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In the answer to "Why is ground water rising on these forest rings?" it is written:
these rings are too numerous and too regular in shape to be related to either kimberlites or (even more bizarrely) Oklo-type natural nuclear reactors
How is the regularity of forest rings assessed? How many near-rings and other shapes have been found?
From my correspondence with Dr. Hamilton, who is one of the leading researchers on forest rings (emphasis mine):
They can be irregular (see figure below) but it is not very common. You are right, the circularity is why we notice them and there are probably lots that we don’t notice because they are not round. However, the mechanism of formation that we have postulated ought to produce circular shapes. All point-source force fields in an isotropic medium will produce a circle if the area of influence is large enough. We argue that the source of energy is the electrochemical potential (redox) difference between the inside and outside of the ring; and the force is an electrical field. The groundwater and glacial overburden have fairly isotropic electrical conductivity (the inverse of resistance) and therefore circularity is a requirement if the current density is sufficient.
This photo was taken near from northeastern Ontario north of Kapuskasing. The circular ring is ~150m across.
