As far as I've seen, and heard, forests do not regrow their tiles. This would be very annoying for people who lived in the forest, Surrounded by endless trees, only for them to be clear cut and the trees and forest tiles gone forever. So for this, I propose a new feature for decay; if a wooden object decays enough, it causes a forest to appear, and when it decays entirely, it mosses over and has a chance of giving birth to a sapling. Also, pre-planted forest trees could slowly generate forest tiles in a radiating circular pattern. This would allow ruins of towns to become taken over by plants due to noone living there, and also thus causing interesting ruins sites.
Just a thought, I always found it annoying when forest tiles never came back. If this is already implemented, lock this post.