Crater Forest
Crater Forest is home of the elusive treemen - who are hunted for their precious barks. The forest can be reached by foot from mountain pass (hint: jump in the river and head down the waterfall) or by going to the 2nd level of the mine and heading out west.
Be warned, the path to the forest is plagued with yetis who love to beat the shit out of overzealous new players. Be particularly aware of the Ashen Yeti, a new breed of yeti who hits very, very hard. It's recommended you avoid him until you have solid dodge and good beat soaks.
The forest itself is irradiated, so remember to bring some rad-q tablets.
Treemen
If you're a mid level player looking for good cash and solid XP, you'll most likely end up grinding trees for some time. Treemen give great XP, and you can scavenge their barks to sell to the old chinese man (who is in Botany Bay) for good cash and the 'Lumberjack' job.
Treemen are slow, but they hit very very hard. Initially as a mid level player, you will probably want to tackle trees with other players in parties of 2 or 3. Treemen do vicious slash/stab damage, so it's recommended you have good slash/stab gear, tire armor and leech hide are two easily acquirable armors that give good protection against slash/stab. Treemen will almost always attack on sight, so you should powerattack almost the instant you enter the tile. If you can stun the treeman from the get go, you/your party can make very short work of him.
Treemen are also weak to fire, so having a weapon or someone who has a weapon that deals fire damage will make your bark hunting alot easier.
Silicate treemen
These assholes are treemen who are affixed to one tile and act as a sort of beacon to other trees when they are engaged. If a player enters the tile, the silicate will grab onto the player, not allowing him to leave, and then will proceed to call in treemen left in the area, who will arrive almost instantly.
The silicate is a pussy, so the real problem is fighting off 2 or more trees at a time which can be a struggle even for tougher players.
Note that there is no real reason to bother yourself with killing silicates (other than the initial kill achievement of course) as they don't have barks to scavenge. If you can, you're better off leaving them alone.