The Tree of Harmony is a minigame added in the Everfree Forest update in which players donate Element Shards to try to remove the Plunderseed Vines from the Tree of Harmony by donating Shards. Each branch of the Tree (representing each of the Mane 6 respective Elements) can be unlocked. When worldwide players achieve 100% of Shards on each bar for each element on the "Save the Tree of Harmony and Save Equestria!" Event page, an Element branch is unlocked. To access the Events page press the 'Events' tab in the bottom left corner of the game screen. Once all of the Branches have been unlocked a special limited edition Tree of Harmony decoration is rewarded to all game players. A treasure box that has six locks and requires six keys to open it appears when a player has destroyed the vines and has saved the Tree of Harmony. After the Tree is saved and Tirek is defeated, the cave is viewed as a normal background image and can't be accessed anymore. This is a global event.
Once you have saved the Tree of Harmony, don't expect to be out of the woods just yet. For as soon as you complete the Tree of Harmony Event, the Tirek Event begins. And you'll have to once again, collect as many shards as possible.
So make sure you're packing enough elemental shards in your inventory.
Note: You don't need all the Mane Six to cure the Tree.