Science

How fairy circles form in Shanghai’s salt marshes

Dry areas of Namibia and Australia are well known for their fairy circles—barren rings that dot grasslands and are thought to result from termites and water scarcity. But moist regions have fairy circles, too. A new study reveals what is driving their formation in the salt marshes of eastern China.

In coastal marshes near Shanghai, fairy circles are ephemeral—appearing for a few years before they give way to wide expanses of well-rooted grass. The circles of grass often bump up against each other and have areas of open mud in the center.

Scientists theorized that nutrient availability might be driving their formation. To find out, they created a computer model that simulated the growth of fairy circles using two inputs: the slow accumulation of sulfide, a toxic byproduct of bacterial growth, and the depletion of nutrients in the center of rings as they expanded. To their delight, the model re-created the nestled rings seen throughout the marshes (see video, above). To verify their findings, the scientists then applied fertilizer to the rings’ dense outer edges and sparsely vegetated centers; whereas the boost in nutrients made little difference to the well-established outer rings of vegetation, it caused the areas inside the rings to flourish.

Because the grasses in the center take root first, they face more competition for nutrients as successive plants spring up around them. That causes them to die first and leave behind newer, living rings, the scientists report this month in Science Advances. Sometimes a bird’s eye is the best way to understand microscopic interactions on the ground.

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