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America's Amazing Carbon Sink

An article in the October 16, 1998 issue of Science (see also ABSNews.com report) by S. Fan and others presents evidence that North America's second-growth forests south of latitude 51 absorb 1.7 (SE = 0.5) billion tons of atmospheric carbon annually. This claim, which is based on a study of north-to-south and west-to-east atmospheric carbon dioxide concentration gradients, is in sharp conflict with estimates derived from measurements of forest production.

According to the Food and Agriculture Organization (FAO) of the United Nations (2), forest production in the whole of North America (including the vast boreal forest, which lies north of latitude 51), is just under one billion cubic metres of harvestable stemwood per year. The FAO estimates North America's total timber harvest at just under 0.6 billion cubic metres of stemwood per year. Net accumulation of standing timber is thus estimated to be approximately 0.4 billion cubic metres per year, representing the accumulation of approximately 100 million tons of carbon. To this might be added up to 150 million tons of carbon contained in the 0.6 billion cubic metres of harvested timber on the optimistic assumption that all is converted to products which either have a life time comparable to that of a forest, or are indefinitely recycled without loss.

An annual increment of one hundred million tons of carbon in harvestable timber implies an accumulation of approximately 150 million tons of carbon in whole-tree biomass (i.e., including leaves, branches and roots as well as harvestable stemwood). In addition, carbon accumulates in forest soils in the form of leaf and branch litter, dead stems and roots. The accumulation of soil organic matter can exceed that in living biomass by a factor of as much as two. Thus, based on the FAO data, one can estimate that North America's forests comprise a sink for, at most, 150 million tons each in living forest biomass and harvested wood products, plus 300 million tons in forest soil organic matter, for a total of 600 million tons annually. But this implies a North American carbon sink only one-third the size of that inferred in the Science paper.

If the Science paper is correct, it implies that ground-based estimates of forest yield have overlooked two-thirds of North America's annual accumulation of timber (i.e., growth less timber harvested), or about 1.2 billion cubic metres. It might seem difficult to overlook that much wood--more than a cubic kilometer added to inventory each year. However, the undetected additional volume would be spread over 0.8 billion hectares, and would thus amount to no more than a millimeter or two in annual mean tree diameter growth, or just one or two cubic metres of timber per hectare.

Given that ground-based estimates of forest yield are dependent on plot measurements made over many years, some dating back more than half a century, they may not reflect the impact on tree growth of recent changes in climate and atmospheric chemistry. As the authors of the Science paper note, a small rise in temperature, accompanied by increases in atmospheric carbon dioxide concentration, nitrogen deposition rates and precipitation may provide a more potent stimulus to forest production than has yet been generally recognized.

But the results reported by Fan et al. are based on a limited data set and need confirmation with results from a more comprehensive atmospheric sampling network than that available at present. In the meantime, the study is bound to stir controversy. The United States and Canada, as signatories of the Kyoto agreement, must take steps to regulate their net carbon emissions. However, with combined emissions from fossil fuel combustion of 1.6 billion tons per year, i.e., less than the inferred forest carbon sink, both countries may be net absorbers of carbon.

Whatever the true magnitude of the North American forest carbon sink, the report by Fan et al. emphasizes the central role that forests play in regulating the composition of the atmosphere. If a massive North American forest carbon sink is confirmed, it will have major and, on the whole fortunate, implications; among them that timber, a vital industrial raw material, will be in abundant supply for the foreseeable future. North America alone could, if necessary, supply all of present world timber demand on a sustainable basis.

Further, the result implies that the productivity of second-growth forests in North America is so large that there is no urgent economic justification for allowing further inroads into North America's remaining stands of old-growth timber until their role both as a carbon reservoir in the global carbon cycle and in the maintenance of biodiversity has been clarified.

References

(1) Fan, S., M. Gloor, J. Mahlman, S. Pacala, J. Sarmiento, T. Takahashi and P. Tans. 1998. A large terrestrial carbon sink in North America implied by atmospheric and oceanic carbon dioxide data models. Science 282:442-446.

(2) Food and Agriculture Organization of the United Nations, 1992, 1993 and 1995, cited in: Sohngen, B., R.A. Sedjo, R. Mendelsohn and K.S. Lyon. 1996. Analyzing the economic impact of climate change on global timber markets. Resources for the Future. http://rff.org/proj_summaries/files/sedjo_climate_timber.htm.


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