I remember it very distantly from my childhood but I really enjoyed it and would love to be able to revisit it. I just remember that the world was one giant tree, and it begins in the "Root Realms" and eventually our main cast of characters has to venture up the trunk of the tree to the branch realms above that have never been explored. As for the characters I really don't remember much about them at all. I recall a group of avian people that lived in one of the root realms I think. Oh, and I believe that each realm championed a specific element or something, like the fire realm and the rock realm and so on, but I'm not 100% on that.
I think this is "The Great Tree of Avalon" trilogy by T.A. Barron.
The Seven Realms, also called the Root-Realms, are the roots of the great tree of Avalon, and where the majority of creatures live. The root realms are Woodroot, Wateroot, Airoot, Shadowroot, Mudroot, Stoneroot, and Fireroot.
Edit: The avian people you remember were the Eaglemen. Note that this trilogy was part of a larger series about Merlin, who planted the tree in an earlier book. It's possible you may be remembering a different book in the larger series, but as this trilogy focuses specifically on life in the tree I'm putting it up as the answer.