Hello RED, the best advice for improving the long term health of any tree growing in a lawn is to carefully remove a circle of truf around the base of the tree with as big a radius as possible (or as much as you find visually pleasing). There are a great many very fine feeder roots in this area of top soil so be careful to cut a very thin thickness of soil with each sod of turf. You will then have the trunk of your tree surrounded by a bare circle of earth say 6-10' in radius, over the bare earth spread aged woodchips to a depth of 2/3" leaving a space around the trunk itself (having any mulch piled up against the stem of the tree is a very bad idea for tree health).
This layer of mulch will gradually breakdon and in the process produce the perfect environment for healthy tree roots and the symbiotic micro soil organisms crucial to effective absorption of soil minerals and water.
Any large surface roots will normally be prodiced by the tree in this areaq close to the stem (unless very adverse soil conditions exist in the growing site ie; highly compacted or waterlogged soils)and creating the mulched area will avoid mower damage as well as eliminate tripping hazards.