Maybe some evergreen-style trees around a big shrub-shape. You're right, they wouldn't worry about aesthetics so much, so you'd only need a hint of a forest, but you could dress it up. Otherwise you could do something ancient/unrefined like a single overly large tree for each forest, a single mountain for a range, etc... and make the *size* of the icon indicate the size of the forest/mountain/whatever... ?

I'm not much help, sorry

Big like for the idea and the map so far, though!