Vegetation tricks: Solving the ‘fairy circle’ mystery, sans 1 suspect

WASHINGTON: The strange, barren spots pepper the vast Namib Desert, which stretches from southern Angola to northern South Africa. They are known as “fairy circles,” and for a natural phenomenon with such a whimsical name, scientific debates over their origins have been heated. “The to and fro between opposing camps has often been nothing less than vitriolic,” said Michael Cramer, an ecophysiologist at the University of Cape Town who has studied fairy circles.

Despite decades of research, no consensus exists about the origin of the mysterious formations. Theories have included poisonous gases, noxious bushes and plant-killing microbes or fungi. Two of the explanations — the circles are made by termites, or they result from plants competing over limited water — have dominated the scientific debate. “Each publication has been hailed as having finally ‘solved’ the fairy circle ‘mystery’ in the popular media,” Cramer said in an email, calling such reporting an approach that “is not the norm for science.”

A rigorous study published in October will not end this fight, but it does seem to give the water-related hypothesis a clear lead over the termite theory. “Plants are forced to create these circles to redistribute water to maximise their chances of survival,” said Stephan Getzin, an ecologist at the University of Göttingen in Germany and an author of the study. “We call it ecosystem engineering.” The Namib Desert is one of the driest places in the world, usually receiving only a few inches of rain each year. Researchers first proposed in 2004 that plants, in competition for water in this harsh ecosystem, may self-organize into fairy circles — an idea originally adapted from pattern-formation theory developed by mathematician Alan Turing.

Over the past decade, Getzin and others have published more than a dozen papers in support of the hypothesis, known as plant water stress. For their latest study, Getzin and his colleagues spent three years examining fairy circles at 10 study sites across 620 miles of desert. One of those years, 2020, was a drought, while 2021 and 2022 were exceptionally rainy — a lucky break that permitted the researchers to compare different conditions, Getzin said.

They used soil moisture sensors to collect continuous readings every 30 minutes of water content in the sand in and around fairy circles. They also examined hundreds of individual grass shoots and roots excavated at various intervals from within the circles and the surrounding areas. After rain, the researchers found that grasses germinated both inside and outside fairy circles, but that within about 20 days virtually all of the young shoots inside a circle had died. They also found that the top 8 inches of soil within fairy circles quickly dried out, something they hypothesize is caused when established plants surrounding fairy circles actively draw water toward them.

Plants are constantly transpiring — or losing water — through their leaves. Their roots, meanwhile, take water in. In Namibia’s sandy soil, this creates a vacuum effect that moves water from the interior of fairy circles toward the plants’ roots at the circle’s fringe and beyond. “It’s similar to you opening your window in winter and the warm air instantly moving out,” Getzin said.

The new paper also speaks to the termite hypothesis, which has been championed by Norbert Jürgens, an ecologist at the University of Hamburg in Germany. He reported in 2013 that fairy circles were in fact generated by sand termites that damage grass roots.