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.
