I Googled 'tree' on my smartphone and one of the first definitions I came across was:
Something constructed in the form of, or considered as resembling, a tree, consisting of a stem, or stock, and branches; as, a genealogical tree.
That's from the 1913 edition of Webster's.
Now I'm sober, I understand what the definition is driving at - that 'tree' is sometimes used metaphorically (or perhaps I mean analogously?).
But I have to say that defining a tree as something "in the form of... a tree" does not help resolve drunken arguments. And really, what else are dictionary definitions for?