A tree's root system is the most vital organ on a tree. When roots fail to work properly the tree will eventually decline and die. A few common mistakes made by tree owners is to build or pave over roots, excavate on and around the tree trunk, park or store equipment and/or toxic material over the root zone.
The attached photo is of a magnolia showing signs of stress due to trailer and building materials invading the root zone. Actually, in this case it is the neighbor of the tree owner doing the damage.