Unlike people, your trees do not heal like humans do. When persons are hurt they generate new tissue to fix or repair the problem. Trees on the other hand, simply produce new tissue that goes over the top.
These giants are little trees that have grown a new tree on the outside every year it has been alive. Whatever damage we do to that tree remains there. Like people, trees have a protective skin on the outside called bark. Just like skin, bark helps to keep the bad things out and good things in. Your trees also have a structure for mechanical support just like we have muscles and bones. This structure is wood. It is more flexible than bone but less flexible than muscle. Trees also store food and other important things in their wood. Trees do not get fat, but sometimes they can grow too fast.
Trees can, and often do, die of starvation and trees can suffocate. When we damage the bark or the wood, the tree is being seriously hurt. Because the tree cannot heal, these injuries are permanent and often cause problems for many years thereafter. For this reason, it is very important that we take care of the things that we do to and around our trees.
A simple thing like a clip with a whipper snipper, or even over watering the grass, can have permanent and irreversible consequences on your tree.