Greetings Bob,
Most tree identification tags that I see are hardened plastic with UV coating or aluminum tags that have been engraved.
Don't nail the ID tags to the tree, rather hang them from a limb or place them at the base of the tree. Many arboretums utilize wooden stakes (4" x 4" post) and plastic signs to identify woody and herbaceous plants.
To give you some good ideas, I would "Google" Forestry Suppliers Inc., and Voss Signs LLC.; both these companies sell tree identification tags and signs.
Good luck!