Background
The authors note that old-growth forests have generally been considered to "represent carbon sources or are neutral (Odum, 1963, 1965)," stating that "it is generally assumed that forests reach maximum productivity at an intermediate age and productivity declines in mature and old-growth stands (Franklin, 1988), presumably as dead woody debris and other respiratory demands increase."  More particularly, they report that a number of articles have suggested that "old-growth conifer forests are at equilibrium with respect to net ecosystem productivity or net ecosystem exchange (DeBell and Franklin, 1987; Franklin and DeBell, 1988; Schulze et al., 1999), as an age-class end point of ecosystem development."

What was done
Paw U and 13 colleagues used an eddy covariance technique to estimate the CO2 exchange rate of the oldest forest ecosystem (500 years old) in the AmeriFlux network of carbon-flux measurement stations - the Wind River old-growth forest in southwestern Washington, USA, which is composed mainly of Douglas-fir (Pseudotsuga menziesii) and western Hemlock (Tsuga heterophylla) - over a period of 16 months, from May 1998 to August 1999.

What was learned
Over the entire period studied, the scientists report "there were no monthly averages with net release of CO2," and that the cumulative net ecosystem exchange showed "remarkable sequestration of carbon, comparable to many younger forests."

What it means
Paw U et al. conclude that "in contrast to frequently stated opinions, old-growth forests can be significant carbon sinks," noting that "the old-growth forests of the Pacific Northwest can contribute to optimizing carbon sequestration strategies while continuing to provide ecosystem services essential to supporting biodiversity."  This conclusion is also supported by the study of Carey et al. (2001), whose work is described in our Carbon Sequestration Commentary Will Forest Carbon Sink Capacity Fade Away as Trees Age?.  But how can such old trees maintain their photosynthetic prowess so well?

Cary et al. suggest that long-term recruitment and the periodic appearance of additional late-successional species (increasing biodiversity) may be responsible for the phenomenon, infusing the primary unit of concern (the ever-evolving forest "super-organism") with greater vitality than would have been projected on the basis of the characteristics possessed by the unit earlier in its life.  Another possibility is that the growth-promoting effect of the 100-pm (36%) increase in atmospheric CO2 concentration experienced over the lifetimes of the ancient trees inhabiting old-growth forests has nearly compensated for what otherwise would have been the gradual reduction in vigor typically projected for them by most forest ecologists in years and decades past on the basis of the composition of the atmosphere surrounding the units earlier in their lives.

References
Carey, E.V., Sala, A., Keane, R. and Callaway, R.M.  2001.  Are old forests underestimated as global carbon sinks?  Global Change Biology 7: 339-344.

DeBell, D.S. and Franklin, J.S.  1987.  Old-growth Douglas-fir and western hemlock: a 36-year record of growth and mortality.  Western Journal of Applied Forestry 2: 111-114.

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.

Franklin, J.F. and DeBell, D.S.  1988.  Thirty-six years of tree population change in an old-growth Pseudotsuga-Tsuga forest.  Canadian Journal of Forest Research 18: 633-639.

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.

Schulze, E.-D., Lloyd, J., Kelliher, F.M., Wirth, C., Rebmann, C., Luhker, B., Mund, M., Knohl, A., Milyuokova, I.M. and Schulze, W.  1999.  Productivity of forests in the Eurosiberian boreal region and their potential to act as a carbon sink: a synthesis.  Global Change Biology 5: 703-722.