Qualifications
Many folks who become arborists start out as groundsmen with no intention of making a career of physical labor. Qualifications are limited. To be honest, many become groundsmen because they lack the experience, education, or motivation for other work. Those that move on to become climbers need to be adaptable to another type of physical labor that allows them to get around trees efficiently.
The best arborists and a few groundsmen who are experienced enough can get their certification from the International Society of Arboriculture to become "certified arborists" (source). These are the guys who're qualified to write reports necessary for tree removal permits that so many municipalities require.
Many who get a bachelor's degree in horticulture also become certified arborists in a backward sort of way without first becoming a groundsman or climber.