I know this question has been asked before but the SimHQ search feature (least on my phone) is hideous.
When using the Airfield Mods, like the Combined Airfield, does the Workshop setting of “Airfield Complexity” do anything?

Number of trees = Airfield Complexity.

Sorry to be obtuse but even with modded airfields with their handplaced(?) trees?
Guess my thought was they would be independent of the Workshop setting. Presuming, of course, the mods work as I think with things placed there by the modder are always there.

Sorry I'm late to the prom! Panama is correct and to answer your more specific question it does reduce the trees even in modded airfields. That by the way is a good things because it puts the option in the hands of the player to reduce FPS impact.
If you fly Izel custom mod and reduce the airfield complexity in workshops, you will notice all the trees on the last part of the town road disappear as well as a bunch of trees around the parked trucks.

So much for thinking I know how modded airfields work.
Obviously I am not a modder.
Thank you gentlemen!

Switched Airfield Complexity from High to Medium and didn’t experience any FPS gain FWIW.
Always lotsa variables though from mission to mission in a campaign.

That is because you are measuring wrong, it is not about FPS.