In nature, trees drop their leaves, twigs and other debris onto the soil where it decomposes into useable nutrients for the tree. It also helps with erosion and moisture control. In a suburban environment we often blow away or rake up all the leaves and tree litter leaving nothing to decompose for the tree. Mulch is made up of tree trunk branch chippings and leaves. If you do not like the way it looks, but want the health benefits for your tree, you can put down a two to three inch layer of the mulch and then a inch layer of decorative colored bark.