Oracle Text

If you control a Forest, rather than pay Invigorate's mana cost, you may have an opponent gain 3 life.

Target creature gets +4/+4 until end of turn.

Card Rulings

6/8/2016 If another effect causes Invigorate to cost more, you must pay that additional cost even if you pay its alternative cost.
6/8/2016 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.