Didn't find anything about climbers, but they do have pretty strict rules. http://www.dnr.state.md.us/forests/download/rtregs03.pdf This is from the DNR site: "Who must have a license? All tree care professionals practicing in Maryland must obtain a license. Without a license, they may not practice or advertise tree care services in the state. To obtain a license, the applicant must possess adequate and related college education plus one year of experience under a LTE or have five years experience under a Licensed Tree Expert (LTE), then have passed an exam and carry adequate amounts of liability and property damage insurance. The license must be renewed annually every January." Not something I, myself, would mess with unless a lawyer told me otherwise. You or anyone else are free to do what you want.