The World Tree

Not exactly a tree, but nonetheless, a tree in a metaphysical sense.
It's the seed of Faerie into a world. Fairy Tale worlds need it to exist, it gives them the connection to the magic they want/need to function properly. The Tree, when planted, weaves itself into the planet and bridges Faerie with the world it's planets into.

Some trees don't weave well enough into the worlds to bleed over or cause weakspots, but due to Mab's fleeing into Anagod, she disrupted the veil and holes and threadbare places have been rubbed by the 'friction' against the tree and the planet and all the forces between them.

The seeds produced by the World Tree fell into Anagod and became shapeshifters and Elves and Merfolk and the like made their ways over.