Disclaimer
There is absolutely no evidence for this post that i can recall. This is purely speculation.
The heaven trees as we know it aren’t really trees there’s more like spires rooted in the continent in game.
With this…
Possibilities
1. The heaven trees have been holding up the continent.
2. The heaven trees are a connector to the mystic sea or a mental world that has been storing souls and reincarnating them in the continent.
3. The heaven trees are there just to provide phoxene for the benefit of the people.
4. Phoxene is created by the heaven trees to extract magic power from the atmosphere and which most likely also needs another material to be fully produced.
Those are the theories we have been able to make around the heaven trees. So please add more for what there actual purpose is.
Sable
1: Don't think so.2: High likelihood.3: Don't think so.4: Very high likelihood.
Fluttershy
So, it's the combination of Atlas (the Titan in Greek mythology, known for carrying the whole world with his arms) and Yggdrasil? (The gigantic tree in Norse mythology that connects the Nine Realms)