Background
The authors say there have been "many reports of woody plant increase in grasslands and savannas worldwide over the past century," some of which "may cause biome shifts from grassland to scrub forest," citing Briggs et al. (2005), Brook and Bowman (2006), Bowman et al. (2010) and Wigley et al. (2010). And in this regard they additionally note that woody cover in savannas has been thought to be regulated by a number of different variables, including climate, fire regime, herbivory and grazing; but they state that "the phenomenon may also have been influenced by increasing atmospheric CO2," citing the publications of Idso (1992), Polley (1997) and Bond and Midgley (2000).

What was done
To further explore the subject, Buitenwerf et al. analyzed changes in woody-plant density at three sites in South African savannas where the normal disturbance regime (fire and herbivores) was kept constant for either 30 or 50 years, noting that "if global drivers had significant effects on woody plants, we would expect significant increases in tree densities and biomass over time under the constant disturbance regime."

What was learned
The four South African scientists report that for the more arid savannas they analyzed, there was no indication of global drivers promoting an increase in wood density over the period of their study. However, they found that wood density actually tripled in a mesic savanna between the 1970s and 1990s, and that in another mesic savanna it doubled from the mid-1990s to 2010, while "aerial photograph analysis on adjacent non-cleared areas showed an accompanying 48% increase in woody cover."

What it means
In discussing their findings, Buitenwerf et al. say their analysis "has shown significant increase in tree densities and stature that are consistent with global drivers promoting woody thickening." And they conclude that "the only plausible candidate in the experimental areas is increasing CO2 since there were no significant temperature or rainfall trends over the last 50 years."

References
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.

Bowman, D.M.J.S., Murphy, B.P. and Banfai, D.S. 2010. Has global environmental change caused monsoon rainforests to expand in the Australian monsoon tropics? Landscape Ecology 25: 1247-1260.

Briggs, J.M., Knapp, A.K., Blair, J.M., Heisler, J.L., Hoch, G.A., Lett, M.S. and McCarron, J.K. 2005. An ecosystem in transition: causes and consequences of the conversion of mesic grassland to shrubland. BioScience 55: 243-254.

Brook, B.W. and Bowman, D.M.J.S. 2006. Postcards from the past: charting the landscape-scale conversion of tropical Australian savanna to closed forest during the 20th century. Landscape Ecology 21: 1253-1266.

Idso, S.B. 1992. Shrubland expansion in the American southwest. Climatic Change 22: 85-86.

Polley, H.W. 1997. Implications of rising atmospheric carbon dioxide concentration for rangelands. Journal of Range Management 50: 562-577.

Wigley, B.J., Bond, W.J. and Hoffman, M.T. 2010. Thicket expansion in a South African savanna under divergent land use: local vs. global drivers? Global Change Biology 16: 964-976.