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African Tropical Rainforest: Flexing Its Muscles in Times of Stress

Reference
Fisher, J.B., Sikka, M., Sitch, S., Ciais, P., Poulter, B., Galbraith, D., Lee, J.-E., Huntingford, C., Viovy, N., Zeng, N., Ahistrom, A., Lomas, M.R., Levy, P.E., Frankenberg, C., Saatchi, S. and Malhi, Y. 2013. African tropical rainforest net carbon dioxide fluxes in the twentieth century. Philosophical Transactions of the Royal Society B 368: 10.1098/rstb.2012.0376.
The African humid tropical biome constitutes the second largest rainforest region of the world; and, unfortunately, deforestation in this Central African region is accelerating, according to Fisher et al. (2013), in ways that are "similar to deforestation patterns in the rest of the tropics." In addition, as the sixteen scientists continue," humid tropical Africa has also borne the brunt of "the 'great drought' that began in the 1960s in the Sahel [and] lasted well into the 1980s, with 20-40% less precipitation in the 30 years following 1960, relative to before," as reported by Hulme (1992), Olsson et al. (2005) and Giannini et al. (2008).

"Compounding this decrease in precipitation," in the words of Fisher et al., "there has been an overall warming trend in Africa at large," as reported by Hulme (1996a,b) and Lewis et al. (2004). And they go on to correctly state that "given less water and hotter temperatures," we may have expected "increasing water stress to decrease CO2 uptake, and move the biome towards a net source of CO2 to the atmosphere," citing in this regard the study of Phillips et al. (2009). However, they say that "paradoxically," both "models and measurements have shown an increase in vegetation productivity and biomass of 0.3-0.4 Pg C/yr in the African humid tropics since the 1960s," citing the studies of Cao et al. (2001), Nemani et al. (2003), Williams et al. (2007), Alo and Wang (2008), Ciais et al. (2009), Lewis et al. (2009), Malhi (2010), and Pan et al. (2011).

So how did it happen? How did what most people thought to be highly improbable - if not impossible - actually occur? Fisher et al. write that "the recent increase in plant productivity has been attributed to the CO2 fertilization effect," citing a wealth of studies that have come to this conclusion, including those of Amthor (1995), Lloyd and Farquhar (1996), Cao et al. (2001), Lewis et al. (2004), Friedlingstein et al. (2006), Stephens et al. (2007), Ciais et al. (2009), Lewis et al. (2009), Malhi (2010), Ballantyne et al. (2012) and Higgins and Scheiter (2012). And in regard to a more recent African study that has yet to appear in print, they write that its authors "found that gross primary production increased over the past 30 years even though soil moisture decreased."

Such is the growth-promoting power of atmospheric CO2 enrichment!

Additional References
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Amthor, J.S. 1995. Terrestrial higher-plant response to increasing atmospheric [CO2] in relation to the global carbon cycle. Global Change Biology 1: 243-274.

Ballantyne, A.P., Alden, C.B., Miller, J.B., Tans, P.P. and White, J.W. 2012. Increase in observed net carbon dioxide uptake by land and oceans during the past 50 years. Nature 488: 70-72.

Cao, M., Zhang, Q. and Shugart, H.H. 2001. Dynamic responses of African ecosystem carbon cycling to climate change. Climate Research 17: 183-193.

Ciais, P., Piao, S.L., Cadule, P., Friedlingstein, P. and Chedin, A. 2009. Variability and recent trends in the African terrestrial carbon balance. Biogeosciences 6: 1935-1948.

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.

Giannini, A., Biasutti, M. and Verstraete, M.M. 2008. A climate model-based review of drought in the Sahel: desertification, the re-greening and climate change. Global and Planetary Change 64: 119-128.

Higgins, S.I. and Scheiter, S. 2012. Atmospheric CO2 forces abrupt vegetation shifts locally, but not globally. Nature 488: 209-213.

Hulme, M. 1992. Rainfall changes in Africa: 1931-1960 to 1961-1990. International Journal of Climatology 12: 685-699.

Hulme, M. 1996a. Recent climatic change in the world's drylands. Geophysical Research Letters 23: 61-64.

Hulme, M. 1996b. Global warming. Progress in Physical Geography 20: 216-223.

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.

Lewis, S.L., Malhi, Y. and Phillips, O.L. 2004. Fingerprinting the impacts of global change on tropical forests. Philosophical Transactions of the Royal Society B 359: 437-462.

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

Malhi, Y. 2010. The carbon balance of tropical forest regions, 1990-2005. Current Opinion in Environmental Sustainability 2: 237-244.

Nemani, R.R., Keeling, C.D., Hashimoto, H., Jolly, W.M., Piper, S.C., Tucker, C.J., Myneni, R.B. and Running. S.W. 2003. Climate-driven increases in global terrestrial net primary production from 1982 to 1999. Science 300: 1560-1563.

Olsson, L., Eklundh, L. and Ardo, J. 2005. A recent greening of the Sahel: trends, patterns and potential causes. Journal of Arid Environments 63: 556-566.

Pan, Y., Birdsey, R.A., Fang, J., Houghton, R., Kauppi, P.E., Kurz, W.A., Phillips, O.L., Shvidenko, A., Lewis, S.L., Canadell, J.G., Ciais, P., Jackson, R.B., Pacala, S.W., McGuire, A.D., Piao, S., Rautiainen, A., Sitch, S. and Hayes, D. 2011. A large and persistent carbon sink in the world's forests. Science 333: 988-993.

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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.

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Archived 7 May 2014