Dogs display aggressive behaviors for numerous reasons.  Knowing your dog is a key element of recognizing true vicious behavior.  Truly vicious dogs are not welcome in the park and can be identified through a temperament test.  Vicious behavior is defined as biting.  Barking off, on the other hand, is a warning and if calmed will not escalate.
If your dog barks off another dog, it is important to show your dog the other dog is not a threat.  First, give your dog the “No!” command.  Then accept the other dog into your pack.  This is easily done by petting the other dog.  Displaying affection to the dogs and reassuring both of them no one is a threat will lower any tension the dogs have.  Dogs are pack social and will follow your lead.