The fairy ring mystery, solved

By Angela Heathcote 21 April 2021
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A team of Australian scientists may have answered the nagging question of how fairy rings form.

For years, rings of spinifex grass, which come together to form honeycomb patterns that span large swathes of Australia’s arid landscape, have perplexed scientists. Everything from lack of water to plant-destroying insects have been offered up as explanations. But a new study says the fairy circles may take their famous form due to microbes.

The scientists suggest that the hollow centre of the fairy rings is caused by a build-up of pathogenic soil microbes eating away at older parts of the spinifex, while new seedlings gather at the edge where there are fewer pathogens in the soil.

Author of the paper, published in the Australian Journal for Botany last month, Angela Moles, says she was inspired by a similar study on a species of Dutch swamp grass, which, like the spinifex grass, started out as a hummock and over time formed into rings. The study’s conclusion was that soil pathogens were causing the die-back.

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With this in mind, co-author of the paper Neil Ross collected soil from the inside and outside of the spinifex rings. Back in the lab, they planted seeds in both unchanged and sterile soil, comparing the seedling emergence and growth differences. 

Seedling emergence was significantly higher in the unchanged soils from outside the rings, while seedling emergence from soils inside the rings was higher in sterilised soil, suggesting that microbes in the soil are having a negative effect on seedling success. This means that seedlings that can’t handle these pathogens within the clump of spinifex may sprout beyond them, forming the fairy rings. 

Interestingly, while sterilising the soil from inside the rings increased recruitment, sterilising soil outside the rings did the opposite. “What this shows is that microbes vary through space,” Angela says. “Evidence from our study and other studies shows that microbes can have substantial effects on the type of plants that can survive in an ecosystem, the growth rate and productivity of the plants, and can even affect the shape of the plants.”  

She added that understanding which microbes are beneficial or pathogenic is important. 

“If we can separate the good microbes from the bad, we can use this knowledge to help improve the success of arid land restoration projects.”