Fridge Horror: The heroine is the sole surviving member of the Mana Tribe and the only one left eligible to become the Mana Tree. She actually has to become the tree after the final battle, despite her falling in love with the hero. Keep in mind that the Mana Tree is the source of all peace and life in that world, and did I mention she's the last of her race?
Also, if you consider Secret of Mana the direct sequel to this game, guess what happens to the Mana Tree? Gets blown to bits by a Lost Superweapon dug up by the Empire, that's what.