Forest rings shape regularity
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.