Answer: Some types of trees naturally run their roots along the soil surface and the roots should be allowed to continue to grow that way. Adding topsoil is not a good idea as it may smother the tree roots and possibly kill the tree. A thin layer of an inch or two of organic mulch would probably not harm the tree as long as you are not covering thick woody roots or touching the bark on the trunk. If you would prefer a ground cover you might try English ivy (Hedera helix) which is usually far more able than lawn to compete with trees for light, water, and nutrients.
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