What was done
The authors used repeated sequences of digitized and geo-referenced historical aerial photography to determine the nature of any changes that might have occurred, over the latter half of the 20th century, in the status of small patches of closed forest that are scattered throughout the vast eucalypt savannas that stretch across northern Australia, which patches represent "the only known habitat of one of Australia's most threatened species, the critically endangered Carpentarian rock-rat, Zyzomys palatalis." In doing so, they focused on five sites situated within a 37-km radius on the Wollogorang pastoral property in the southwest of the Gulf of Carpentaria, Northern Territory. For one of these sites, aerial photography was available from 1952 and 1995, while for the other four sites it was available from 1947, 1972 and 1997.

What was learned
Brook and Bowman report that "at all five sites over the 50-year study period there was a substantial increase (mean = 42%) in the coverage of closed forest in the landscape."

What it means
In discussing their results, the two Australian (Charles Darwin University) researchers say that "two factors may plausibly explain the expansion of closed forests." First, they note that "eco-ethnographic records stress the skilful use of fire by Aboriginal people in protecting isolated and locally resource-rich closed-forest patches." Second, they state that "the recent global increase in atmospheric CO2 may be changing the competitive balance between savanna and forest by enabling C3 trees to grow fast enough to escape the fire trap presented by flammable C4 grasses," noting that "theory, field observations and meta-analyses prove that CO2 enrichment of the atmosphere changes profoundly the assimilation and growth of C3 tropical plants (reviewed by Malhi and Grace, 2000)," and that elevated CO2 "is known to disadvantage C4 grass relative to C3 woody plants (Bond et al., 2003), and may thereby shift the dynamic vegetation equilibrium further toward a C3-dominated system."

References
Bond, W.J., Woodward, F.I. and Midgley, G.F. 2005. The global distribution of ecosystems in a world without fire. New Phytologist 165: 525-538.

Malhi, Y. and Grace, J. 2000. Tropical forests and atmospheric carbon dioxide. Trends in Ecology and Evolution 15: 332-337.