Enchant Forest or Plains
Whenever enchanted land becomes tapped, its controller loses 2 life.
Corrupted Roots can enchant only a Forest or a Plains. It checks the enchanted land’s subtypes, not its name. If at any time the enchanted land is neither a Forest nor a Plains (due to Unstable Frontier’s ability, perhaps), Corrupted Roots is put into its owner’s graveyard as a state-based action.
Corrupted Roots doesn’t care why the enchanted land becomes tapped. If that land changes from being untapped to being tapped for any reason, the ability will trigger.