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Laser scanning reveals underestimation of above-ground biomass in Wytham Woods test site

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New evidence suggests assumptions of tree size-to-mass scale-invariance is not well-justified for this kind of forest, which has wider implications for climate mitigation actions.

Wytham Woods test site
Wytham Woods test site

UK forests could store almost double the amount of carbon than previous calculations suggest, with consequences for the understanding of carbon stocks and climate actions, according to a paper published today based on data produced in MetEOC.

The paper, Laser scanning reveals potential underestimation of biomass carbon in temperate forest, published in the journal Ecological Solutions and Evidence presents new empirical evidence showing that the fundamental assumption of tree size-to-mass scale-invariance is not well-justified for this kind of forest. The article goes on to suggest that testing the underlying assumptions of allometric models more generally is an urgent priority due to its wider implications for climate mitigation.

The study was a collaboration between researchers from the National Physical Laboratory, UCL, UK’s National Centre for Earth Observation (NCEO), the Universities of Ghent, Oxford and Tampere, and Sylvera.

The study made the national and international news, with a particularly strong publicity showing across the BBC (featuring on BBC News, BBC Radio 4, and online). It was also featured in Belgian news VRT.

Headline findings

  1. Above ground biomass (AGB) is 409.9 t ha−1; 1.77 times more than current allometric model estimates.
  2. The current allometric models underestimated partly due to the limited calibration data (i.e. under-representatiion of the range of trees) used to derived these models but also as a result of the assumptions engrained in their formulation.
  3. New empirical evidence is presented that the fundamental assumption of tree size-to-mass scale-invariance is not well-justified for this kind of forest.
  4. The underlying data and allometric models discussed in the paper are widely used in the UK and, to a lesser extent, in Europe with suggesting that there are substantial biases in current AGB estimates, with knock-on implications for climate mitigation and policy.

Implications for considering role of forests in tackling climate change

The team used a laser scanner to measure hundreds of locations throughout a 1.4 ha area of Wytham Woods, Oxford. The laser scanner emits millions of light pulses and records the time taken by each to return to the sensor after it has reflected from objects in the scene. Subsequently, the position of everything in the local environment can be known to very high accuracy.

Wytham Woods test siteThey then used statistical modelling to calculate the mass and volume of the trees, and subsequently the carbon storage capacity of the area, and compared this to the findings of previous models.

The authors say that their study brings into question the certainty of estimates of forest carbon storage across the UK, particularly for the largest and most carbon-heavy trees, which are currently based on widely used models that estimate tree mass from the trunk diameter. It is likely that previous studies have been greatly underestimating forest biomass across the UK.

Study lead author Professor Kim Calders (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 50 cm in diameter, which are 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 place 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.”

Wytham Woods

Currently, the UK’s biomass stock reporting to the Food and Agriculture Organization of the UN is based on these allometric models, which the authors say have very likely resulted in significant under-reporting.

The authors say that their study could have implications for the role of forests in tackling climate change, with the potential underestimation of forest carbon stocks having both positive and negative consequences for climate policy.

Study co-author Professor Mat Disney (UCL Geography and the National Centre for Earth Observation) said, “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 hectare of woodland lost, we’re potentially losing almost twice the carbon sink capacity we thought.”

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

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