If you over farm the yield should drop to almost nothing and trees should stunt and never grow beyond their first harvestable tier and reach it much more slowly. Pollution should also diffuse out more from it's emitters and air pollution should flow around a basic prevailing winds system that covers the map, Gradually forming large smog clouds if there is not enough nature to properly sink it. A city without greenery should also become a smoggy wasteland if you place too many pollution emitting crafting stations and no parks in it.