Fairy rings are circles of mushrooms which are best noticeable when the fruiting bodies are out. In fact, in woodlands, fairy rings can only be seen when the fruiting bodies are out because otherwise there is not much to see on the ground – like this ring of Trooping Funnel (Clitocybe geotropa) in Sailor’s Grove. This fungus lives in symbiosis with tree roots. As the fungus grows, the circle becomes bigger while the host tree remains in the centre of the ring.
In grasslands, the picture is different. Fungi might affect the grass growth – some improve the growth while others inhibit it. The mycelium (thread-like underground part of fungus) of Fairy Ring Champignon (Marasmius oreades) is so tight that the water cannot penetrate, and the grass dries out. The grass is greener on the inside of the circle because the fungus decays there producing nutrients which the grass utilises. These rings became apparent in the lawn near the lake after a spell of dry weather in April 2020.
Many thanks to Claudi Soler from Herts Branch of British Naturalist Association for identifying the fungi and directing me towards Fungi (Collins New Naturalist Library, book 96) for further information.