Carbon stocks in harvested wood products

 

Harvested wood and wood products

A description of wood as a material reads like the stuff of science fiction. A complex, fibrous bio-polymer that is produced naturally as a by-product of the growth of shrubs and trees on this planet, wood in its many forms can vary in density by almost as much as an order of magnitude. It can be supple enough to weave into baskets and fences, rigid and strong enough to build houses, offices, even aircraft and once pulped it can be formed into paper with wide-ranging qualities. Chipped into a variety of particles and combined with glues, it can be machined into soft, sound-absorbent blocks or hard, decorative mouldings for use in furniture. Wood is also used as a feedstock in chemical production, and of course as a direct source of energy. Most recently even brands of margarine have been made from it. This brief list hardly does justice to the range and subtlety of applications for wood. Other materials such as metals and plastics can also be put to an impressive diversity of uses. Compared to these materials, however, wood is almost unique in being renewable – it is possible to literally re-grow what we use.

Carbon stocks in harvested wood products in the UK

The prime and arguably only source of information on UK carbon stocks and dynamics in the UK is Alexander (1997). Sources of data relevant to the assessment of HWP carbon stocks, either by direct inventory or as input to models, were exhaustively reviewed and defendable, conservative estimates of carbon stocks in different wood product categories and landfill were derived. The development of carbon stocks in wood products in primary and secondary use in the UK over the period 1965 to 2000 is shown below. The total carbon stock in viable HWP in the year 2000 is estimated as between 80 and 90 MtC, of which approximately 65% is accounted for by carbon in structural timber in domestic and commercial buildings (with the estimate for commercial buildings believed to be particularly conservative). If carbon in non-structural timber (eg internal door frames, furniture, card and paper) in domestic and commercial buildings is included, the proportion of the total viable HWP carbon stock accounted for rises to 85%, and if stocks in fencing are also included, over 90% is accounted for.

However, it it is thought that the carbon in wood disposed (currently and historically) to landfill in the UK constitutes an even larger pool than that associated with wood products in use. Estimates of carbon stocks in HWP in landfill are much more uncertain than for viable HWP, and have been estimated for the year 2000 by Alexander (1997) at 216 MtC, and to be increasing by up to 5 MtC/yr.

At present, the wood products pool in the UK (excluding landfill) is estimated to be increasing at a rate of 0.44 MtC/yr. The fact that carbon stocks associated with HWP bear comparison with those for living biomass in UK forests is a consequence of the relatively small area of forests in the UK compared to the volumes of wood consumed and the dominant contribution (about 85%) from imported timber.

Global socks of carbon in harvested wood products

At the global scale, the contribution from harvested wood products to the forest carbon sink is much less important - global stocks of carbon in HWP are approximately 5 GtC representing less than 1% of carbon stocks in biomass, and this stock is only rising at 1/30th of the rate that vegatation carbon stocks are increasing as a result of enhanced growth. Changes in carbon stocks associated with harvested wood products is one of the activities that may in future form part of national carbon accounting. Wood products have a broader role in the carbon balance beyond simply contributing to forest carbon stocks.

The role of harvested wood products in climate change mitigation

For wood products to sequester carbon, the size of the reservoir must grow – simply replacing old products with new does not increase the amount of carbon in the wood products pool. There is only a limited potential for this reservoir to grow as, the market for wood and wood products is finite. However, wood products can make a far more important contribution by substituting for materials such as concrete and steel which have high fossil fuel emissions associated with their production. The importance of this role is compared with that of carbon sequestration in forest biomass in the example given below. This highlights how, in the long term, emissions reductions resulting from the use of wood represent an ongoing benefit, compared to the one-off gain in forest biomass.

A simulation of carbon stock changes in a forestry system together with carbon emissions avoided through reduced fossil fuel consumption arising from wood utilisation. The simulation was produced using the carbon accounting model CO2fix and represents a ‘typical’ commercial, even-aged stand of Norway spruce growing in Europe on a rotation of 75 years and subjected to periodic thinning. In this example soil carbon represents the largest contribution to the stock of carbon in the forest ecosystem in the timescale considered. The contribution made by wood products harvested from the stand to overall carbon stocks is relatively small compared to stocks in forest biomass and soil. The carbon stock in wood products harvested from the stand rises over several rotations to a maximum, after which no further accumulation takes place. This is due to the relatively short lifetimes of wood products compared to the rotation period of the stand from which they are harvested. However, as more and more wood products and bioenergy are harvested through thinning and felling activities, a succession of opportunities arise to use harvested wood to avoid consumption of fossil fuels. As a result, the magnitude of the emissions reduction through avoided fossil fuel consumption accumulates indefinitely, in sharp contrast to the cycling of carbon stocks in the biomass, soil and wood products pools. By the end of the third rotation the emission reduction achieved is similar in magnitude to the sum of carbon stocks in biomass, soil and wood products and in subsequent rotations will greatly exceed them. Redrawn in adapted form from Nabuurs (1996).