African Tropical Rainforest: Flexing Its Muscles in Times of Stress
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.
"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!
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