Sorry Donny Brook, but that's not everything. In Vanilla/PTW it takes 10 turns to chop a forest with a non-industrious worker. In Conquests it takes only 4. With Industrious workers it takes 5 turns in Vanilla and 3 in Conquests. The power graphs don't take this into account (it got written before Conquests existed, so why would it?). This "When two or more towns can overlap the same square of forest, the harvest bonus will go to the town that is working the forest square at the moment the forest is harvested." also doesn't come as correct.
I think it best to chop a forest in every square that you can. It can work out as a nice boost even to plant and chop forests, so long as you've managed to keep track of which squares you've chopped and which you haven't (screenshots might help here).
Addendum: Pre-sanitation and rails, in *regular* grassland squares, planting a forest can give you an extra shield, or on grassland food bonuses like cattle, wheat, and game also if you don't need the food for growth.