How spinifex grasses got their ring shapes

Scientists say they may have solved a longstanding mystery of how Australia’s iconic spinifex got its distinctive ring shape. It seems the grasses die off in the middle due to a build-up of pathogenic soil microbes.

“People generally think about the beneficial effects of soil microbes, which can help plants access water and nutrients,” says Angela Moles from the University of New South Wales, senior author of a paper published in the Australian Journal of Botany. 

“However, there are lots of pathogenic microbes in soil too.”

Spinifex (Triodia spp) grows in arid and semi-arid zones, covering nearly a fifth of the continent, and plays important ecological, cultural and economic roles. 

The shrub-like grasses provide habitat and food for lizards, birds and small mammals, for instance, and fuel wildfires that help regenerate the landscape. Indigenous Australians have traditionally used them for tool making, medicine, food and fibre and pastoralists use them for grazing.

Scientists, for their part, have been curious about the intriguing ring-forming species.

“They start out growing as a nice hummock, but then the centre parts of the plant die back, and the plant expands from the outer edges, often forming rings metres across,” says Moles. “Even though this is a beautiful and widespread phenomenon, nobody had a convincing explanation for why the spinifex grasses might die back in the middle.”

Some theories include water availability, ants, termites and depletion of soil nutrients. Moles and first author Neil Ross had an inkling that soil microbes might impede seedling emergence and growth in the centre of the grass.

To test this, Ross collected soil from inside and outside the rings, then planted seeds from a pervasive spinifex species (Triodia basedowii) in soil that had live microbes or soil that was sterilised. 

In soil with live microbes, they found that seedling emergence was 41% higher outside the rings than inside. But in sterilised soil, seedling growth was 46% higher inside the rings than in rings with live soil. 

This suggests die-back in the older centre of the plants might result from a build up of pathogenic soil microbes over time, while new seedlings grow better around the outside with fewer soil pathogens.

The first study to demonstrate the effect of soil microbes in an arid environment, the authors say it “contributes to a growing body of evidence suggesting that plant-soil feedbacks are important to a wide range of ecosystems worldwide”.