Mulching is one of the most beneficial things that can be done for the health of a tree.  Mulch can reduce water loss from the soil, minimize weed competition, and improve soil structure.  Mulch can give landscapes a handsome, well-groomed appearance.  However, if mulch is too deep or if the wrong material is used, it can actually cause significant harm to trees, or proper application important.  Mulch should be applied in a 2 to 4 inch layer.  The mulched area should extend as far out from the tree as practical for the landscape site. (From the International Society of Arboriculture)