As this enchantment enters, choose a creature type.
Creatures you control are the chosen type. The same is true for creature spells you control and creature cards you own that aren't on the battlefield.
All Forests and all Saprolings are 1/1 green Saproling creatures and Forest lands in addition to their other types. (They're affected by summoning sickness.)
When this creature enters, exile all lands.
When this creature leaves the battlefield, return the exiled cards to the battlefield tapped under their owners' control.