I saw it like the entire planet was like a single giant organism - Eywa - and that many of its inhabitants therefore had those neural interfaces (could call them axons, perhaps?) for communicating and merging with each other, including the trees. As if every creature on Pandora was like a standalone, self-sustaining part of the whole, but could connect to others if needed. The reason the Na'vi could save memories in the trees might be that they interfaced with them just like they did with the Ikran, for example, and their thoughts would be passed from tree to tree across the 10^12 trees on Pandora, probably remain in some of the trees at any given time, and never be "forgotten" - like a message travelling through an infinite loop of redirections. Maybe the Na'vi were the only ones intelligent enough to realize that the link could be used across species in this way, and to "drive" other creatures. However, I found it interesting that other creatures had two axons, while the Na'vi only had one. Theories?