For the public
Ask an arborist
Ask an arborist is public web forum where you can pose your questions about trees to our panel of arborists. The forums archive is easily searchable and with tens of thousands of messages there's every chance that the answer to your question is already here, but if not, it's free, it's easy, so want not pose your question?
For the professional
UK Tree Care mailing list
The UK Tree Care mailing list is one of livliest forums on the internet for arborists and other tree care professionals. Known to its members as the UKTC, it is an email based discussion group that has been running since 1999. With a fully searchable web based archive it is an invaluable source of information about arboriculture and urban forestry.