Explaining the history of Australia's vegetation

The new understanding will help researchers better predict the likely impact of climate change and rising CO₂ levels on these critically important plants. Called 'C₄ plants' after their alternative photosynthetic pathway, these plants include a wide variety of native tropical, subtropical and arid grasses as well as saltbushes. C₄ crops include sugarcane and corn.

"C₄ plants evolved to be able to photosynthesise under warm, dry, and low CO₂ 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 C₄ dominated continent today, little is known about the initial C₄ 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 C₄-dominated ecosystems first rose to prominence in Australia.

"In many regions around the globe, C₄ plants became prevalent between six and eight million years ago, which is thought by some to be the result of falling global atmospheric CO₂ 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 C₄ plants did not expand at this time in spite of regionally arid conditions and falling atmospheric CO₂, both of which should have promoted C₄ vegetation. Instead, C₄ vegetation expanded across the landscape only 3.5 million years ago, several million years later."

The authors say that the rise of C₄ 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 C₄ 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 CO₂ and regional changes in seasonality of rainfall is likely to play an important role in the distribution of C₄-dominated ecosystems. Rising CO₂ will place C₄ plants at a disadvantage, while rising temperatures, and changes in the season and amount of rainfall, could favour them.

"In Australia, C₄ plants are critical to grazing, soil carbon storage and biodiversity. We need to understand the factors that are likely to influence their survival in the future, to provide a basis for future conservation of these important plants."