According to documentation (PDF) we obtained from Japan’s Ministry of Agriculture, Forestry and Fisheries, in 1973 an area of land near Nichinan City was designated as “experimental forestry” and one of the experiments was to try and measure the effect of tree spacing on growth. The experiment was carried out by planting trees in 10 degree radial increments forming 10 concentric circles of varying diameters.

Part of what makes the crop circles so alluring are their concave shape, which was an unexpected result of the experiment that would suggest tree density does indeed affect growth. The trees are due to be harvested in about 5 years but officials are now considering preserving the crop circles.

https://www.thisiscolossal.com/2018/12/tree-crop-circles-emerge-in-japan/