I already prefer Peggado's suggestion to just the spell spawning trees from a concept perspective, but I have some reservations. There's potential technical obstacles as these spawn points are not standardized for spawning something on the edge and keeping it there. I am interested to hear other opinions.
I agree that Isengard might rely too heavily on being able to harvest trees, although practically speaking it's rarely an issue. Devastation is paired well with Palantir to steal the enemy's tree and the Timber Yard can be used in places where you went for a mineshaft, especially with the recent buffs.
I wouldn't mind giving the Timber Yard a small buff. There are multiple possibilities there, we could reuse the old slave mechanic from 3.8.1, the Timber Yard could be upgradable to provide a global buff of some sort, or it could train some unit.
I think it's stupid that the spell only works on certain maps.
All maps should have tress, please report any that don't as bugs