Carbon sequestration in UK woodlands twice as high as expected

Research from University College London (UCL) suggests that the carbon sequestration capacity of UK woodlands is potentially double that of previous estimates.

Carbon sequestration occurs in forests by capturing carbon dioxide from the atmosphere and turning it into biomass via photosynthesis. The new research infers that UK forests could store around double the amount of carbon that previous calculations suggest, potentially revealing novel ways to combat climate change.

The collaborative study, ‘Laser scanning reveals potential underestimation of biomass carbon in temperate forest,’ involved experts from the UK’s National Centre for Earth Observation (NCEO), the Universities of Ghent, Oxford and Tampere, the National Physical Laboratory, and Sylvera.

The researchers believe their findings could have significant implications for combatting climate change. However, the potential underestimate of forest carbon storage could have positive and negative consequences for climate policy.

Professor Mat Disney, the study’s co-author from the UCL Geography and the National Centre for Earth Observation, explained: “Forests currently act as a carbon sink in the UK. However, whilst our finding that the carbon storage capacity of typical UK woodland could be nearly double what we previously thought might seem like a purely positive outcome, in practice, this means that for every ha of woodland lost, we’re potentially losing almost twice the carbon sink capacity we thought.

“This has serious implications for our understanding of the benefits of protecting trees in terms of climate mitigation – and deforestation and afforestation targets more broadly.”

Current carbon storage may be underestimated considerably

The team believes that their investigation brings the accuracy of forest carbon sequestration in the UK into question, as estimates for the largest and most carbon-heavy trees are predominantly based on models that estimate tree mass from the trunk diameter. This means that UK forest biomass has likely been underestimated considerably.

© iStock/LukeP77

Professor Kim Calders, the lead author of the study from Ghent University, said: “Currently, most estimates of forest carbon stocks are based on simple allometric models that assume that a tree’s size and mass increase at a steady rate. Our findings show that relying on these models is problematic, as they are not representative of UK forests.

“While the models work well for trees smaller than around 50cm in diameter, which is fairly uniform in terms of their size and volume, this isn’t what we see for larger, heavier trees. These are far more complex when it comes to structure – and they vary hugely across places and species.

“It’s vital that we’re able to reduce uncertainty in forest carbon estimates, given that land use and forest protection and restoration, in particular, constitute a quarter of countries’ current commitments to their Paris Agreement targets.”

The UK’s biomass stock reporting to the Food and Agriculture Organization of the UN is currently based on these allometric models, which has likely caused the inaccuracies.

How effectively can UK forests store carbon?

To obtain a more accurate carbon sequestration estimate, the researchers performed 3D terrestrial laser scanning (TLS) analysis in a 1.4 ha section of Wytham Woods in Oxfordshire. TLS is a type of remote sensing technique in which millions of laser pulses are emitted to capture the environment and tree structures in 3D. The method was used to analyse the amount of aboveground biomass (AGB) of 815 trees.

Statistical modelling was then employed to calculate the mass and volume of the trees and the carbon storage capacity of the area. The results were 77% higher than previous estimates (410 t ha-1 of biomass vs 232 t ha-1).

Yadvinder Malhi, a co-author of the study from Oxford University, concluded: “Wytham Woods belongs to the University of Oxford and has witnessed over 70 years of detailed scientific research. This research shows how new approaches can yield surprises in even well-studied forests, with profound consequences for our understanding of forests and their role in tackling climate change that apply across the UK and beyond.”

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