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How Does Timber Sequester Carbon?

chopped timber

 
 
Timber and sequestered carbon

Timber is a renewable material, which can be sustainably produced with good forest management. Schemes to monitor and ensure sustainable forest management such as FSC® and PEFC have been widely taken up by the industry.

During forest growth, the trees sequester carbon dioxide by photosynthesis. The timber and other biomass are all derived from this process of converting carbon dioxide and water into sugars - these become cellulose and other components of the tree.

When the forest is harvested, much biomass is left on the site (roots, leaves, brash etc) and the timber is the main crop. The cellulose and other structural polymers in the timber still contain the CO2 removed from the atmosphere by the tree. This is not released back into the atmosphere until the timber either decomposes or is burnt for energy recovery at end of life.

There is increasing interest in quantifying the amount of sequestered carbon stored within timber - for example in long-term applications such as decking, cladding, structural timbers or joinery. Typically, the quantity of stored carbon is related to the weight of timber. Timber and biobased materials (e.g. paper, plywood etc) are the only materials which offer this storage of sequestered carbon.

LIFE CYCLE ASSESSMENT

For all materials, a process called life cycle assessment (LCA) can be used to quantify the amount of carbon dioxide which is emitted during manufacture. It includes other greenhouse gases as carbon dioxide equivalents.

In the case of timber, this CO2 would be associated with processes such as the forest management, harvesting, transport of logs, sawmilling, kiln drying etc. LCA studies allow comparison of the balance of carbon dioxide emitted in manufacture of one material with another, e.g. timber, PVC, steel, concrete, by comparing a functional unit of each material.

Timber compares favourably with many construction materials on carbon emissions. When timber is used in place of these other materials, a substitution effect is seen, reducing the total embodied carbon of the building. Recent studies by the BioComposites Centre, and by the NHBC, have shown that the combination of materials used in timber framed housing have lower carbon emissions than the materials used in masonry housing.

 
In addition, some studies have used an LCA approach to compare the quantities of stored carbon in timber with LCA data for the carbon emissions in manufacturing the same quantity of timber - this indicates the carbon balance. Caution is needed as different approaches are taken in accounting for the duration of storage, and experts are still to agree on methods. However, such studies indicate very negative emissions for timber itself, and studies evaluating systems which use timber (e.g. in housing) also demonstrate a benefit. This result led the Committee on Climate Change in the UK to favour the use of timber in long term applications over its use in biomass energy.
 

Dr Morwenna Spear FIMMM is a Research Scientist at the BioComposites Centre, Bangor

Dr Morwenna Spear

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