"The land gives of itself. Its abundance provides, even in times of chaos."

Sylvan Advocate

Vigilance
As long as you control six or more lands, Sylvan Advocate and land creatures you control get +2/+2.

Zendikar Rising Commander

Duel Decks: Elves vs. Inventors

Oath of the Gatewatch

Oath of the Gatewatch Promos

A “land creature” is a permanent that’s both a land and a creature.

Damage remains marked on creatures until the turn ends. If Sylvan Advocate’s ability stops applying (because Sylvan Advocate leaves the battlefield or you no longer control six or more lands), then any land creatures that needed the toughness bonus to stay alive will be destroyed.