Thursday, 3 January 2008

Forestry for carbon dioxide fixation. Philip Stewart. 1978

This is a prescient technical look at how we should be using trees to absorb atmospheric carbon. Published 30 years ago. Nobody was listening then.

By P. J. Stewart

[First published in the Commonwealth Forestry Review, vol. 57 no. 4, 1978. Slightly edited for style; content unchanged. The rate of industrial emissions of CO2 have since increased greatly, and the volume of forests has been reduced, but the biofuels programme may well have increased the rate of emission from forests and forest soils. I was not the first person to draw attention to the role of forests in the planetary carbon budget; the idea was mentioned in an article by Freeman Dyson ‘Can we Control the Carbon Dioxide in the Atmosphere?’, Energy, vol. 2, pp. 287-291. However, I believe I was the first to suggest specific forestry systems and to emphasize the importance of conserving wood and wood products.]

The world’s forests and forest soils contain between two and a half and five times as much carbon as the atmosphere, according to recent estimates (Woodwell et al. 1978). The way that humankind treats woodland can therefore have a major effect on the amount of carbon dioxide in the air. It has been estimated that deforestation may currently be adding at least as much of the gas to the atmosphere as the combustion of fossil fuels (Hutchinson, 1954; Woodwell and Houghton, 1976). The matter is therefore an important one.
The rate at which coal,oil and natural gas are being burnt is known quite accurately: 5 gigatons of carbon per year. The rate of release by deforestation is much less certain, with estimates ranging from 1 to 12 gigatons. Only 2.3 remain in the atmosphere (equivalent to an increase of 0.8 parts per million added to the current level of about 330 [2007: now about 2 p.p.m. added to 382] – as against 290 a century ago), the rest being absorbed by the top100 metres of the oceans. It appears however that this surface layer can only lose its carbon dioxide to deeper waters at a very slow rate and might rapidly become saturated. Current stocks of carbon are estimated at 7000 gigatons in fossil fuels, 765 in terrestrial biomass, 1000 to 3000 in soil organic matter, 700 in the atmosphere, 600 in marine biomass and 600 dissolved in the surface layer of the oceans, with a relatively inaccessible 38,000 deeper in the oceans. All these quantities are dwarfed by the millions of gigatons locked up in carbonate rocks and sediments, the ultimate destination of carbon on our planet.
Climatologists are concerned at the prospect of a rapid increase in carbon dioxide levels, though the extent and precise nature of the consequences is controversial (Smith, 1978; Siegenthaler and Oeschger, 1978). The gas absorbs and re-emits infra-red radiation, contributing to the ‘greenhouse’ effect of the atmosphere. Warmer air and more energy would mean increased amounts of water vapour – the other main infra-red absorber – and a magnification of the effect. There is also the disturbing possibility that the surface temperature of the sea might rise enough to reduce the rate at which carbon dioxide is removed from the atmosphere by the oceans.
It appears that the effect on temperatures is likely to increase with distance from the equator and that the polar ice caps might melt, raising world sea levels and slowly drowning coastal towns and regions. Nobody can tell whether this would be wholly undesirable (Kellogg 198). We might find ourselves on an altogether warmer and wetter planet, with luxuriant vegetation and diminished deserts. We might move out of the succession of ice ages and into a new and balmy equilibrium. On the other hand we might be overcome by changes too great and too rapid for us.
The purpose of this article is to examine the consequences for forestry and forest utilization if governments were to decide to reduce the rate of increase in atmospheric carbon dioxide levels. Unless some way is found of injecting the gas directly into the ocean depths, it is likely that the main effort would fall on foresters, since about 90 % of the world’s biological organic matter is found in forests and forest soils. To give an idea of the scale of activity required, it is enough to compare the net annual addition of carbon to the atmosphere, 2.3 gigatons, with the carbon content of forest vegetation, 700 gigatons, and forest soils, say 1000 gigatons (a minimum estimate). It would be sufficient to increase forest carbon by about 0.2% per annum to cancel the atmospheric effect; this should be well within the bounds of possibility.
It is important to not two points. Firstly, it is not growth rates that count but the difference between these and destruction rates; high increment obtained at the cost of exposing the forest soil to faster oxidation would mean a net loss, and high increment balanced by rapid exploitation would not necessarily accomplish anything. Secondly, the destruction rate is not the rate at which trees are felled but that at which wood and humus are oxidized; timber removed from the forest and preserved in the form of buildings, furniture, paper and other artifacts is still fixing carbon oxide as long as it is not burned or allowed to rot. The oak beams of medieval churches are as much part of the stock of wood carbon as are standing trees. Failure to take account of this has perhaps flawed many estimates of the contribution of deforestation to atmospheric carbon dioxide, but there are no satisfactory statistics on the average duration of wood preservation after felling.
