Mulch helps keep roots cool in summer and warm in winter. It keeps weeds down and helps hold in moisture. A "moat" of mulch also protects urban trees from cuts and nicks caused by lawn-mowers and weed-whippers. Studies have shown that wood-chip mulch can nearly double the rate of tree growth during the few years after planting. Mulch even fertilizes as it breaks down.
Proper Mulch Technique
A 2 to 3" deep layer of mulch should be spread in a circle as far around the base of a young tree as possible, but at the very least 2 or 3 feet out. The right way to mulch is depicted below on the left; the wrong way to mulch is seen below on the right.
Mulch should be spread out in the shape of a saucer, not a mound, as is the common mistake. The saucer shape will hold and distribute rain water to a tree's roots more effectively. Mulch mounded up against a tree's trunk can cause girdling roots and other problems.