OAR 629-025-0500 section 5 of designated recreation areas subsections b and c restrict the use and carry of loaded firearms. These "designated recreation areas" are trailheads, campsites, boat ramps, and a few more. They say they get this authority from ORS 530.050 I'm assuming section 12, because there is no express authority given to restrict firearms.
Maybe a legal eagle can weigh in on this, but it sounds relatively valid. I'm assuming that this does not apply to CHL holders. Maybe ORS 166.173 or ORS166.170 section 1. ORS 166.170 might be used to argue that the forestry dept. doesn't have the authority to restrict carry considering that ORS 530.050 doesn't give "express authority" to regulate this act.