A team of researchers from the University of Oxford, FERA Science, Sylva Foundation and the Woodland Trust has calculated the true economic cost of ash dieback (ADB). The predictions, published in the journal Current Biology, are staggering. I tried to find out the facts behind this claim, but this was not easy because the only way to read the full article is to pay $31.50 to the publishers, Cell Press. I was reluctant to pay.

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It seems that everyone else was also reluctant to pay because, although the headline and published summary has been widely reported and repeated in newspapers, television and social media, I have searched the internet but only been able to get snippets of information about how the authors have reached the sum of £15 billion. Eventually curiosity got the better of me and I paid up to download the paper.

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The total putative cost of £15 billion comes from several components which I have outlined below.

1. Loss of ecosystem services

The largest cost of ash dieback disease (£9 billion) is apparently due to the loss of ecosystem services e.g. water and air purification and carbon sequestration.

It appears that the authors have calculated the value of ecosystem services provided by a single ash tree and multiplied that by the number of ash trees that they estimate will be killed by ADB.

The authors estimate that 95% of ash trees in Britain will be killed by ADB within 10 years based on published scientific papers.

So far I have hardly noticed a single mature ash tree killed by ADB disease. Plenty of ash trees are slowly dying from PHYSICAL die back due to stress caused by drought, flooding, root disturbance etc. Fungal root infections can then kill the whole tree, but ADB disease (previously known as Chalara) is a leaf infection that generally only affects the fast growing shoots of young ash trees. Young ash trees are not great contributors to ecosystem services and their loss will be quickly replaced.

The loss of ash trees in woodlands is estimated to cost £4 billion in lost ecosystem services, but individual trees within a woodland will always grow and die. As long as the habitat stays as woodland, the same ecosystem service will continue to be provided. The ecosystem service will only be lost if the woodland is clear felled or burnt to the ground.

The first paragraph of the article reads ” invasive tree pests and diseases present some of the greatest global threats to forests”. Pests and diseases kill trees, they don’t kill woodlands and forests unless they are monocultures.

A further £5 billion of ecosystem services are apparently lost when non-woodland ash trees suffer from ADB.

An isolated ash tree may stop providing its ecosystem services when it dies, but all trees die of something at sometime and a dead ash tree arguably supports more wildlife than a living one. Fungi and insects are able to colonise the dead tree because they are no longer repelled by the tree’s natural defences. Birds nest in the hollow trunks and young trees sprout up as more sunlight reaches the ground.

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2. Safety felling of dead and dying ash trees

Local authorities, utility providers and private landowners are said to be facing enormous bills (£5 billion) to fell and clear for safety reasons all the dead and dying ash trees that have been attacked by ADB disease within 25 metres of a public highway.

This astonishing claim is based on the lifespan of an ash tree being 300 years and ADB disease is the only reason why any ash tree would fail to reach that age! (Life expectancy of 300 years estimated from Ecoflora [S5] and Thomas P. (2000) “Trees: Their Natural History” [S6]. Even age distribution assumed). This shows an amazing lack of knowledge about tree population dynamics.

Roadside ash trees have suffered from physical die back long before Chalara appeared on the scene. Ploughing of the adjacent field has weakened many shallow rooting ash trees. More recently the 2018 drought caused extensive die back to ash trees which only became apparent in Summer 2019. The symptoms of ADB disease cannot be distinguished from physical die back in mature trees.

In recent years cash strapped local authorities have cut down on the felling of dangerous roadside trees so a backlog has developed. Little owls, Athene noctua, which like nesting in such trees, are benefiting.

As local authorities tackle this backlog the extra work being provided for tree work contractors is surely good for the British economy.

All the other supposed costs of ash die back disease amount to £1 billion of which the greatest is the cost of replanting trees that have died from ADB. But replanting woodlands is only necessary when ash has been planted as a monoculture.

If new woodlands are planted as an intimate mixture of several species rather than in single species blocks (mini monocultures) the ash trees would be less likely to suffer from ADB and, even if they did die, surrounding trees would soon fill any canopy gaps.

The extra cost of government grants to replace trees suffering from ADB disease could have been avoided if new woodland planting and management had been based on sound ecological principles.

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Scientific research into ADB disease

There is no doubt that many millions of pounds have been spent on research since ADB disease was discovered in Britain in 2012. The Government has decided to invest this money to breed resistant strains of ash which will eventually be reintroduced to the British countryside. Whether these trees will be able to cope with other environmental pressures is debatable. We should be widening the genetic base of our ash trees not narrowing it

Meanwhile, nature has been practising natural selection on the billions of young ash saplings that grow naturally on uncultivated ground throughout Britain. My personal observations are that where ADB disease has been present for 5 years or more, ash trees tolerant of the disease have outgrown and shaded out their susceptible siblings and are well on their way to becoming the dominant trees.

Studying ash trees and ash dieback disease “in the wild” involves continuous observations over a number of years, but may be more useful in understanding the natural world than dissecting the genes of Fraxinus excelsior and Hymenoscyphus fraxineus (ADB disease) in the laboratory.

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