Use mowers and string trimmers carefully around your trees and shrubs. Routine lawn care can cause high-impact damage to the protective bark of trees and shrubs. A tree’s life support system of conduction tissues is located directly under the bark. Wounds can stop the flow of nutrients, invite invasive insects and diseases and ultimately cause the tree to decline. Protect the bark area with two to three inches of organic mulch, taking care not to place it directly against the trunk.