Background
It was long believed that forests reached their maximum level of productivity at an intermediate age and that productivity declined in mature and older stands (Franklin, 1988), as woody debris and other respiratory demands increased, which was presumed to lead old-growth forests to ultimately become carbon neutral or actual sources of carbon (Odum, 1963, 1965). Hence, little thought was historically given to keeping old-growth forests intact as a means of reducing the rate-of-rise of the air's CO2 content.

What was done
The authors conducted a literature survey to test the contrary hypothesis that forests continue to acquire and sequester carbon from the atmosphere for literally hundreds of years, compiling data from 519 plot studies conducted throughout the world's boreal and temperate forests (30% and 70% of the studies, respectively), but skipping the tropics because of the low number of tropical sites that possessed the net ecosystem production (NEP) and forest age estimates needed for their analysis.

What was learned
Luyssaert et al. report that "in forests between 15 and 800 years old, the NEP is usually positive; that is, the forests are CO2 sinks." In fact, they say that "young forests rather than old-growth forests are very often conspicuous sources of CO2 because the creation of new forests (whether naturally or by humans) frequently follows disturbance to soil and the previous vegetation, resulting in a decomposition rate of coarse woody debris, litter and soil organic matter that exceeds the net primary production of the regrowth."

What it means
In discussing the implications of their findings, the team of American, Belgian, British, French, German and Swiss researchers writes that "because old-growth forests steadily accumulate carbon for centuries, they contain vast quantities of it," and that "they will lose much of this carbon to the atmosphere if they are disturbed, so carbon-accounting rules for forests should give credit for leaving old-growth forests intact," and, we would add, for letting them continue to sequester even more carbon.

References
Franklin, J.F. 1988. Pacific Northwest Forests. In: Barbour, M.G. and Billings, W.D. (Eds.) North American Terrestrial Vegetation. Cambridge University Press, New York, New York, USA, pp. 104-131.

Odum, E.P. 1963. Ecology. Holt, Rinehart and Winston, New York, New York, USA.

Odum, E.P. 1965. Fundamentals of Ecology. Saunders, Philadelphia, Pennsylvania, USA.