If you control a Forest, rather than pay this spell's mana cost, you may have an opponent gain 3 life.
Target creature gets +4/+4 until end of turn.
If an effect says that an opponent can't gain life, you can't have that player gain life to pay Invigorate's alternative cost. If an effect instead replaces life gain, such as that of Sulfuric Vortex, you may choose to pay the cost even though the cost's action is replaced with nothing.