The tree is likely magical. It isn't one tree - that would be fairly preposterous - it is a tree braid made out of many trees. The walls and gaps inside would therefore be edged with almost natural wickerwork - many branches and trunks weaving together to lend strength and flexibility with little twigs sticking out at semi-random intervals, probably with little, glowing flowers on them that get replaced occasionally (and incidentally leaving a soft, petal floor in the tunnel).
The tree(s) themselves can for instance be related to the sequoia - it's a good hue and general texture to have.
You could also have the characters be shrunken slightly (like the opposite of Enlarge Person) in order to better fit in the trunk.
There's also plenty of room for other (giant) pests - there might be some parts that have (giant) tree wasps and their hives in them, there will likely be other insect hives that could be encountered, and at the 'ground floor' there might be the top of (giant) ant hives.
It is a bit odd that a treant would be using something like Life Leech Gauntlets - they're usually Chaotic Good, and using Life Leech anything isn't usually a good act whatsoever.
The treant might have asked the elves for help with something, but was rebuffed by the elven council and is therefore trying to build its power to deal with the problem on its own. Maybe its forest is threatened by an invasion, or there's a ruin there which is corrupting the little forest animals that get lost inside the ruin - and they're coming out as forever-hungry zombie forest critters.
