Discovered this accidentally, and it seems like a bug/unintentional exploit: If you go through the Dwarven Halls and enter the Animated Forest via the upstairs that lead to the left side of the AF, and then go down again (without killing any trees) you get the Treehugger achievement ("went through the animated forest without killing any trees") - I guess it's technically sort of correct - I left the AF without killing any trees, but I didn't go through all of it! I guess the achievement should only be gained after entering through the down stairs that lead to the right hand side of the AF and then leaving the level through the stairs on the left hand side.