A region would have a certain percentage of each tree that can grow in the region locked down. So someone could kill all the fruit trees in a region, but couldn't plant more than a certain percentage of non-fruit trees, thus only fruit trees could replace those killed fruit trees.
And lock down the spice trees in the spice region, so the trolls leave them alone. :)
It might help slow down the harassment of killing all the trees of a region and replacing them with a single species.