A groundbreaking new study reveals that the impact of ash dieback disease on greenhouse gas emissions is far more profound than previously understood. Beyond the well-documented loss of living trees and decreased atmospheric CO₂ absorption, the disease triggers significant carbon release from soils in affected woodlands. This critical discovery emphasizes how expanding tree diseases globally could severely undermine forests’ role in climate change mitigation efforts and jeopardize current net zero strategies.
Ash dieback, caused by the invasive Hymenoscyphus fraxineus fungus, has decimated millions of ash trees across Britain. Researchers from the UK Centre for Ecology & Hydrology (UKCEH), in collaboration with Lancaster University, the Woodland Trust, and the University of Oxford, quantified not only the carbon forfeited through diseased biomass but also a previously overlooked mechanism: the degradation of soil organic carbon. This soil carbon loss manifests as increased greenhouse gas emissions from the woodland floor, compounding the environmental damage far beyond aboveground symptoms.
Using data collected via the Bunce Survey—a long-term ecological monitoring effort initiated in 1972 and repeated in 2001 and 2022—the research team conducted comparative analyses of soil carbon stocks in plots with and without ash dieback infestation. The results revealed an alarming trend: over a five-year span from 2016 to 2021, British woodland soils afflicted by ash dieback emitted approximately 5.8 million tonnes of CO₂, a figure that rivals half the annual carbon sequestration capacity of all broadleaf forests in Great Britain. This soil-based carbon emission represents a “triple whammy” that exacerbates climate impacts beyond tree death and reduced photosynthesis.
Lead ecologist Dr. Fiona Seaton highlighted the complexity of the carbon cycle disturbances induced by tree disease. “Our findings demonstrate that the presence of ash dieback disrupts belowground carbon storage and cycling processes,” she explains. Such disruptions may involve diminished root exudates, altered microbial communities, and accelerated decomposition of organic matter, all contributing to enhanced release of soil carbon. These belowground impacts have been overlooked in prior climate models and forest management plans, underscoring an urgent need to recalibrate projections and mitigation frameworks.
The implications extend beyond carbon dynamics to threaten ecosystem stability on multiple fronts. Soil organic carbon forms the foundational energy source sustaining diverse belowground organisms, including fungi, bacteria, and invertebrates intrinsic to nutrient cycling and soil structure integrity. The depletion of this organic matter compromises soil fertility and impairs ecosystem services essential for forest resilience. Moreover, widespread ash mortality diminishes habitat availability for numerous woodland fauna reliant on ash trees, further destabilizing biodiversity networks.
The study underscores a daunting prognosis: with an estimated nine million ash trees already lost and projections of up to 100 million more succumbing over the coming three decades, the cumulative threat to woodland carbon storage is immense. As the disease reduces the capacity of forests to sequester carbon, it simultaneously accelerates carbon release, creating feedback loops that could intensify atmospheric greenhouse gas concentrations and hamper climate stabilization targets.
Chris Nichols of the Woodland Trust emphasized the intertwined threats posed by tree diseases and habitat loss. “Ash dieback is not simply a conservation issue—it is increasingly apparent that its ramifications extend into climate change resilience. Protecting and managing our woodlands in light of such challenges is vital to uphold both biodiversity and carbon sequestration functions,” Nichols said. The Woodland Trust’s investment in research and conservation efforts is therefore essential to inform adaptive strategies.
The Bunce Survey’s longitudinal data have been instrumental in exposing shifts in woodland structure and function over the past five decades. Alongside the impacts of ash dieback, this dataset reveals trends toward shadier woodlands with denser canopies composed of fewer but larger trees. Such ecological transformations interplay with climate pressures, land-use changes, and biotic threats, necessitating a comprehensive understanding of their combined effects on forest carbon dynamics.
To further complicate matters, the study highlights the limited current knowledge about how other emergent tree diseases might similarly influence belowground carbon processes. Future work must incorporate soil health metrics and microbial interactions alongside traditional aboveground assessments to fully grasp the breadth of forest carbon feedbacks in a changing environment.
The research was conducted as part of an expansive collaboration funded by the Woodland Trust and the EU Horizon Europe research and innovation programme. Publication in Global Change Biology marks a significant contribution to the field, illuminating an important dimension of forest ecology that demands urgent attention. As policy-makers and environmental managers strive to meet ambitious net zero goals, integrating these findings into land management and disease mitigation frameworks will be critical.
Ultimately, this study presents a vital call to action: forests are not just carbon sinks but complex, dynamic systems vulnerable to disease-induced perturbations that ripple through carbon cycles above and below the ground. Recognizing and addressing these hidden pathways of carbon loss will be pivotal in safeguarding forests’ climate mitigation potential in the decades ahead.
Subject of Research: Effects of ash dieback on soil carbon cycling and greenhouse gas emissions in British woodlands.
Article Title: Forest topsoil organic carbon declines under ash dieback.
News Publication Date: 20-Aug-2025.
Web References:
References:
- Seaton et al. 2025. Forest topsoil organic carbon declines under ash dieback. Global Change Biology, DOI: 10.1111/gcb.70430.
- Bunce Survey report, UKCEH, 2024.
Image Credits: UK Centre for Ecology & Hydrology (UKCEH).
Keywords: Trees; Plant diseases; Climate change mitigation; Anthropogenic climate change; Carbon emissions; Soil science; Soils.
