Where does the warriors series take place?

In the forest, though in Dawn, the clans move to the lake territory.

Actually, to be more specific, the main part of the story-thunderclan territory-takes place in a forest just out side of the twoleg place. River clan, the territory that borders the river. Shadowclan the marshy part of the forest. and Windclan on the prarie(sp?).

The rest of the expanded series takes place in a simalar territory, but around a lake instead of the "fourtrees"

In the series through dawn, the cats live in "The Forest" until it is desroyed by twolegs, then they move to "The Lake."