The so-called mushroom circle is actually a reflection of the mushroom growth circle! That is, the direction of mycelium, just like on the culture medium, mycelium will grow from the inoculation block and grow radially around, and it is the same on the ground. (If you don't understand here, you can go to the microbiology laboratory! )
From the initial growth, the mushroom group formed the center of the later mushroom circle. The mushrooms grown in the second year always move outward than those grown in the first year, and so on ... In the next n years, we will see a bigger mushroom circle!
The reason why this happens is that the top growth tissue of mycelium (that is, the mycelium tissue at the edge of mushroom circle) has stronger growth and fruiting ability! However, the mycelium growth ability of the middle part (which is also related to the nutritional status of the soil) is decreasing year by year, which is the same as changing the cultivation materials every time you cultivate mushrooms!
That's about it. If you want to elaborate, you can write a paper! I hope it helps you!