University of Adelaide-led research has uncovered the history of when and why the native vegetation that today dominates much of Australia first expanded across the continent.

The new understanding will help researchers better predict the likely impact of climate change and rising CO 2 levels on these critically important plants. Called 'C 4 plants' after their alternative photosynthetic pathway, these plants include a wide variety of native tropical, subtropical and arid grasses as well as saltbushes. C 4 crops include sugarcane and corn.

"C 4 plants evolved to be able to photosynthesise under warm, dry, and low CO 2 conditions, with a special ability to take advantage of summer rainfall," says lead author Jake Andrae, PhD candidate in the University's School of Physical Sciences and Sprigg Geobiology Centre. "As a result, they dominate the vegetation of Australian tropical, subtropical, and arid regions today.

"But despite being the most C 4 dominated continent today, little is known about the initial C 4 expansion in Australia."

The researchers analysed fossilised leaf waxes and pollen preserved in marine sediments. They measured the chemical signatures from these remnants, to reconstruct how and when C 4 -dominated ecosystems first rose to prominence in Australia.

"In many regions around the globe, C 4 plants became prevalent between six and eight million years ago, which is thought by some to be the result of falling global atmospheric CO 2 concentrations during this time," says project leader Dr Francesca McInerney, Australian Research Council Future Fellow at the University of Adelaide.

"Surprisingly, in north-west Australia C 4 plants did not expand at this time in spite of regionally arid conditions and falling atmospheric CO 2 , both of which should have promoted C 4 vegetation. Instead, C 4 vegetation expanded across the landscape only 3.5 million years ago, several million years later."

The authors say that the rise of C 4 plants in Australia was likely the result of a strong summer monsoon that developed around that time.

"The difference in the timing of the expansion of C 4 plants in Australia from other parts of the globe demonstrates that regional climate changes are important in driving vegetation change," Dr McInerney says.

"In the future, the interaction between global atmospheric CO 2 and regional changes in seasonality of rainfall is likely to play an important role in the distribution of C 4 -dominated ecosystems. Rising CO 2 will place C 4 plants at a disadvantage, while rising temperatures, and changes in the season and amount of rainfall, could favour them.