Renewable and non-renewable resources

[heilkitty]

E.g.:

1. When forest is used for certain number of turns, it automatically transforms to some other, less productive terrain. When that terrain is not used for some turns, it transforms back. (Currently ecological impact of humans is pretty much limited to global warming and nuclear winter, the agriculture and stuff appears to be sustainable and "green".) Support of various conditions would be good for this feature (forest with slash-and-burn agriculture wears out faster than grasslands with irrigation).
2. When terrain with oil is used for some turns, the oil disappears for good.

[Lachu]

It be great, if forest don't go back once player irrigate terrain.