How to detect doodad entering region?

Is there any way I can detect if a doodad enters a region?

A doodad can't move, so where's the point?

I'm making a few triggers spawning TONS of trees for some random doodad placement, but i don't want the middle to have any extra trees in it.

Best modify the creation. First compute a random location, then check if it is in the middle and only if not create the tree.

Alright, thanks, I'll see if I can.

EDIT: It worked! Thanks for idea man.

Another way is just to remove all the trees in the middle area after they have been placed.

That wouldn't of worked, I had pre=placed trees that couldn't be removed, though that would work.