When I worked in a vocational training capacity, it was brought to my attention that residential/commercial tree workers and utility tree workers were not recognized in Federal documents as "Skilled Labor" trades.
Many tree workers are self taught, or taught on the job by self-taught coworkers. The formalized training from big companies is varied and often inconsistent.
Although there are several avenues of study and training for tree workers, I believe the ISA's Certifed Arborist program has been the most recognized, respected, and desired nationally.
The idea that tree work be recognized as a skilled trade is a critical one, and currently the CA through the ISA is your best bet.
If you have a job that can be taken by someone, anyone, with no training or experience, then you don't have a job worth having. The idea that arborists have a defined skill set and knowledge base is critical to our inclusion in the rapidly shrinking blue-collar middle class.
I often make sarcastic comments regarding the ISA and it's various certifications; but for many individuals those credentials are the only path to career growth available.
Personally:
On paper, the certifications I have earned have gotten me jobs.
On paper, the certifications I have let lapse have barred me from opportunities I could have otherwise pursued.
On the ground, in the field... the combination of experience, skill, and certification have earned me the respect of co-workers, been responsible for promotions, put me in leadership roles, and allowed me tremendous flexibility to move within the industry.