Answer: It's generally not recommended to put topsoil over the roots of a tree but 6" of bark mulch should be okay, providing you don't place plastic beneath it. You'll want the mulch to be water and oxygen penetrable so the roots of your liquidambar will continue to get the moisture and air it's used to getting. Try not to pile the mulch material up against the trunk of the tree where it could hold too much moisture and rot the bark of your tree. Best wishes with your landscape!
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