Mulches provide one of the greatest benefits to trees and shrubs. In nature this occurs as a natural event, with leaves and twigs falling to the ground and naturally decomposing and enriching the soil. As this occurs, beneficial fungi called mycorrhizae begin to colonize along the smaller, finer absorbing roots, forming a symbiotic relationship. This root and fungi relationship helps roots to absorb moisture and nutrients more readily. This beneficial relationship helps to keep roots and trees healthy and further reduces environmental stress caused by weather or other events.
(Note: A live spore of these fungi is now available for landscape use. Search: mycorrhizae inoculants).
As we try to replicate this in our residential and commercial landscapes, we must use caution. All to often I see poor and inappropriate applications of mulch that can cause more harm than benefit. This is especially true when applied in excess. Please keep in mind that mulch products such as recycled rubbers and plastics, and even stone, that do not decompose, do absolutely nothing to improve soil structure or microbial activity that will benefit roots.