Planning on a witness not showing up in court is a poor, Poor legal strategy. It's outright stupidity.
The Park Ranger will drive a government vehicle to court and get paid for the trip, not a bad way to make overtime for a GS-5.
What's your plan if the Park Ranger shows up in court?
Plead guilty or no contest. Pay the fine and court fees and 420 miles worth of gas?
Plead not guilty? Want to show your pictures of the shot up sign? Do you understand evidentiary law? Do you know and understand Federal Law and USFS regulations regarding posted signs and notices? What are you going to do if the judge tells you to appear in court the following Wednesday?
Talk to a lawyer.