, Grass Wood in Wharfedale.

Rather than go straight to the wood, Geoff took me first to Threshfield Quarry, a spectacular, disused limestone quarry a few miles north of Skipton.

Geoff Garrett at Threshfield Quarry

From a ridge of the quarry, we could look across Wharfedale to Grass Wood and see it in its landscape setting.

Grass Wood, as seen from Threshfield Quarry

“Look at the wood,” said Geoff, “The dark green on the fringe is sycamore. Now scan across into the main body where there’s a lighter shade of green and a more open structure.  That’s ash.  Focus in on that. Imagine the ash all gone, then ask yourself what would be left.”

Ancient semi-natural woodland – the most important type of woodland for biodiversity – covers about 1% of the Yorkshire Dales National Park.  About 80% of this woodland is made up of ash.

“Grass Wood is an incredibly important ancient semi-natural ash woodland, partly because it has been there so long – probably since the last Ice Age – and partly because of the cohort of plants and animals that it supports,” Geoff said.

“Ash dieback will create opportunities for other trees, but other trees won’t have the same qualities as ash – the same dynamic of light intensity.  The thing about ash is that it takes forever to come into leaf, then at the first hint of a frost in the autumn, all the leaves drop off.  This means certain plants can thrive such as water avens, St John’s wort and celandine.  Dog’s mercury and herb paris grow in Grass Wood – plants which take many years to spread.  It is this mix which makes an ashwood so special.”

On the way down from the ridge, we saw a long-dead ash.  This had not died from the effects of Hymenoscyphus fraxineus.  Nevertheless, Geoff said it was a vision of what was to come.

“This is what we’ll end up seeing throughout woodlands in the Dales”

Another ash by a drystone wall looked as if it was ailing, but this wasn’t ash dieback disease.

“You’ve got to be careful when diagnosing ash dieback,” said Geoff, “because ash trees can die of all sorts.  When ash dieback was first confirmed in Britain, many people rang us up to report cases, but they were often mistaken.  They’d seen ash trees suffering from canker or wind damage.

“A single, isolated ash in a field is unlikely to be affected by dieback.  The wind will blow away the fungal spores before they can settle.  It is ash woodlands that are most heavily affected because of the high loading of spores in the air.”

From the quarry we drove the couple of miles to Grass Wood.   First we went into the lower part of the wood, and immediately the impact of ash dieback could be seen.  The young ash trees were bare.

In Lower Grass Wood

To explain how the disease had taken hold, Geoff picked up a shoot from the floor.  The fungus would develop on shoots such as this, or on fallen ash leaves, before producing fruiting bodies – just a few millimetres high – in the spring.  The spores from these fruiting bodies would infect ash trees via the leaves.

Fungi will develop on shoots and leaves which have fallen to the wood floor

The disease blocks the vessels in the tree along which nutrients flow.  The leaves fall off, the branches die back to the trunk, often forming a lozenge-shaped scar, before the disease spreads upwards to affect the crown.

“The thing to look for is a bronzing brown of the branches, starting at the tips,” said Geoff

It was time to ask some hard questions.  Geoff has worked in the National Park Authority’s woodlands team for thirty years.  Had the coming of ash dieback ruined his life’s work?  And will it really turn out as bad as it currently seems?

He answered “no” to the first question:  “Over the years, our tree planting schemes have been ash woodlands.  But we’ve always planted a mix, using hawthorn, holly, hazel, alder, and sometimes oak. So even if we lose the ash out of the woodland we’ve planted, we’ve still ‘future proofed’ and got the right structure of trees when and if ash makes a comeback.”

It was a “yes, but” to the second question:  “The vast majority of native woodland in the Yorkshire Dales National Park is ash woodland on limestone. The ash is the dominant tree, just as it is in the Lake District and the Peak District.  From a landscape perspective, the ash is absolutely iconic.  Losing it will be nothing short of disastrous.  Big trees with big trunks will last longer. It could be years before we see them dying back.  Ash trees are not going down without a fight, but the consensus is that they are losing the fight.  The disaster will unfold over the next 20 years or so.

“But it may well be that ash keeps regenerating and dying – over and again – until the ‘Darwin effect’ kicks in and trees with immunity begin to grow.  Important research is being carried out by the Forestry Commission to try to identify and develop ash trees that are most tolerant to the disease. My expectation is that some ash trees will be resistant to the disease.”

Ash – the ‘absolutely iconic’ tree of the Yorkshire Dales

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