Mana Tree

In lands observing this Law, every use of magic and presence of magical items adds to the overall mana of the land. Once there is enough potency of a sufficient amount of mana types, a great tree will spawn and heavily increase the overall amount of mana available.
Law originates from The Legend of Mana. All the plant-based beings (sproutlings, flowerlings, and even Lil'Cactus) got a dandelion-like seedpod growing out of them when there was enough mana, and they all ascended to become the Mana Tree. So if you have a civilization of sproutlings, ents, dryads, and florans, you're probably going to lose them.