Timber LCA: biogenic carbon storage, sustainable forestry and end-of-life

Environmental assessment of wood products is one of the most contested areas in LCA methodology. The reason is simple: a tree draws carbon from the atmosphere as it grows, and that carbon is stored in the product. How you account for that storage decides whether the result comes out negative or positive.

Accounting for biogenic carbon

EN 15804+A2 tracks biogenic carbon as a separate indicator and records it in both directions: carbon bound into the product appears negative in A1, and carbon released at end-of-life appears positive in the C modules. Over the full life cycle the two balance out, unless the product is permanently stored.

That explains why most 'carbon negative timber' claims are incomplete. A declaration looking only at A1-A3 counts the storage but not the release. In a full cradle-to-grave account, timber's advantage is not storage but low production energy relative to alternative materials.

The value of temporary storage

That is not to say storage has no value. Timber used as a structural element for fifty years keeps that carbon out of the atmosphere for fifty years. In climate policy terms, that is time bought towards abatement targets. Quantifying the benefit requires dynamic LCA methods, though, and EN 15804 does not bring those into standard reporting.

The sustainable forest management condition

Treating biogenic carbon as net zero depends on the harvested forest regrowing. In a sustainably managed forest, harvest stays below annual increment and the carbon stock is maintained. Where harvesting drives deforestation the situation reverses and land use change emissions enter the calculation.

• Origin certification (FSC, PEFC) is a methodological requirement, not a marketing choice
• Harvest intensity and rotation length create a need for regional data
• Forest road and harvesting machinery fuel sit within A1
• Kiln drying is the largest energy line in the wood product chain
• Adhesives and resins are small by mass but fossil-derived

Drying and adhesives: the hidden burden

Raw timber's low production energy rises quickly in processed wood products. Kiln drying takes substantial thermal energy to bring moisture from around sixty per cent down to twelve. In particleboard, MDF and glue-laminated timber, adhesives enter the picture; urea-formaldehyde or isocyanate resins are fossil-based and carry impact well above their mass share.

The end-of-life scenario decides the result

What happens to timber at end-of-life shifts the total result more than for most materials. Burned with energy recovery, the stored carbon is released but displaces fossil fuel. Sent to landfill, anaerobic decay produces methane — whose effect is far higher than CO₂. Reused or recycled, the storage period extends.