Those are good, but if you're looking for an actual canopy of trees, I too have been looking for something like that.
I have not done it yet, but the best I can come up with would be to construct a number of trunks that sit on the table. Then, cover it with a mesh of some kind covered in flock and moss. Different sections so that they can be lifted as the figures enter the area.
This is similar to the way that model railroaders build up a dense forest along the back of a layout, and I dont see any reason that it wouldn't work here.
If you do it, I'd love to see how it works out. It might be a bit before I get to it, though this has sort of inspired me again!