Tree of Harmony

The Tree of Harmony is a magical tree which holds the Elements of Harmony. It is located in the Everfree Forest, near Princess Celestia and Princess Luna's old castle. The center of the tree has a star shaped like Twilight Sparkle's cutie mark, while Celestia and Luna's cutie marks are on the trunk.

Princess Twilight Sparkle - Part 2
In the second part of season four's two-part premiere, Twilight Sparkle experiences a flashback where she learns that Princess Celestia and Princess Luna discovered the Tree of Harmony thousands of years ago. The princesses took the Elements of Harmony from the tree and used their magic to defeat Discord.

According to Celestia, the tree controls and contains all that grows in the Everfree Forest, even without the Elements in it.

Twilight Sparkle and her friends discover that the Tree of Harmony has lost its magic, allowing Discord's plunderseeds to sprout and grow all over the Everfree Forest, slowly killing the tree. Twilight and her friends give up the Elements of Harmony and return them to the tree, recharging its magic and restoring the forest.

When the Tree of Harmony regains its magic, a flower sprouts at its roots. The flower blooms to reveal a chest with six locks. Princess Celestia tells Twilight that she doesn't know where the keys are. Celestia assures Twilight that it is a mystery she will not be solving alone.

Twilight's Kingdom - Part 2
The tree is seen again with the Elements of Harmony still on its branches with the mysterious chest in front of it. At the episode's climax, the tree activates the elements and connects them with the rainbow power that came from the chest. The tree then transports the chest into the ground in Ponyville, which sprouts into Twilight's new castle. The new castle partly resembles the tree itself.

Other depictions
In The Journal of the Two Sisters, Star Swirl the Bearded show a young Luna and Celestia the tree. The tree had the three cutie marks before Celestia and Luna received theirs. It is implied that Star Swirl knew that Twilight was going to be the third cutie mark.

Miscellaneous
Albero dell'Armonia