According to Build Green: Wood Can Last for Centuries, the same principles of decay that are applicable to buildings also apply to wooden decks, fences, boardwalks, pergolas, gazebos, planters, and playground structures.
Authors Carol Clausen and Samuel Glass tell us that the greatest decay hazard exists at the ground line. Pressure-treated wood approved for use in the ground is recommended. A lesser, but still important, decay hazard exists where horizontal rails, stringers, or timbers are joined together and where vertical boards fasten to the structure. Both of these locations collect water and dry slowly. Use preservative-treated wood in these areas and keep vertical boards off the ground.