The forests are where most of the action takes place in Jade Cocoon. You access them via one of the two gates in the Jade Throne Room - the Kumari or Kaya gates.
You start off in the Wormhole forest, accessed via the Kumari gate, until you retrieve all four Forest orbs. Once this happens, the Dark Forests, and its gate, the Kaya gate are unlocked.
At the end of the main game, the Kaya gate is shut.