Rising CO2: what's in store for the savannas?

Rising levels of carbon dioxide may well change fundamental aspects of Australia’s savannas: trees could out-compete grasses, the grasses, while growing better, may be less nutritious for the animals that feed on them. A new study outside Townsville will measure some of these changes so land managers can plan for the future. By Kate O'Donnell

CSIRO’s Andrew Ash and Mike Whiting at one of the study’s plots. The pipe ringing the plots sends carbon dioxide over the vegetation. Two plots simulate current carbon dioxide levels (370 parts per million), two will have 450 ppm—levels we’ll be living with in about 30 years—and the last two 550 ppm, CO 2 , the level predicted for 2050.

The study, a collaboration between CSIRO Sustainable Ecosystems, James Cook University and Queensland Nickel Industries (QNI), is the first of its kind not only in the savannas, but also Australia. Called a Free Air Carbon Dioxide Enrichment study (FACE) it will pipe different levels of carbon dioxide over six 15 m round plots, each containing native grasses and trees, over the next five years.

While there are a number of FACE studies in operation overseas, only a couple involve natural or semi-natural systems. “Most are looking at crops, plantation forests and improved pastures,” explained CSIRO’s Andrew Ash, one of the leaders of the study. “From existing work we can probably plot the growth of individual species. But we can’t predict the response of what will happen between species in a real ecosystem. That is the real unknown, and one of great importance, that this study allows us to do.”

The project’s co-leader, JCU eco-physiologist Joe Holtum, will study potential impacts on plant physiology: from changes in competition between trees and grasses and between the grasses themselves, to changes in soil composition and plant defence mechanisms. In the long term, he hopes to set up collaborative studies to examine what might happen to the insects that depend on the flowers and seeds of the plants.

Another issue the study will shed some light on is one of carbon storage by looking at carbon flow through the ecosystem. Australia’s savannas are important stores of carbon, and it is estimated they currently contain 33 per cent of Australia’s terrestrial carbon overall, a proportion which may increase as CO ₂ levels rise.

The study is situated next to QNI’s nickel plant atYabulu, 20 km north of Townsville. QNI has given substantial support to the project, providing land, infrastructure, electricity and CO ₂ —itself no small contribution considering the project will use 1 to 1.5 tonnes of CO ₂ per day over the life of the experiment, a prohibitive cost for the researchers to meet themselves.

The study area is dominated by tussock perennial grasses with the main species being Kangaroo grass ( Themeda triandra ), Golden beard grass ( Chrysopogon fallax ) and Wanderrie grass ( Eriachne obtusa ). The plots do not have mature trees or shrubs in them so two native species from the immediate area, Acacia holosericea (soap bush) and Eucalyptus crebra (narrow leaf ironbark) were planted to study tree-grass interactions.

Work performed on the effects of elevated CO ₂ on savanna grasses—again by CSIRO and JCU—suggests that the grasses grow better because they use water more efficiently under higher amounts of CO ₂ . If this is correct, there may be more water available in the soil, and shrubs and trees could do better.

One of the potential results, says Andrew, might be an increased woody layer across the savannas. Another is that with greater water-use efficiency, savanna systems might become less prone to the effects of drought, and produce a more stable supply of forage from year to year. The FACE study will help answer these questions.

“We’re also looking at the effect of grass quality and quantity from the pastoral perspective,” said Andrew. The experiment simulates grazing effects and high nutrient growing conditions as well as a more natural low-nutrient regime. “We know that under high CO ₂ grass grows more, but it could be at the expense of forage quality. You might be able to grow more, but animals might not do as well.”

The grazing simulation will also help give a better picture of just how grazing affects carbon storage. Apart from the woody layer, most carbon is actually stored under perennial grasses. So, if these grasses are overgrazed, a lot of carbon will be lost from the system.

Joe will be examining the physiology of how the trees and grasses compete against each other under different conditions. “We want to see what the new equilibrium is, to see how they function physiologically,” he explained. “That will eventually relate to the rates of growth, and the productivity of the whole savanna.” Another variable is competition between grasses. For example, if annual grasses start performing better, they may start out-competing perennial grasses.

The project leaders are both keen to develop collaborative research with other institutions and scientists. One such collaborative study that might begin soon is on plant defence mechanisms. Plants growing under high CO ₂ tend to allocate carbon and nitrogen—the building blocks of their protein—differently. As the plant generally does better, it doesn’t need to put as much nitrogen into protein. Instead, the extra nitrogen may go towards producing more defensive compounds against the creatures that eat it, which may result in quite different food ranges for the animals and insects that live off the plants. Stephanie Brown, a JCU/TS–CRC Honours student, has already begun measurements of seedlings and will be spending the year measuring plant responses to elevated CO ₂ .