Any arborist you hire must understand how to work on trees, but also how trees work .
Trees have remarkable built-in defenses against rot.
An arborist who works with these defenses can improve any tree worth saving. The best way you can be sure of good tree work is to understand the problems and the solutions, and be there to make sure the work is done right. The arborist you hire must understand tree biology. He must know what to do, and what to avoid. A good arborist will understand these things, and can discuss them in a way that makes sense.