The prickly crunch of dry, brown grass is a familiar sensation in the Australian summer. But despite its ubiquity, little is known about when and why grass turns brown, and what it means for Australia’s extensive grasslands in a changing climate.

ARC Georgina Sweet Australian Laureate Fellow, Distinguished Professor Belinda Medlyn and colleagues at 91���� are embarking on an Australian Research Council Discovery Project to answer key questions about how grasslands respond to changes in temperature and rainfall, and what implications that has for the carbon they store in the soil.

Grasslands make up around 40% of the planet’s terrestrial surface; from the vast prairies of North America, and the wind-swept steppes of Eurasia, to the pampas of South America and savannahs of northern Australia.
They’re also vital for cattle farming. “There’s a lot of interest around how to maintain pasture grasses during hot and dry periods, what kinds of pasture grasses, what will be able to survive and continue to perform when we have hotter and drier conditions,” Medlyn says. “The grasslands are really interesting ecosystems, valuable in their own right, but they also support a huge industry.”

The project was inspired by research conducted in the North American grasslands of Wyoming, which looked at the effect of increasing carbon dioxide levels and heating on the grass. Several vegetation models were applied to try and predict the responses in this experiment, but Medlyn says the models didn’t adequately capture the impact of the grasses browning-off as the soil started to dry out. The probable reason was that a lot of models of the effects of carbon dioxide levels, temperature increases, and rainfall decreases, were originally developed for forests, which don’t brown-off in the same way that grass does.

Browning of grass in dry conditions means that the grass isn’t photosynthesising and therefore won’t be capturing carbon or producing new grass. “If you’re trying to predict how much productivity you have in a grassland, you need to be able to predict that browning process,” Medlyn says.

Medlyn and her group are deploying devices known as ‘phenocams’, which are essentially time-lapse cameras that record changes in the grass colour over time. “By looking at those images, we get much more information about what’s going on with the grasses than if we would just go and measure what had happened once a year.”

The project also involves field and glasshouse experiments using technologies that enable researchers to subject grasses to different concentrations of carbon dioxide, and levels of heat and water to see how they respond over time.

One key challenge with grasslands is that so much happens below the surface, says Professor Elise Pendall, a soil ecologist and one of the four Western researchers leading the project.

“In grasslands, roots make up around 75% of the biomass,” Pendall says. Grass growth crowns sit right at the soil surface, with the leaves growing upwards from them, and the roots extending down. Study often involves digging pits to extract the entire plant and root systems. If that isn’t possible, researchers take soil cores so they can extract the roots and measure factors such as biomass, respiration rates, carbon content and nutrient content.
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Roots are vital for grass’s survival during hotter, drier conditions, Pendall says. “They can get thicker when it gets warmer, and that might be to reduce water loss, but we want to know if that compromises their ability to absorb water.”
One of Medlyn’s post-doctoral researchers, Dr Jinyan Yang, is developing a model that will pull together all the information gathered in the project, and which can be used to predict grassland productivity, greening and browning under different climatic conditions.

“Then, you can start exploring different species and figure out what kinds of species might be able to cope best with different climatic conditions,” Medlyn says.

She points out that many Australian grasslands, particularly in the country’s interior are already well adapted to hot, dry conditions.

“Part of our work is really looking at how those grasses managed to survive during those hot dry conditions, what traits enable them to keep going during those dry periods,” she says.

Pendall gives the example of a project in Tasmania, which compared soil carbon storage underneath kangaroo grass and wallaby grass. Kangaroo grass sequestered much more carbon, and this sequestration was shown to be fairly resilient to modelled climate change conditions.

In addition to storing carbon, grasslands also play an important role in stabilising Australia’s fragile soils and preventing erosion. The deepening drought in central Australia is contributing to a loss of grasslands that is, in turn, enabling larger and more regular dust storms.

In all, the work is feeding into a bigger research project; Medlyn’s ARC Laureate Fellowship-supported efforts to model Australian vegetation as a whole, from forests to woodlands to grasslands.