"Back in 1973 scientists began a project in ‘experimental forestry,’ with the aim of learning about the spacing of trees and its effect on growth. They planted circles of cedars near Nichinan City in Miyazaki Prefecture, with smaller inner circles expanding incrementally into larger radiuses to create 10 perfect rings.
As you can see in the photo below, the experiment seems to show clear results. The concave shape suggests that the outer circles of least density promote a higher growth, with the tree size getting steadily smaller as the density increases."
Japanese Experiment That Took Half A Century Ended In Amazing Tree ‘Crop Circles’ | Bored Panda
https://www.boredpanda.com/tree-crop-circles-half-century-miyazaki-japan/