The following organizations and websites are a good place to start searching for the information you need related to trees.
The City of Atlanta Arborist Division is responsible for protecting trees on private property throughout the City. If you have a tree to be removed, you must obtain a permit from this office.
This page details everything you need to know about tree removal procedures in the City of Decatur.
This is the professional organization for individuals and companies which take care of your trees in Georgia.
This is the certifying organization for tree professionals of all types. If you couldn't find a certified arborist on the GAA website directory, the ISA's listings may help.