Too Much Mulching Can Kill
Don't mound mulch up against the trunks of trees and shrubs. These volcano-shaped piles may look good to some, but they can kill your trees.
Instead, spread the mulch about 3 to 4 inches deep in a circle at least 3 feet in diameter around the young trees and shrubs. Then brush away mulch at the center of the circle so that it is several inches form the base.
A slight depression in the center is OK. Think lunar crater--not Mt. Vesuvius.
As the trees grow, extend the mulch to their dripline (the edge of the canopy).
Trees benefit more from this kind of extensive mulching. Piling mulch deeper and higher does not help the tree, and in fact will kill a tree.