Cheers guys and thanks Hessi, the tutorial was very helpful.
I turned off shadows on the trees and made some overlays to fake them. They are not quite perfect yet, and there is a little of what looks like z-fighting where my displacements hit world brushes, but it's certainly an improvement.
[Blocked Image: http://www.code303.com/files/images/static/statictreeshadows.jpg]