New research led by University of Göttingen scientists shows that grass death in fairy circles occurs immediately after rainfall due to plant water stress but not due to termite activity.
Fairy circles are patches of bare soil within arid grasslands known from southwestern Africa around the Namib Desert and from Western Australia.
They are devoid of vegetation and often surrounded by a fringe of tall grasses. Although seedlings are sometimes found in these patches after rainfall, they usually do not survive, leaving the patches completely bare for most of the time.
In northwest Western Australia, the typical diameters of the circles are about 4 m (13 feet), and in Namibia, the diameters increase with aridity from 4 m in the south to 6 m (20 feet), and even 10 m (33 feet), further to the northwest.
While the origin of Namibia’s fairy circles has been puzzled over since the early 1970s, the Australian fairy circles were only discovered in 2014.
Up to now, scientists offered several theories to explain the honeycomb-like pattern formed by these circles.
Some believed termites or ants nibble away at the roots of the grasses and kill them. Others suspected toxic carbon monoxide gas may rise from soil under the circles and kill the vegetation. And a third camp thought that the barren areas simply arise of their own accord under certain conditions.
“The sudden absence of grass for most areas within the circles cannot be explained by the activity of termites because there was no biomass for these insects to feed on,” said lead author Dr. Stephan Getzin, a researcher in the Department of Ecosystem Modelling at the University of Göttingen.
“But more importantly, we can show that the termites are not responsible because the grasses die immediately after rainfall without any sign of creatures feeding on the root.”
In their study, Dr. Getzin and colleagues followed the rains along the Namib between 2020 and 2022 and assessed the cause of the grass death within fairy circles at different time intervals after grass-triggering rainfall.
To assess whether termite herbivory was the cause, they used grass excavations and observations on the roots and shoots.
“The data show that about 10 days after rainfall, the grasses were already starting to die within the circles while most of the interior area of the circles did not have grass germination at all,” they said.
“Twenty days after rainfall, the struggling grasses within the circles were completely dead and yellowish in color while the surrounding grasses were vital and green.”
“When we examined the roots of the grasses from within the circles and compared them to the green grasses on the outside, we found that the roots within the circles were as long as, or even longer than, those outside. This indicated that the grasses were putting effort into the growth of roots in search of water.”
“However, we found no evidence for termites feeding on roots. It was not until 50-60 days after the rainfall that root damage became more visible at the dead grasses.”
When they analyzed the data on soil-moisture fluctuations, the researchers found that the decline in soil water inside and outside of the circles was very slow after initial rainfall, when grasses were not yet established.
However, when the surrounding grasses were well established, the decline in soil water after rainfall was very fast in all areas, even though there were almost no grasses within the circles to take the water.
“Under the strong heat in the Namib, the grasses are permanently transpiring and losing water,” Dr. Getzin said.
“Hence, they create soil-moisture vacuums around their roots and water is drawn towards them.”
“Our results strongly agree with those of researchers who have shown that water in soil diffuses quickly and horizontally in these sands even over distances greater than 7 m (23 feet).”
“By forming strongly patterned landscapes of evenly spaced fairy circles, the grasses act as ecosystem engineers and benefit directly from the water resource provided by the vegetation gaps.”
“In fact, we know related self-organized vegetation structures from various other harsh drylands in the world, and in all those cases the plants have no other chance to survive except by growing exactly in such geometrical formations.”
The study was published in the journal Perspectives in Plant Ecology, Evolution and Systematics.
Stephan Getzin et al. Plant water stress, not termite herbivory, causes Namibia’s fairy circles. Perspectives in Plant Ecology, Evolution and Systematics, published online October 20, 2022; doi: 10.1016/j.ppees.2022.125698
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