In European folklore, arcs of mushrooms
—known as fairy rings—were considered a portal
into a different world.
These mesmerizing patterns are also seen in the grasslands of Namibia
where meters-wide, barren circles speckle the landscape.
To ecologists, these shapes are enchanting, but not for their magical properties.
They give important clues about the health of an ecosystem—
and how they might respond to change.
Most research on these self-organizing patterns are in systems where fairy circles persist
long-term.
But in salt marshes, these shapes are part of early development—a stepping stone to
establishing widespread expanses of vegetation.
Now, researchers have gained more insight into these fleeting patterns.
Researchers used experimental testing and mathematical modeling
to uncover how fairy circles form in salt marshes.
First, they picked a young marsh to observe.
They settled on the mudflats of Chongming Island and Nanhui tidal flat near Shanghai.
While marshes in many places have been shrinking due to sea-level rise, some marshes in China
have expanded by hundreds of meters a year.
Human development and the expansion of farmlands pushing more sediment into the Yangtze river
has expanded shorelines.
During an aerial survey of the area, scientists observed rings that nestled inside each other,
and that these rings often merged.
In arid environments, rings repel each other.
They used mathematical models to recreate these patterns
and found two mechanisms driving them.
One, the accumulation of the toxic chemical sulfide, created when bacteria decomposes
dead plant matter.
The other, nutrient depletion in the center of the rings from heightened competition with
plants.
Less competition with plants expanding outwardly means more available nutrients.
To test if these ecological processes were driving patterns, scientists sampled from
different areas of fairy circles—
the center of rings, on the rings, and the bare, outer edge.
They found higher concentrations of sulfide in the center samples than on the rings and margins.
Although the measured amounts were not high enough to kill plants, researchers say sampling
during hotter times of day could yield lethal
They did find conclusive evidence that a lack of nutrients played a major role in the formation
of these rings.
In experiments on the salt marsh, adding nitrogen fertilizer to the center patch increased their
growth, while adding it on the rings hardly gave a boost.
Researchers say this study provides a good starting point for figuring out what could
limit the expansion of marshes in other areas.
Since marshes serve important functions—from buffering against storm surges to capturing carbon—
understanding the mechanisms that underlie their success is crucial.
Taking a bird’s eye view of other marshes could reveal the secrets to their success.