Repeated use of a string line weedeater around the base of a tree can do harm to what appears to be a "tough" tree trunk. The velocity of the line action can harm the living tissue just below the tree's outer bark and open trunk wounds that will subject the tree to destructive, invasive pathogens. Use care when weedeating around trees; A better, protective measure would be a mulched area around the base of the tree.
Trimming off branches, if not done properly, can leave branch stubs that may allow entry of destructive insects and disease pathogens. Conversely, cutting a branch too close to the tree trunk can remove the "branch collar" which provides a degree of protection to the open wound. Trees actually have protection ability built right into the branch collar, making it unnecessary , even undesirable to "paint" the cut.
Use caution around trees when applying herbicides; some herbicides can leach into the root zone and be picked up by tree roots, causing tree damage. Because they do not last forever and are vulnerable to changes in their surroundings, our job is to exercise responsible and sustainable custodianship to one of our environment's most valuable resources.
The best time to plant a tree is 25 years ago, the second best time is now........a very wise person