Fairy rings.
These intriguing features are found by the thousand in southern Africa and range from 2-12 metres across. They consist of a barren circle of ground surrounded by grasses and have puzzled scientists for years. Explanatory theories have included meteorites, toxic patches of soil, degassing, and the activity of sand termites, but new research by Cristian Fernandez Oto from Brussels Free University suggests that they may be an emergent property of plants roots competing for water in the transition zone between arid and fertile regions. They occur in a 2400 Km band where the two regions meet and are stable for up to six decades, with an estimated average span of 35 years.
The team used computer modelling to show how plants would interact in such climate zones, taking into account the fact that their root systems are much larger than the surface expression of the plant. They posit that when moisture spreads into an area the plants gradually enclose the barren area, and the circle is the zone between the root systems. As the circle closes, the plants cannot grow any further together, and the barren zone remains neutral territory. The model correctly predicts that the circles should be larger in more arid areas, as the root systems grow larger to reach more scarce water.
The sand termite (Psammotermes allocerus) theory is also interesting, Norbert Juergens of Hamburg University has suggested that termites are bioengineering the desert, with these rings a visible consequence of their work. He found that the circles remained moist, even in the dry season, and the plants remained active all year round. He suggested it may be some sort of insectile transformation of their ecosystem. All new rings were found to be inhabited by termite colonies, and nearly all older ones. He suggests that they eat the roots of plants to create the circle, that then becomes a water trap as water percolates into the sand. The surrounding plants then use that water, and are consumed by the termites in a sort of agricultural cycle. Normally when rain falls, plants grow and die swiftly, this system allows them to endure over the long term, allowing a sustainable harvesting rather than boom and bust based termite economy. This allows the termites and plants to survive long droughts in an unusual kind of symbiosis.
Either way fairy rings are both long lived and dynamic: satellite images show some appear and vanish over a span of a few years, while most remain stable. Whatever their cause, I'm sure we haven't understood the full story yet. Because of their remoteness, there is little scientific interest in researching them, so a fuller answer may take some time to emerge.
Loz
Image credit: N. Juergens
http://www.technologyreview.com/view/516406/fairy-circle-mystery-solved-by-computational-modelling/?utm_campaign=socialsync&utm_medium=social-post&utm_source=twitter http://www.kqed.org/news/story/2013/03/28/118535/whats_behind_the_fairy_circles_that_dot_west_africa?category=science http://www.amusingplanet.com/2013/04/mysterious-fairy-circles-of-namibia.html Original paper, paywall access: http://arxiv.org/abs/1306.4848 http://news.sciencemag.org/sciencenow/2012/06/mysterious-fairy-circles-are-ali.html