This is just an idea that came across my mind:
Since it's possible that Lucky Tree event drops diamonds using nectar,
likewise, then there should be an option to save progress of personal lucky tree level
using the same diamonds for the next event.
That being said, this is the part that is for the devs to decide how increasingly expensive it is to save
the lucky tree as it levels up. But a general guideline is that the exp built up during the current event will be lost
as this only saves the level that the tree has made it up to during the current event.
If there is fear that this system is being abused, then VIP restriction can be imposed for tree levels above 3.