Ash dieback is triple whammy for net zero plans

Ash dieback and other tree diseases are resulting in significantly more greenhouse gas emissions than previously thought because a large amount of carbon is escaping from woodland soils, a study has found. This is in addition to carbon losses from tens of millions of dying trees and reduced removal of CO₂ from the atmosphere due to the widespread deaths of mature ash trees.

The research team led by the UK Centre for Ecology & Hydrology (UKCEH) says that the exponential rise in tree diseases across the world are likely to hamper forests’ ability to help meet net zero plans.

They estimate losses from soils alone due to ash dieback in British woodlands resulted in 5.8 million tonnes of CO₂ emissions over five years (2016-21). This is equivalent to over half the amount of CO₂ removed from the atmosphere by broadleaf woodlands in Great Britain in a year, or alternatively the total amount of CO₂ emitted by cars travelling on Scotland’s roads annually.

While the loss of carbon from diseased trees themselves and reduced removal of CO₂ has been previously noted by scientists, the authors of the latest study say a third impact – the effects on carbon cycling and storage in the surrounding environment – have not been considered before.

“Tree disease is a triple whammy for climate change mitigation,” said UKCEH ecologist Dr Fiona Seaton, who led the study, published in Global Change Biology.

“A lot of future net zero plans assume a certain level of forest carbon sequestration but this is threatened by the exponential increase in tree diseases such as ash dieback in Britain and across the world.”

Widespread deaths of trees

An estimated nine million trees have so far died in British woodlands as a result of ash dieback, with up to 100 million others expected to be lost over the next 30 years. The disease is therefore going to continue to have significant impacts on woodlands’ ability to store and remove the greenhouse gas and mitigate climate change.

Organic carbon underpins the entire soil food web, so any loss will have negative impacts upon ecosystem services. But the study authors, from UKCEH as well as Lancaster University, the Woodland Trust and University of Oxford, say there is still limited knowledge about the impact of tree diseases on carbon cycling, especially below ground.

They say the difference in the amount of greenhouse gas released and absorbed across woodlands in response to tree disease needs to be better understood.

Jeopardising wildlife

Chris Nichols of the Woodland Trust, a coauthor of the study, said: “Ash are among the most common trees in the UK, and the imported dieback fungus is one of the biggest threats facing British woodlands. Not only does it jeopardise the wildlife that depend on ash, and risk costing the UK billions of pounds in the future, but we are increasingly understanding what dieback could mean for climate change resilience. This kind of research is vital in refining our approach to woodland conservation."

The research was carried out as part of an extensive survey of the physical and chemical composition of Britain’s broadleaf woodlands. The Bunce Survey, led by UKCEH, was carried out at multiple plots on around 100 sites across Britain in 1972, 2001 and most recently in 2022. This unique study shows how our woodlands have changed over the past 50 years due to climate change and land management.

The researchers measured the amount of soil carbon in topsoil in the surveyed woodlands, and compared how soil carbon levels changed in plots with and without ash dieback over the three surveys.

The research was funded by the Woodland Trust and the EU Horizon programme.

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Media enquiries

For an interview with one of study authors or further information, please contact Simon Williams, Media Relations Officer at UKCEH, via simwil@ceh.ac.uk or +44 (0)7920 295384.

Paper information

Seaton et al. 2025. Forest topsoil organic carbon declines under ash dieback. Global Change Biology. DOI: 10.1111/gcb.70430. Open access.

Notes to editors

The estimates for soil carbon losses and deaths from ash dieback mentioned in the press release relate to ash trees in British woodlands rather than all individual ash trees everywhere.

The Bunce survey, examining around 100 broadleaf woodlands in different landscapes across Britain, was initially undertaken in 1971 by the late Professor Bob Bunce and the subsequent two resurveys led by Professor Simon Smart of UKCEH. It has found that woods are shadier due to a lack of management over time, with fewer canopy gaps, resulting in woodland composed of fewer but older and larger trees.

A recently published separate analysis of data form the study found that UK woodlands are increasingly under threat from changes in land management, climate, diseases and deer populations – and vegetation is responding differently, requiring tailored action to protect biodiversity.  

About the UK Centre for Ecology & Hydrology (UKCEH) 

The UK Centre for Ecology & Hydrology (UKCEH) is a leading independent research institute dedicated to understanding and transforming how we interact with the natural world.  

With over 600 researchers, we tackle the urgent environmental challenges of our time, such as climate change and biodiversity loss. Our evidence-based insights empower governments, businesses and communities to make informed decisions, shaping a future where both nature and people thrive. 

ceh.ac.uk / BlueSky: @ukceh.bsky.social / LinkedIn: UK Centre for Ecology & Hydrology

About the Woodland Trust

The Woodland Trust is the largest woodland conservation charity in the UK with more than 500,000 supporters. It wants to see a world where woods and trees thrive for people and nature.

Established in 1972, the Woodland Trust now has over 1,000 woodlands in its care, covering more than 30,000 hectares. Access to the Trust’s woods is free so everyone can benefit from woods and trees.

The Trust has three key aims:

• protect ancient woodland, which is rare, unique and irreplaceable

• restoration of damaged ancient woodland, bringing precious pieces of our natural history back to life

• establish native trees and woods with the aim of creating resilient landscapes for people and wildlife