Mulch around trees acts to conserve soil moisture, buffer soil temperature, control weeds and protect tender tree trunks from damage inflicted by string trimmers. It also provides organic matter and nutrients to soil. Optimally, mulch should extend out to the dripline of a tree and be no more deeper than about 3 inches. Read more about mulch in this WSU fact sheet.

ISA mulching diagram
This graphic by the International Society of Arboriculture (ISA) shows how to apply mulch correctly: avoid applying against the trunk of the tree. Graphic: ISA, Bugwood.org