From Consumer Wiki
An arborist is licensed with the California Department of Consumer Affairs, Contractors State License Board under the category of "Landscaping Contractor" (C-27 License). As an arborist, the individual may graft trees, shape, or plant trees. An arborist is not a tree trimmer.
Contractors State License Board
9821 Business Park Drive
Sacramento, CA 95827
(916) 255-3933 (fax)