To the Brood

Each green permanent you control looses all abilities, and becomes a colorless enchantment with "At the beginning of your upkeep, put a 1/1 colorless Eldrazi Scion creature token onto the battlefield. It has "Sacrifice this creature: Add {c} to your mana pool."
The rich forests gave up the growth of plant life and the ability to support of animal life in favor of becoming a place for something much different to thrive. Whether they did it willingly or not is another story.