The Great Forest

The Great Forest's first ability means, for example, that a 2/3 creature will assign 3 damage in combat instead of 2.

The Great Forest's first ability doesn't actually change creatures' power; it changes only the value of the combat damage they assign. All other rules and effects that check power or toughness use the real values.

A plane card is treated as if its text box included "When you roll {PW}, put this card on the bottom of its owner's planar deck face down, then move the top card of your planar deck off that planar deck and turn it face up." This is called the "planeswalking ability."