Have you ever noticed older lawns are higher than the sidewalk? They didn’t start out that way, thatch, and the lawns natural way to create soil, built that up. This can become a real problem for a Tree.
The trunk flare must be exposed (we get into the reason why in the Tree Evaluation Booklet).
If the flare is covered it can kill a tree over a 15 year time period. See University Evidence
Girdling root and other tree killing issues occur when trees are planted too deep or the trunk flare is covered. The root is strangling the vascular system.
The vascular system is just below the bark, so lawn mowers and weed trimmers can damage the trees arteries when they accidentally tear through that bark.
Easy answer, get that tree well around the tree. But do not tear out the grass if there are roots present. If the tree is mature you will do less damage with glysophate sprayed to kill the grass and let it die off, then mulch the area (see Green Team Clients Free Booklet).