The carbon accounting blind spot and why we need to keep large trees

The logic behind burning woodchips for fuel and treating this as a carbon zero sum game has to be questioned. Even before the burning imported wood pellets from Canada at Drax Power Station in Yorkshire is considered.

When examined this all raises an uncomfortable question about how we account for carbon emissions. We know that trees absorb carbon dioxide as they grow and release much of that stored carbon when they are burnt.  

If (outside forestry) trees are encouraged to grow and absorb more carbon, when the time comes from them to be felled (hopefully allowing them to compost down insitu or elsewhere, returning the carbon and nutrients  to the soil), it will be decades in the future, when we are (hopefully) past this climate emergency.

Why are we giving enough consideration to the carbon we could keep out of the atmosphere by protecting existing trees and allowing them to continue growing? asks Robert Wilkins at Ruskins, the tree and soil specialists.

In April 2025, the House of Commons Public Accounts Committee reported that approximately £22 billion of government support had been provided for biomass power and heat since 2002, including around £6.5 billion for Drax. The committee raised concerns about the assurance processes used to demonstrate that biomass was being sourced sustainably.

The accounting rules explain part of the controversy. Carbon dioxide released when biomass is burnt for energy is recorded in the land-use, land-use change and forestry sector, rather than counted again in the energy market. This is intended to prevent double counting. It does not mean the emissions disappear or that burning wood is automatically carbon neutral.

The actual climate impact depends on where the wood comes from, what would otherwise have happened to it, the emissions involved in processing and transporting it and how quickly the carbon released is absorbed again. The key question is whether burning the wood delivers a better outcome than the alternatives, including leaving trees standing and allowing them to continue growing.

When a tree is burnt, much of its stored carbon is released into the atmosphere immediately. If another tree is planted, it takes decades  to grow and meaningfully absorb carbon. The period required to recover the lost carbon depends on the species, growing conditions and how the forest is managed.

Forest Research, the UK’s forestry research agency, warns that accounting rules alone cannot establish whether forest biomass provides a climate benefit. The assessment must consider changes in forest carbon stocks alongside emissions from harvesting, processing and transport.

Drax says its wood pellets include sawmill residues, forestry residues and low-grade wood. These materials can have different environmental consequences depending on their origin and alternative uses. Genuine residues that would otherwise decompose are not necessarily equivalent to trees harvested specifically for fuel. This matters because the argument is not that all biomass is harmful, but that its carbon benefits must be demonstrated rather than assumed.

Why large trees matter

The same accounting debate raises a wider issue as to how do we value the carbon already stored in trees outside commercial forestry, including those in parks, gardens, streets, hedgerows and development sites?

A tree is not a fixed carbon store. While it grows, it can continue absorbing carbon dioxide and adding to the carbon held in its trunk, branches and roots. A study published in Nature in 2014 examined 403 tree species across tropical and temperate regions. It found that biomass accumulation increased with tree size for most of the species studied, demonstrating that large trees can continue to make a substantial contribution to carbon storage.

This does not mean every large tree captures more carbon annually than every young tree. Species, health and growing conditions all matter. But it does challenge the assumption that removing a large tree and planting several smaller ones automatically produces a better carbon outcome.

A large tree represents carbon accumulated over many years, as well as the potential for further growth. Replacement planting is important, but it cannot immediately recreate the carbon store that has been lost, and young trees are not guaranteed to survive.

Trees outside traditional woodland are particularly relevant to development. The Forestry Commission’s Urban Forestry and Woodland Advisory Committee Network reported in 2025 that trees outside woodland account for approximately 30% of England’s tree cover.

These trees can be vulnerable to removal when land is developed or infrastructure installed and their value extends beyond carbon. Large trees provide shade, support wildlife, intercept rainfall and help moderate local temperatures. Their roots and surrounding soils are also part of the wider ecosystem.

For developers and landowners, the starting point should be to understand what is already there and whether it can be retained. Early assessment of tree condition, root protection requirements and soil conditions can help project teams consider alternatives to removal, including layout changes or specialist tree moving where appropriate. The objective is not to prevent development. It is to ensure that the environmental cost of removing existing trees is understood before decisions are made.

Keep carbon in useful materials

Where trees must be harvested, the way their timber is used also matters. Burning wood releases much of its stored carbon quickly, while using suitable timber in construction, furniture or other durable products can retain carbon for longer.

Forest Research estimates that the UK’s wood-product carbon pool contains approximately 140 million tonnes of carbon. Longer-lived products generally retain carbon for longer than products with short service lives.

Timber products are not permanent carbon stores and their eventual disposal must be considered. Nevertheless, using wood in a durable product can offer a different outcome from burning it for energy. The choice should reflect the material’s best use and the full carbon consequences.

Net zero requires us to balance greenhouse gas emissions with removals, but the way emissions are recorded should not be confused with proof of an environmental benefit. The particular debate surrounding Drax demonstrates why biomass needs careful assessment. The carbon accounting framework is designed to avoid double counting, but the real climate impact still depends on the wood source, supply chain and changes in forest carbon stocks.

For the construction industry, developers and landowners, the lesson is straightforward, because reducing carbon means protecting existing carbon stores as well as seeking new ways to capture it.

We at Ruskins believe that large trees should be assessed for their existing carbon, future growth, soil relationships and wider environmental contribution before removal is considered. Where a tree can safely remain, protecting it may preserve benefits that replacement planting will take years to recreate.

Net zero should not make the loss of an existing carbon store appear inconsequential. If a large tree can continue growing, why release its stored carbon now and rely on a replacement to recover it over the decades that follow?