I really like magic involving trees, so when I encounter such magic in a serie I just gotta analyse it. While doing so, I noticed some things. For those who have forgotten about any of these magics: Great Tree Arc was used by Azuma of Grimoire Heart and Green Magic is used by Walrod Seeken, the Wizard Saint who was involved with founding Fairy Tail over a century ago.
The first thing I noticed while comparing them is the Great Tenrou Tree. This tree bestows a "divine" protection to FT members who happen to be on Tenrou Island, granting them all sorts of advantages which can be summarized as plot armor. However, what actually caught my attention was the shape of the Tenrou Tree. The trunk seems to consist of several trunks who swirl around eachother, giving the impression that the tree has a twisted trunk. What's the big deal with the trunk's appearance? Well, it closely resembles the tree created by Walrod Seeken to transport Natsu and the others to the Sun Village.
The interesting thing is that Walrod was once a member of FT, so it isn't too difficult to arrive at the conclusion that it could've been him who grew the Tenrou Tree in the first place.
The second thing that caught my attention is the fact that those who use these magics become more tree-like. Azuma became a complete tree after "overusing" his magic, while Walrod seemed like a normal human during FT's founding and now he clearly looks more like a walking broccoli than a human.
Therefore I think it's possible that these two magics are in fact one and the same magic with enormous potential and versatility. I know Walrod said he doesn't have any offensive moves and that he isn't anything special, but he jokes around all the time, so when push comes to shove he is likely a force to be reckoned with. It's possible that the protection the Great Tenrou Tree offers to FT members was implemented by Walrod and not by Mavis, so if such powerful blessings can be created through Walrod's magic, it makes one wonder just how much he may be able to do with his magic. He may be able to turn pretty much any situation into his favor by merely growing a tree which provides a special blessing.
Another possible clue about this magic's potential is the fact that Precht taught it to Azuma. He might have wanted to "create" a second Walrod in order to have these powerful blessings at his disposal when the situation required it. After all, Precht was aware of the powers the Great Tenrou Tree has and Azuma was already on the Island before Grimoire Heart detected Zeref (they only detected him after he nearly killed Natsu, Evergreen and Elfman), perhaps Azuma was there to study the Tree created by Walrod.
Taking all these things into account, I dare say Great Tree Arc and Green magic could be the same, but whether that's the case depends on the answer to the following questions:
- Was the Great Tenrou Tree created by Walrod?
- If so, was it him who established the "divine" protection?
- If he did, does that mean he can bestow other blessings as well?
- Was Precht inspired by Walrod's magic when he trained Azuma?
- Could this be why he send Azuma to Tenrou Island before they found out Zeref was there?
If all these answers come together, just how freaking powerful is Walrod? Just how strong can those 3 other Gods of Ishgar, who still rank above him, be?