Wood products

 

Wood products are a carbon reservoir and can act as a carbon ‘sink’ if the size of this reservoir is increased by making use of more wood products. However, wood has a far more significant role to play. Because it is a renewable and relatively energy-efficient source of material, carbon dioxide emissions can be reduced by using wood in place of more energy-intensive materials.

Product substitution

Manufacturing products from wood instead of other more energy-expensive materials is recognised as having the potential to reduce greenhouse gas emissions. This potential role of wood products can be illustrated by this example of the energy required to manufacture an everyday kitchen spoon from either wood, stainless steel or plastic:

Making the spoon from wood requires about one thirtieth of the energy needed to make a steel or plastic spoon, and the associated greenhouse gas emissions are also lower. The suggestion is that if we actively chose to make kitchen spoons out of wood rather than steel or plastic, we could cut greenhouse gas emissions by up to 95% per spoon. It's important to stress that this example is intended to illustrate the principle of product 'substitution' – in the real world the choice of materials for making goods is very complex. There are many examples of products that could never be made out of wood, or for which wood is not the most suitable material. Some products, such as buildings, will always need to be made out of a mix of materials. However, research into the environmental impacts of building construction suggests that increasing the use of wood in place of other materials (where this is practical) could cut greenhouse gas emissions by between 40% and 80% per building, depending on the type of building.

The principle of 'product substitution' is well understood and there have been a number of 'life cycle analysis' research case studies. However much less is known about the actual potential for different economic sectors or regions of the world or within countries to reduce emissions through increased use of wood, and there are no formally derived estimates of global potential.

Wood products as a carbon stock

Apart from product substitution, wood products contain carbon removed from the atmosphere by trees. Measures that encourage increased use of wood products should result in carbon stocks in wood products increasing, with the result that carbon is sequestered. The main factors determining the carbon stock in a particular wood product are:

• The amount of wood contained in an individual product.
• The number of products of this type retained in service.

Both these quantities are determined entirely by humankind, limited only by the availability of suitable wood. When combined with the service life of the wood product, which ultimately depends on natural processes of decay and attrition but may also be strongly influenced by human intervention, these quantities determine the magnitude of the in-flow of carbon in the form of harvested wood, which is also mediated by humans. Thus in essence, humankind decides the level of stocks of a particular wood product and then controls the in-flow (harvesting/production) and out-flow (disposal/destruction) accordingly.

This picture of the dynamics of carbon stocks and stock changes in wood products contrasts sharply with that for other terrestrial and also aquatic carbon pools. In these cases, rates of in-flow and out-flow of carbon to and from a pool are usually the result of natural processes, and the stock of carbon in the pool is formed by the balance between quantities of carbon that enter and leave. Any attempts by humankind to control carbon stocks are usually limited to influencing the natural processes of in-flow and out-flow, for example by establishing stands of trees on sparsely vegetated land or by reducing disturbances made to soil. In contrast to the situation for wood products, it is very rare for humans to be able to exercise complete control over these processes.

However, compared to forest biomass and forest soils, and to emissions from burning fossil fuels, carbon stocks (and stock changes) in wood products are very small. Carbon stocks in longer-lived wood products in primary and secondary use around the world are estimated to be about 2–3 gigatonnes of carbon. Estimates including stocks in short-lived products and landfill are more approximate, but current evidence suggests that total world carbon stocks in short-lived and long-lived wood products and landfill amount to no more than 25 gigatonnes carbon. This is equivalent to about 2–7% of the carbon in biomass in world forests, and just 1–2% compared to carbon in forest biomass, litter and soils under forests. Research studies also suggest that the wood products carbon pool is currently expanding (i.e. sequestering carbon) at a rate of no more than 0.1 gigatonne per year. This should be compared with the global estimate for release of carbon from vegetation to the atmosphere of 1.6 gigatonnes per year, primarily due to deforestation (and not counting absorption due to vegetation growth). The value for global carbon sequestration in wood products should also be compared with the estimated global emissions of carbon dioxide to the atmosphere due to burning fossil fuels of 6.3 gigatonnes per year.

Although the contribution of wood products as a carbon stock appears to be small, there may be specific situations where the sequestered carbon is a beneficial add-on to other potential benefits of using wood as a material such as product substitution.