Thanks for the reply Bustr.
I finally found a workaround. I was was trying to make a custom version of the ww1af tile because I wanted to lay things out differently. By habit, I didn't want to mess with the original, I just copied everything with a different named tile. It worked fine except on my airfield tile, the trees were sprouting. Even after building clutter mask. Weird!
I eventually found it I found if I renamed my new tile to ww1af and just overrode the old one the trees vanished, but the field showed my changes.
So somehow the TE knew to clear the trees out from under a tile named "ww1af" but not under the exact same data named "ww1af2".
But since I don't need to use both simultaneously, I'll take that as a win.
