Is there anything preventing wind power from being above forests? Every wind farm I've seen has been on empty fields but it doesn't seem impossible that you could use the land for crops or even trees.
Efficiency of wind turbines increases with height above 'ground'. I expect for this purpose the top of the tree canopy would be the ground, so turbines would have to be higher by the amount of the forest canopy height, at least. I expect there are more issues with wind flow over tree canopy vs a crop field, but I suspect the height issue alone is enough to make the economics pretty bad.
Not only that, but the trees would provide habitat for all manner of birds which, to not put too fine a point on it, would end up diced by the turbine blades.
The land owner would make more money 1) cutting the trees and selling the lumber, 2) turning the land into farm land or grazing land, and then finally 3) erecting a wind farm than just doing #3 by itself.
Better to situate land-based wind turbines on hills, which usually don't have many trees.