Background
Citing the work of Bowman et al. (2001), Banfai and Bowman (2006), Bowman and Dingle (2006) and Brook and Bowman (2006), the authors write that "a large research program in the Australian monsoon tropics has concluded that monsoon rainforests have expanded within the savanna matrix" since "the middle of the last century," while noting that a similar trend "has been emulated throughout the tropics worldwide," citing the work of Schwartz et al. (1996), Guillet et al. (2001), Puyravaud et al. (2003) and Wigley et al. (2009). And they say that this phenomenon has been "suggested to be linked to a long-term trend towards wetter climates, atmospheric CO2 enrichment, and changed fire regimes," although they remark that the observed forest expansion in Australia "is remarkable, being contrary to the widely accepted view that fire limits the extent of rainforests."

What was done
Bowman et al., as they describe it, "review these findings with particular consideration of [their] analytical and evidentiary basis and plausibility."

What was learned
At the conclusion of their study, the three researchers write that they "consider it most likely that the expansion of rainforest patches is related to global climate change via increased rainfall and/or the CO2 'fertilizer effect'," since the expansion of Australia's monsoon rainforests occurred "despite hostile fire regimes." And they note that this conclusion "is consistent with the recent finding from South Africa that strongly contrasting localized land management practices had little effect on the dramatic expansion of forest patches into the surrounding savanna in the latter half of the 20th century, strongly implicating global environmental change."

What it means
This study presents yet another significant piece of evidence for atmospheric CO2 enrichment being the cause of the great range expansion of woody plants that has been occurring in tandem with the historical rise in the air's CO2 content all around the world, as may be seen by perusing the many items we have archived under the several subheading of Range Expansion (Woody Plants) in our Subject Index.

References
Banfai, D.S. and Bowman, D.M.J.S. 2006. Forty years of lowland monsoon rainforest expansion in Kakadu National Park, northern Australia. Biological Conservation 131: 553-565.

Bowman, D.M.J.S. and Dingle, J.K. 2006. Late 20th century landscape-wide expansion of Allosyncarpia ternata (Myrtaceae) forests in Kakadu National Park, northern Australia. Australian Journal of Botany 54: 707-715.

Bowman, D.M.J.S., Walsh, A. and Milne, D.J. 2001. Forest expansion and grassland contraction within a Eucalyptus savanna matrix between 1941 and 1994 at Litchfield National Park in the Australian monsoon tropics. Global Ecology and Biogeography 10: 535-548.

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.

Guillet, B., Achoundong, G., Happi, J.Y., Beyala, V.K.K., Bonvallot, J., Riera, B., Mariotti, A. and Schwartz, D. 2001. Agreement between floristic and soil organic carbon isotope (¹³C/¹²C, ¹⁴C) indicators of forest invasion of savannas during the last century in Cameroon. Journal of Tropical Ecology 17: 809-832.

Puyravaud, J.-P., Dufour, C. and Aravajy, S. 2003. Rainforest expansion mediated by successional processes in vegetation thickets in the Western Ghats of India. Journal of Biogeography 30: 1067-1080.

Schwartz, D., de Foresta, H., Mariotti, A., Balesdent, J., Massimba, J.P. and Girardin, C. 1996. Present dynamics of the savanna-forest boundary in the Congolese Mayombe: a pedological, botanical and isotopic (¹³C/ and ¹⁴C) study. Oecologia 106: 516-524.

Wigley, B.J., Bond, W.J. and Hoffmann, M.T. 2009. Thicket expansion in a South African savanna under divergent land use: local vs. global drivers? Global Change Biology 16: 964-976.