Background
The authors write that increasing CO2 concentrations "would be expected to favor trees over grasses by increasing the growth rates of trees (C3 plants) relative to tropical grasses (C4 plants), which are less responsive to CO2 (Ehleringer et al., 1997), and more generally by allowing faster-growing tree seedlings to escape the 'fire trap' in fire-prone grasslands and savannas (Bond and Midgley, 2000; Bond et al., 2003; Kgope et al., 2010)." And they say that "the large (~ 100 ppm) increase in CO2 concentration over the last glacial-interglacial transition has also been proposed as a major cause of the increase in global forest cover shown by pollen records (Street-Perrott et al., 1997; Archer et al., 1995, 2001; Bond and Midgely, 2000; MacInnis-Ng et al., 2011)."

What was done
To further investigate this phenomenon, which is detectable in the δ¹³C of leaf-wax biomarkers, Bragg et al. extracted such biomarkers from sediment cores collected along an offshore South Atlantic transect, in order to derive a record of vegetation changes in subequatorial Africa.

What was learned
The seven scientists report that their efforts paid off, and that their data suggested "a large increase in C3 relative to C4 plant dominance after the Last Glacial Maximum." And "using a process-based biogeography model that explicitly simulates ¹³C discrimination," they showed that "precipitation and temperature changes cannot explain the observed shift in δ¹³C values," which led them to conclude that "the physiological effect of increasing CO2 concentration is decisive, altering the C3/C4 balance and bringing the simulated and observed δ¹³C values into line."

What it means
In further discussing their findings, Bragg et al. say their results "have implications for the interpretation of the trend towards increased tree density in savannas, known as 'woody thickening'," noting that "an increase of CO2 concentration from 280 to 380 ppm should have increased the potential growth rates of C3 plants during the industrial era by ~ 15 to 20%." And they add that "a continued increase from 380 ppm to 550 ppm should cause a further increase of similar magnitude in the potential growth rates of C3 plants," which increase "would be expected to increase the competitive ability of C3 plants" and "further increase woody plant cover today, just as it did during the worldwide reforestation after the Last Glacial Maximum."

References
Archer, S., Boutton, T.W. and Hibbard, K.A. 2001. Trees in grasslands: biogeochemical consequences of woody plant expansion. In: Schulze, E.-D., Heimann, M., Harrison, S.P., Holland, E.A., Lloyd, J., Prentice, I.C. and Schimel, D.S. (Eds.). Global Biogeochemical Cycles in the Climate System. Academic Press, San Diego, California, USA, pp. 115-137.

Archer, S., Schimel, D.S. and Holland, E.A. 1995. Mechanisms of shrubland expansion: land use, climate or CO2? Climatic Change 29: 91-99.

Bond, W.J. and Midgley, G.F. 2000. A proposed CO2-controlled mechanism of woody plant invasion in grasslands and savannas. Global Change Biology 6: 865-869.

Bond, W.J., Midgley, G.F. and Woodward, F.I. 2003. The importance of low atmospheric CO2 and fire in promoting the spread of grasslands and savannas. Global Change Biology 9: 973-982.

Ehleringer, J.R., Cerling, T.E. and Helliker, B.R. C4 photosynthesis, atmospheric CO2 and climate. Oecologia 112: 285-299.

Kgope, B.S., Bond, W.J. and Midgley, G.F. 2010. Growth responses of African savanna trees implicate atmospheric [CO2] as a driver of past and current changes in savanna tree cover. Austral Ecology 35: 451-463.

Macinnis-Ng, C., Zeppel, M., Williams, M. and Eamus, D. 2011. Applying a SPA model to examine the impact of climate change on GPP of open woodlands and the potential for woody thickening. Ecohydrology 4: 379-393.

Street-Perrott, F.A., Huang, Y., Perrott, R.A., Eglinton, G., Barker, P., Khelifa, L.B., Harkness, D.D. and Olago, D.O. 1997. Impact of lower atmospheric carbon dioxide on tropical mountain ecosystems. Science 278: 1422-1426.