@oracle exactly my thoughts :D Magic's icon is used abundantly in Equestian art, perhaps due to Celestia's influence, and now we seem to know its origin. Others may share that icon as part of their cutie mark as seen in Shining Armor thanks to their exposure to the symbol and its commonly understood meaning, but personally I think Twilight's cutie mark was a sign or her foreordination to the role of guardian of the elements and the tree itself. Why it's the main symbol on the tree seems obvious to me since Magic or Friendship is the most powerful force in their world. The real question I think is why are the sun and moon present on the tree? Maybe it's the symbol of the ones who planted the tree. Maybe they're the symbols of the previous element berers like how 5 more symbols have been added for the most recent supporting bearers. #spoilers
