I recently received some information regarding a controlled burn taking place at the Gulf Islands National Seashore, located along the Gulf Coast of Mississippi and the Florida panhandle.
The communications used during this controlled burn were using 168.6125 MHz, in P-25 digital mode, using a Network Access Code (NAC) of N293.
For more frequencies used by the National Park Service, check out the NPS page on the blog site:
http://mt-fedfiles.blogspot.com/p/national-park-service.html