Tree Well

A trees number one enemy is that lawn mower or line trimmer damaging the cambium layer just below the bark. This is the area that carries nutrients and water. The second enemy is the grass growing around the roots competing for water and nutrients.
Simple solution is to use Glyphosate

(Round-up type product) to create a tree well -  simply spray it and let it die back (you can use the line trimmer to get rid of the dry dead grass look and get down to the thatch brown look)