Ash dieback – a fatal disease of Britain’s native ash trees (Fraxinus excelsior) – is one of the worst tree disease epidemics the UK has ever seen. The disease is caused by a fungus that originated in Asia but is thought to have arrived in Europe on exotic plants in the early 1990s, where it has devastated native ash species which have very little natural immunity.

Ash dieback has since spread ferociously throughout Europe due to airborne spores and trade in ash saplings which have no visual symptoms of the disease. In 2012, the disease was confirmed in the UK and later shown to have been imported on saplings to multiple sites across the country. It is now found throughout the UK. There’s no cure and very few trees show signs of long-term resistance.

The environmental impacts of the disease are likely to last a long time, but as our new paper explains, they’ll also carry a shockingly high economic cost.

There are 150m mature ash trees in the UK, making ash one of the most common native tree species in the country. We estimate that ash dieback will kill at least 95% of ash trees and cost the UK economy £15 billion – a cost one third greater than that reported from the foot-and-mouth disease outbreak in 2001. Half of this cost will arise in the next ten years.

Putting a monetary value on ecosystem services – the beneficial effects that trees provide for people and the economy - helps people understand the scale of the problem. Roughly £10 billion worth of ecosystem services will be lost as ash trees disappear.

Losing these services will have wide-ranging consequences. Less carbon dioxide will be absorbed from the atmosphere and the risk of flooding will increase. Studies have also shown that losing trees from a community is linked to poorer physical and mental health among the people who live there. Tackling climate change calls for an enormous effort to plant trees but ash dieback will rob the UK of using this valuable native species.

Clearing up dead and dying ash trees will carry another major cost, particularly where they present a risk to human safety. Stricken ash trees are prone to shedding limbs or collapsing completely, either directly due to the ash dieback fungus or a secondary pathogen such as honey fungus infecting the weakened tree. More than 4m ash trees line Britain’s roadsides. Felling these will be expensive and involve road closures and power and communications outages as work is carried out.

Ash trees in towns and cities will need the same treatment. A major national replanting effort could reduce the total cost of losing ash trees by as much as £2.5 billion, but a diverse mixture of native species will need to be planted to improve the resilience of new trees to pests and diseases. Replanting should also be carefully managed to ensure habitats are connected throughout the landscape.

Rising from the ashes

Exotic disease is not a problem limited to ash trees. People move plants – and unwittingly, their diseases – around the world at rates that far outstrip natural disease spread. The international trade in plants, travel and climate change are all contributing to an acceleration in the rate of new tree diseases emerging and spreading.

More tree pests and diseases have arrived in Britain in the last 40 years than at any time before then. As more native species are threatened, the effects will combine and multiply. Losing most ash trees will be bad enough, but what if the UK loses oak next, or birch? The idea of a landscape largely devoid of trees is appalling, and the economic costs incalculable.

People aren’t powerless in this story though. The science is clear that the largest pathway for spreading tree diseases is the international trade in live plants and soil. Stricter controls on this trade could better protect our trees for generations to come.

Most countries prioritise the value of trade in live plants over the risks to their native flora. Our paper shows that the value of the annual trade in ash saplings amounted to only 2% of the estimated cost of ash dieback.

The costs of restricting trade and improving border controls have long been used to block the introduction of stronger biosecurity measures for plants. But we now know that the costs of diseases like ash dieback have been wildly underestimated and this new evidence demands an urgent rethink.

The health of native trees, in fact of all wildlife, needs to be valued far more highly. We must recognise not only the essential benefits that the natural environment provides for us, but how severe the consequences are for society when new pathogens are spread.