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Old-Growth Forests of Tropical Africa
Volume 12, Number 26: 1 July 2009

For many years it had been assumed that forests reached their maximum productivity at an intermediate age, and that productivity declined thereafter in mature and old-growth stands, ultimately becoming neutral in terms of carbon sequestration or even negative, as they changed from carbon sinks to carbon sources. In recent years, however, this view has been radically revised, as several studies have demonstrated that (1) very old stands of trees still function as sinks of atmospheric carbon, and that (2) those very old carbon sinks are flexing their muscles and growing at an accelerating rate. (See our Editorial of 20 April 2005.) What is more, evidence for this phenomenon continues to pour in from around the world, the newest contribution coming from Africa.

Lewis et al. (2009) recently invested a great amount of time and effort in documenting changes in aboveground carbon storage in "79 permanent sample plots spanning 40 years (1968-2007), located in closed-canopy moist forest, spanning West, Central and Eastern Africa," based on data obtained from more than 70,000 individual trees spread across ten countries. This work revealed, in their words, that "aboveground carbon storage in live trees increased by 0.63 Mg C ha-1 year-1 between 1968 and 2007," and that "extrapolation to unmeasured forest components (live roots, small trees, necromass) and scaling to the continent implies a total increase in carbon storage in African tropical forest trees of 0.34 Pg C year-1."

In discussing their results, the 33 researchers say the observed changes in carbon storage "are similar to those reported for Amazonian forests per unit area, providing evidence that increasing carbon storage in old-growth forests is a pan-tropical phenomenon," and they report that "combining all standardized inventory data from this study and from tropical America and Asia together yields a comparable figure of 0.49 Mg C ha-1 year-1," which equates to "a carbon sink of 1.3 Pg C year-1 across all tropical forests during recent decades," which can account for roughly half of the so-called missing carbon sink, which has been long sought but never found (but is now, perhaps, half-found).

As for what the driving force is that seems to have breathed new life into old trees, Lewis et al. write in the concluding sentence of the abstract of their paper that "taxon-specific analyses of African inventory and other data suggest that widespread changes in resource availability, such as increasing atmospheric carbon dioxide concentrations [our italics], may be the cause of the increase in carbon stocks, as some theory (Lloyd and Farquhar, 1996) and models (Friedlingstein et al., 2006; Stephens et al., 2007; Ciais et al., 2008) predict."

We agree. The CO2-induced Greening of the Earth continues!

Sherwood, Keith and Craig Idso

References
Ciais, P., Piao, S.-L., Cadule, P., Friedlingstein, P. and Chedin, A. 2008. Variability and recent trends in the African carbon balance. Biogeosciences 5: 3497-3532.

Friedlingstein, P., Cox, P., Betts, R., Bopp, L., von Bloh, W., Brovkin, V., Cadule, P., Doney, S., Eby, M., Fung, I., Bala, G., John, J., Jones, C., Joos, F., Kato, T., Kawamiya, M., Knorr, W., Lindsay, K., Matthews, H.D., Raddatz, T., Rayner, P., Reick, C., Roeckner, E., Schnitzler, K.-G., Schnur, R., Strassmann, K., Weaver, A.J., Yoshikawa, C. and Zeng, N. 2006. Climate-carbon cycle feedback analysis: Results from the (CMIP)-M-4 model intercomparison. Journal of Climate 19: 3337-3353.

Lewis, S.L., Lopez-Gonzalez, G., Sonke, B., Affum-Baffoe, K., Baker, T.R., Ojo, L.O., Phillips, O.L., Reitsma, J.M., White, L., Comiskey, J.A., Djuikouo K., M.-N., Ewango, C.E.N., Feldpausch, T.R., Hamilton, A.C., Gloor, M., Hart, T., Hladik, A., Lloyd, J., Lovett, J.C., Makana, J.-R., Malhi, Y., Mbago, F.M., Ndangalasi, H.J., Peacock, J., Peh, K. S.-H., Sheil, D., Sunderland, T., Swaine, M.D., Taplin, J., Taylor, D., Thomas, S.C., Votere, R. and Woll, H. 2009. Increasing carbon storage in intact African tropical forests. Nature 457: 1003-1006.

Lloyd, J. and Farquhar, G.D. 1996. The CO2 dependence of photosynthesis, plant growth responses to elevated atmospheric CO2 concentrations and their interaction with soil nutrient status. 1. General principles and forest ecosystems. Functional Ecology 10: 4-32.

Stephens, B.B., Gurney, K.R., Tans, P.P., Sweeney, C., Peters, W., Bruhwiler, L., Ciais, P., Ramonet, M., Bousquet, P., Nakazawa, T., Aoki, S., Machida, T., Inoue, G., Vinnichenko, N., Lloyd, J., Jordan, A., Heimann, M., Shibistova, O., Langenfelds, R.L., Steele, L.P., Francey, R.J. and Denning, A.S. 2007. Weak northern and strong tropical land carbon uptake from vertical profiles of atmospheric CO2. Science 316: 1732-1735.