I've heard a few things. The big obstacle with replacing them with new mature trees is the amount of Spike 80 or whatever it is left in the soil. Apparently they are monitoring it pretty closely to see how much washes away or decomposes. I have no idea how long it takes that stuff to break down but I imagine it takes a while.
The problem with putting wood sculptures at the site is one of security. Idiots like setting fire to the trees and if you put wooden anything there, they'll try to burn that too. Which is a shame because that would be cool.