Trees are cannibals. Plain and simple. They eat themselves, and they have adapted a pretty good system in doing so. Think about where leaves and branches fall to when they come off a tree. Those branches and leaves are, in a forest setting, being recycled by either the parent tree, or another nearby tree.
In an urban setting, that all goes out the window. Leaves are raked, branches hauled to the curb for a municipality to chip up and dispose of. So why not try to give your trees a little piece of the paradise like their counterparts in the forest? Mulching does just that. It adds some of that lost organic matter that is hauled away in large trucks. Not only is mulch a great food for trees as it breaks down, but it helps to amend the soil, adding organic matter which improves water-holding capacity, reduces compaction, and increases beneficial microbes in the soil.
-Jonathan R. Bantle