Fairy rings: why doesn't every soil fungus make these?

Or do they?

Fungal fairy rings (picture generated with leonardo.ai)

I usually talk only very briefly about fairy rings in my fungal biology course, and I must admit I have never given them much thought. But during my recent visit to China I talked to some researches at China Agricultural University working on these systems, and they really are quite fascinating.

Fairy rings are enigmatic features of forests and grassland formed by soil fungi. A paper by Gregory (1982) distinguishes tethered fairy rings (that are formed with a tree as the main source of carbon in the center, like for example by ectomycorrhizal fungi) and free fairy rings, which are formed in grasslands and don’t have a central source of carbon. The rings themselves are visible either because of the sporocarps of the fungus or by changes in the vegetation (either more lush or bare soil areas, or both). Some of these rings are likely several hundred years old.

The basic idea is that a fungal propagule can expand spherically in all directions (the intersection of the soil surface with that sphere is a circle); the circle front is the actively growing fungal individual, whereas connections are lost in the rest of the disc. There are a lot of different fungi that can form fairy rings, over 140 species have been documented (Miller & Gongloff 2021), even though most research has focused on a few species.

It turns out that work on fairy rings has a long tradition in soil ecology, with studies dating back to 1807. Nevertheless, it is still not completely clear how the effects on the vegetation manifest. A lot of processes seem to play a role, including plant growth hormone production by the fungus, changes in soil nutrients due to mineralization caused by the fungus, physical changes in the soil, microbes, and others (see a recent modeling study; Salvatori et al. 2023). Also different types of effects are distinguished, dead zone with lush vegetation, only lush vegetation without a dead zone, or no changes to the vegetation at all (only the sporocarps appearing).

What fascinates me is this: why doesn’t every soil fungus form such a ring or band? I am not sure if the answer to this is known.

In principle, every fungus will expand its mycelium in all directions, more or less in a sphere, especially given relatively constant environmental conditions. Thus, in principle every fungus should be forming such a ring. Do they perhaps form a ring, but their effects are not pronounced enough so that we don’t see them? The only reason we know about fairy rings is because of sporocarps or changes in vegetation. What if the fungus doesn’t form sporocarps, and what if the effects of the fungus are somehow not strong enough to affect the vegetation — will they still basically form this ring, but we just don’t see it, and we never will, since we don’t know where it starts?

Maybe the ‘rings’, or expanding mycelia, of other fungi overlap a lot. Think for example of arbuscular mycorrhizal fungi, whose mycelia have a lot of starting points from roots that have been colonized. I think they are very unlikely to form bands. But maybe they are? I don’t see why saprobic fungi shouldn’t all grow in such bands. Maybe some of these bands aren’t close to the soil surface but further down in the soil, then we likely also wouldn’t see their effects; this could happen because competition being too high near the soil surface. Are there deep rings? Or do many fungi get eaten by soil animals, and thus they just never reach the band stage? Is competition among fungi so strong that the bands annihilate each other, and this is why we never see this?

It seems I have definitely been underestimating such rings as an interesting tool to learn about basic fungal biology.

If you have suggestions to any of the points I raised above (or know about studies that have already addressed them), please let me know in the comments!