Green Apocalypses

[Mechalich]

I am working on a campaign in which the entire world is covered in a forest that grows back faster than land can be cleared

The trick here is that land can be cleared by fire which tends to render most defenses the forest has irrelevant, especially if you're willing to burn multiple times in succession - you can clear the freaking Amazon that way (takes about 4 sustained burns). Doing so if not necessarily going to leave any fertile land behind and may in fact produce a wasteland, but if the forest is super dangerous, surrounding your settlement with a burned-out wasteland is almost certainly preferable.

As a result, for this to work you need some means of fire suppression. Possibly this can be simply having your world being extremely wet - there are places on Earth where it rains all the time - but then you have rain world and not forest world. So you're probably looking at a supernatural means of fire suppression. One method might be to have gaseous organisms or spirits that normally live in the atmosphere but can descend lower at need. If they have bodies composed of inert gases they can choke out fires.