There is a third meaning of wood, which no longer exists because all woods are now gardens, and that is the madness where the woodwose, the wild men, are. When Birnam wood comes to high Dunsinane hill MacBeth's madness has flowed over all the orderly world. This gives a third and most interesting interpretation, that the purposeful questing of the trees for light and water hides the madness of the system as a whole.