The overall strategy that would make the greatest possible contribution to fixing carbon dioxide might therefore be to grow wood as fast as possible, to harvest all of it, and to make it last as long as possible after exploitation. It is easy to conceive of a situation with the greater part of the world’s wood held in the form of durable goods and a relatively small part in the form of lightly stocked, fast-growing forests. However, this is unlikely to be feasible if only for economic reasons. To produce wood at such growth rates would be very costly, but the abundance of the supply would make for a low unit price and would discourage preservation and recycling. In any case, the system might cause too rapid a loss of humus by exposing the forest floor, cancelling out the faster fixing of carbon in wood. Nevertheless, fast-growing plantations have a useful part to play and should not be condemned.
Rather than a general policy of fast growth and much more effective wood preservation, it is likely that a whole range of measures would be expedient, each appropriate to particular situations. The following possibilities are all of interest:
 Halt deforestation as far as possible, except in special circumstances, and intensify agriculture on existing farmland rather than clear new areas; tariffs might be placed on the imports from countries known to deforest without good cause.
 Increase the stocking of existing forests, and close the canopy where it is open; careful study would be needed to find the point of maximum biomass, which might not coincide with that of maximum timber volume.
 Favour high density woods where these are exploitable; it might be necessary to learn to think in terms of tons rather than cubic metres per hectare.
 Maintain secondary vegetation, especially where it has a role in protecting the soil and favouring the activity of soil organisms.
 Maximize soil organic matter by appropriate choice of species, maintenance of optimum tree cover, limitation or elimination of grazing, and by erosion-control measures.
 Combat forest fire, which is perhaps the most potent single cause of rapid carbon mobilization.
 Extend the forest area by plantation and encourage the planting of isolated trees, shelterbelts, roadside trees, small woodlots etc.
The preservation of wood after felling is only partly within the ambit of forestry. Certain silvicultural practices could be modifies to postpone the return of unharvested wood and foliage to the atmosphere. The burning of lop and top, in particular, would be something to avoid. Whole-tree harvesting is harder to judge; on the one hand it maximizes the amount of organic carbon converted into durable products, but on the other it reduces the amount returned to the soil. Only careful study could show the balance of advantage in any particular case.
It rests with the manufacturers and still more with the consumers of wood products to lengthen the stay of carbon in solid form once it leaves the forests. It is not proposed to go into detail in the present article. One subject however demands attention: fuel-wood. With the rising price of petroleum there has been a surge of interest in forests as a source of energy, but it might seem that a policy of favouring carbon fixation must seek to minimize the burning of wood. Foresters need to know whether there is a future for fuel plantations, for it affects much current research. A number of broad considerations are offered here.
The use of wood as fuel is not undesirable in a framework of carbon fixation as it seems at first sight. In the case of a forest whose main product is timber, the wood that is burnt would probably otherwise rot, remaining only a few years before returning to the atmosphere. In the case of a fuel-wood plantation, for every ton burnt there are many tons of growing stock on land that might otherwise carry none; the net effect on the carbon dioxide balance is positive.
If fuel-wood – or wood-derived fuel – replaces coal, oil or natural gas, then the fossil fuel can either be left in the ground or be converted into durable goods that may last for many years. The use of fuel-wood is part of a cycle, balanced by photosynthesis, whereas the combustion of fossil fuels is a once-and-for-all mobilization of carbon that otherwise would remain fixed indefinitely. One way to refix the carbon thus freed would be to incorporate it into fuel-wood plantations, thus creating renewable sources of energy.
The use of wood for fuel can thus be incorporated without difficulty into a system of forestry for carbon fixation. It is nevertheless desirable that such use should be as economic as possible. Improved designs of stove and the use of lids on pots, of pressure cookers and of insulation can greatly increase the efficiency of energy conversion. Used wood and paper should be used in preference to freshly grown wood, especially if the alternative is to get rid of it in wasteful bonfires. But these are counsels of thrift that are valid even if there is no anxiety about releasing carbon dioxide.
From all this it is clear that, if asked to do so, foresters could start tomorrow helping to take carbon dioxide out of the atmosphere. A period of research and experimentation would enable them to do so even more effectively. It is up to the climatologists, the informed public and governments to decide what they want done – and the sooner the better.


Hutchinson, G. E. (1954) in Kuiper, G. P. (Ed.) The Earth as a Planet, Chicago Univ. Press.
Kellogg, W. (1978) Effects of Human Activities on Global Climate, World Meteorological Organization, Technical Notes, 151.
Siegenthaler, U. and Oeschger, H. (1978) Predicting Future Carbon Dioxide Levels, Science vol. 199, pp. 188-195.
Smith, I. (1978) Carbon Dioxide and the “Greenhouse Effect”, an unresolved Problem. Report no. 1CTIS/ER 01, IEA Coal Research, London.
Woodewell, G. M. et al. (1978) The Biota and the World Carbon Budget. Science, vol. 199, pp. 141-146.
Woodwell, G. M. and Houghton, R. A. (1976) Biota Influences on the World Carbon Budget. In W. Stumm (Ed.) Global Cycles and their Alteration by Man, 1977, Dahlem Conferenzen, Berlin.