Breath-Testing the Savanna: Australia's Tropical Savannas Are the Focus of World-First Research into the Effects of Rising Carbon Dioxide Levels on a Natural Tropical Ecosystem
Pyper, Wendy, Ecos
At Yabulu, 25 kilometres north of Townsville, scientists from CSIRO and lames Cook University (JCU) have ringed six, 15 metre-diameter plots of knee-high savanna grass and tree seedlings with a perimeter of piping and sophisticated electronic equipment.
For the next five years, this perimeter will regulate the flow of carbon dioxide (C[O.sub.2]), piped across the plots from the neighboring Queensland Nickel Pty Ltd refinery.
Three different concentrations of the gas -- 370 parts per million (ambient), 460ppm and 550ppm -- will be distributed via the wind across two plots each.
Then, at regular intervals, scientists will measure changes in plant performance, species composition, nutrient levels and carbon flow.
Dr Andrew Ash of CSIRO Sustainable Ecosystems says the project, called the Australian Free Air Carbon Dioxide Enrichment Study (OzFACE), will provide the first insights into the impact of rising C[O.sub.2] concentrations on savanna grasses and trees, and on the broader savanna ecosystem.
`C[O.sub.2] levels are likely to be 500-600 ppm by 2050, depending upon our global emissions of greenhouse gases,' Ash says.
`This could have a range of effects, such as increased grass growth, a thickening woody layer, changes in competition between trees and grasses and changes in the flow of carbon through the system.
`During the next five years, this project will help us understand those impacts and allow us to assist land managers and graziers in the savannas to plan for the future.'
Ash and his JCU collaborator, plant physiologist Dr Joe Holtum, already have an inkling of what to expect, thanks to some earlier glasshouse experiments. These experiments involved growing savanna grasses in pots at ambient (370ppm) and high (700ppm) C[O.sub.2], under simulated wet and dry seasons.
`At the end of our trial, we found the control pots (ambient C[O.sub.2]) had dried out at the end of the dry season. But pots under elevated C[O.sub.2] were still moist,' Ash says. `The high C[O.sub.2] plants had also grown 30-40% more because they hadn't suffered drought stress.'
Ash says the reason for this increase in water use efficiency is that under high C[O.sub.2], the stomata (air breathing pores) in the leaves don't need to open as wide to take in C[O.sub.2]. So the plants transpire (lose water) less. While this allows the grasses to grow more, Ash says their nutritional quality could be reduced in a nutrient-limited environment.
`The savanna soils are already low in nitrogen and phosphorus. So increased grass growth will probably reduce the concentration of compounds such as protein, and increase the concentration of fibre,' he says.
`And if temperatures rise, we might expect the nutritional quality to decline further, as higher temperatures stimulate faster growth and the accumulation of indigestible fibrous compounds.'
This outcome could have both positive and negative effects on the beef industry, a large proportion of which is supported by the savannas. `You might be able to grow more and the grasses might be more drought tolerant, but animals might not do as well,' Ash says.
Today's grazing practices could also have an effect on the flow of carbon through the savanna system, affecting its capacity to act as a carbon sink.
The savannas contain 33% of Australia's terrestrial carbon, which is distributed between the trunks, stems, leaves and roots of the plants and within the soil and leaf litter. Preliminary carbon measurements at a number of overgrazed sites showed a 30-60% reduction in organic carbon in the top 10cm of soil. But Holtum says that when grazing practices are adjusted to account for changes caused by rising C[O. …