Fairy Ring - Necrotic or Rapid Growth Zones

Necrotic or Rapid Growth Zones

One of the manifestations of fairy ring growth is a necrotic zone—an area in which grass or other plant life has withered or died. These zones are caused by the mycelia which, during a very dry year, coat the roots of grasses and other herbs in meadows. After some time they are removed by biotic factors from the ground, at which stage a zone on the surface soil becomes visible. Patterns other than the basic ring or arc are also possible: circles, doubled arcs, sickle-shaped arcs, and other complicated formations are also formed by this process. Fungi can deplete the soil of readily available nutrients such as nitrogen, causing plants growing within the circle to be stressed which leads to plant discoloration. Some fungi also produce chemicals which act like hormones called gibberellins, which affect plant growth, causing rapid luxuriant growth.

Long-term observations of fairy rings on Shillingstone Hill, England, further suggested that the cycle depended on the continuous presence of rabbits. Chalky soils on higher elevations in the counties of Wiltshire and Dorset (U.K.) used to support many meadow-type fairy rings. Rabbits crop grass very short in open areas and produce nitrogen-rich droppings. Mushrooms need more soil nitrogen than grass does. A ring can start from a single spore from which the mycelium develops; the fruiting bodies of the mushrooms only appearing later, when sufficient mycelial mass has been generated to support them. Subsequent generations of fungi grow only outwards, because the parent generations have depleted their local nitrogen levels. Meanwhile, rabbits keep cropping the grass, but do not eat the fungi, allowing them to grow through their competition to tower, relatively, above the grass. By the time a circle of mushrooms reaches about 6 metres (20 ft) in diameter, rabbit droppings have replenished the nitrogen levels near the centre of the circle, and a secondary ring may start to grow inside the first.