Looks like it’s pay to play again (and not just on tax day). As of September 2016, visitors to certain fee sites in southern California national forests will need to display a valid recreation pass.
This applies to forest visitors parked in standard amenity recreation fee sites in Angeles, Cleveland, Los Padres and San Bernardino National Forests. This interactive map shows the locations where passes are needed: http://www.fs.fed.us/portaldata/r5/ap/map/
Standard amenity fee sites are those that provide designated developed parking, picnic tables, toilet facilities, security, interpretive signs and trash receptacles. There are several valid passes, including the Adventure Pass and the America the Beautiful pass (also good for national parks). More information on these passes and how to obtain one are available at this link: http://www.fs.usda.gov/main/r5/passes-permits/recreation
If you aren’t visiting an amenity fee site, you don’t need to display a pass on your car.