What was done
The nature of Amazonian ecosystem response to the large-scale environmental changes experienced during glacial-interglacial cycles was investigated via a series of transient, continuous, 21,000-year simulations using a dynamic process-based ecosystem model for three scenarios: (1) real-world glacial-to-interglacial changes in CO2 concentration and climate, (2) the real-world change in CO2 with a constant preindustrial climate, and (3) the real-world change in climate with a constant preindustrial CO2 concentration.

What was learned
During the last glacial maximum, according to the authors, the model suggested that "total above-ground carbon storage in Amazonia was half preindustrial values, indicative of rain forests with markedly lower canopy densities and simpler structures due to lowered CO2 levels, corroborating modeling studies by Cowling (2004) and Cowling et al. (2001)." Thereafter, they say that "biome shifts in ecotonal areas since the last glacial maximum ["the competitive replacement of drought-adapted vegetation (e.g. savanna or deciduous/semideciduous dry forest) by rain forest"] were driven predominantly by climate change, while coincident, increased ecosystem carbon storage throughout the Amazon Basin was driven largely by CO2."

What it means
What do these findings have to do with us today? In answering this question, Beerling and Mayle say "the underlying cause for the observed trend of increasing biomass in long-term Amazonian forest plots over recent years, despite drought-induced El Nino events (Phillips et al., 1998; Baker et al., 2004), has been a subject of considerable debate (Baker et al., 2004; Wright, 2005)," and in this regard they conclude - along with the vast majority of researchers who have addressed the issue (see our Editorials of 7 Sep 2005 and 26 Jul 2006) - that "this biomass increase is part of a long-term historical trend driven by anthropogenically induced rising CO2 levels since the 19th century [our italics]," in a succinct description of the primary basis for the historical and still-ongoing Greening of the Earth.

References
Baker, T.R., Phillips, O.L., Malhi, Y., Almeida, S., Arroyo, L., Di Fiore, A., Erwin, T., Higuchi, N., Killeen, T.J., Laurance, S.G., Laurance, W.F., Lewis, S.L., Monteagudo, A., Neill, D.A., Núñez Vargas, P., Pitman, N.C.A., Silva, J.N.M. and Vásquez Martínez, R. 2004. Increasing biomass in Amazonian forest plots. Philosophical Transactions of the Royal Society of London Series B - Biological Sciences 359: 353-365.

Cowling, S.A. 2004. Tropical forest structure: a missing dimension to Pleistocene landscapes. Journal of Quaternary Science 19: 733-743.

Cowling, S.A., Maslin, M.A. and Sykes, M.T. 2001. Paleovegetation simulations of lowland Amazonia and implications for neotropical allopatry and speciation. Quaternary Research 55: 140-149.

Phillips, O.L., Malhi, Y., Higuchi, N., Laurance, W.F., Nunez, P.V., Vasquez, R.M., Laurance, S.G., Ferreira, L.V., Stern, M., Brown, S. and Grace, J. 1998. Changes in the carbon balance of tropical forests: Evidence from long-term plots. Science 282: 439-442.

Wright, S.J. 2005. Tropical forests in a changing environment. Trends in Ecology and Evolution 20: 553-560.