I'd recommend doing that instead of foamcore for all the trees, not just the small ones.... being a wargamer rather than a role player, metal washers are what we use exlusively just for looks for all our trees (we don't tend to use paper trees, but railroad ones), they're not obtrusive and make playing around them easier as they don't knock over easily.
We bought ours for no more than .25 ea where I'm from. More expensive than foamcore, but not unreasonably so and easier than cutting out foam circles
