Happy to try to help your investigation. And your yard dude shouldn't have any reason to get near your tree except to trim the outside of the rock ring.
If you pull the stuff trying to get going around the edge on the inside, that should make it even more obvious. You could add some mulch such as shredded/chipped wood, leaves, straw, to cover the soil inside the circle to further inhibit stuff from trying to grow. Soon the shade from the tree will also help with that. A mulchy layer will also help maintain the moisture level underneath, retaining moisture longer than bare soil, and keeping the roots a bit cooler, maybe, for how much that is possible in your location. Best luck!
