The Tree is a common symbol in mythologies derived from the Proto-Indo-European religion. It is an enormous Tree that in some way ties the world together and it is either guarded or under attack by a Monster.
Since the PIE religion was never written down, its believers being a preliterate culture, the mythology must be reconstructed based on similarities of its progeny. Nidhogg the Dragon attacks the Root in one, Ladon of the Hundred Heads guards it another, the Lizard waits under it in ancient Iran, and around it the cats play Hauissh to defeat the Serpent.