TCG Rulings

• If the effect of "Spiritual Forest" is resolved once during a turn, it cannot be resolved again even if "Spiritual Forest" is negated and then stops being negated in the same turn.^[1]

• If a face-down Beast-Type, Beast-Warrior-Type, or Plant-Type monster you control is attacked and flipped face-up by battle, the effect of "Spiritual Forest" is applied.^[1]

• You do not choose when to activate this effect. It automatically activates the first time the appropriate type of monster would be destroyed.^[1]

OCG Rulings

• The effect of "Spiritual Forest" is not optional; the player does not choose when to apply the effect. After "Spiritual Forest" is activated, its effect is automatically applied during the first battle in which a monster would be destroyed.^[2]

References

1. ↑ ^{1.0 1.1 1.2} Konami Gameplay FAQ: Stardust Overdrive Sneak Peek -- Card Rulings (Version 1.0)
2. ↑ Konami FAQ: Can the player choose when to apply the effect of "Spiritual Forest"?