In a landscape setting, the growth and health of trees can be dramatically improved by the proper use of mulches. Trees prefer an acidic soil, resulting from mulch covering, much like Mother Nature would provide in a forest setting.
Unfortunately, even organic mulching can be overdone as we see in the "mulch volcano," which occurs when mulch is piled around the trunk of the tree. A thick layer of mulch next to the tree's trunk will keep the area around the tree trunk considerably more moist, thus causing rot and disease problems to occur.
Proper tree mulching resembles a "donut" or "crater" encircling the tree, but not touching the trunk. To properly mulch around a tree, spread a 2-4 inch layer of mulch out as far as the drip line for a finished look with the thickest area of mulch on the outside of the circle.