With over 20 years working in law enforcement (10 of that in Western Wyoming in an area surrounded by NF), I'll chime in with what I recommended back in my patrol days to people when asked or dispatched to a call about litter, property damage, trespassing, etc on public or private land.
As safely as possible (read non-confrontational), get pics/video of vehicles AND plates, along with occupants, especially the driver/s. If you can't get electronic evidence safely, try to at least get good vehicle and person descriptions WITH plate info and last known area/direction of travel. Wait till you're sure they've left the area and then get pics/videos of the trash and/or damage. Try to pick up as much as possible and get it out of there asap. Before leaving the area, try to contact law enforcement (NPS, USFS, BLM, state Park Police or County Sheriff) responsible for the area and turn your info over to them. If it is federal land, the Sheriff's Office may or may not be able to do anything, but they should at least turn the info over to the proper federal or state LE for followup.
Lastly, I'd suggest providing your contact info AND a written statement when you file your complaint. It'll help if an arrest is made or summons issued. These MAY keep you from being subpoenaed if it goes to trial or at least give you the option of providing a deposition if you live out of the area. I realize that going to court is a hassle and some may consider it a safety risk with all the crazies out there nowadays, but if you ARE subpoenaed, PLEASE try to go to court. There are a surprising number of jurisdictions where the DA/County Attorney will throw out the case if a witness refuses to testify without sufficient grounds. Remember, THEY weren't there. YOU were.