Maybe they just grow like trees. Well, what I mean is they start slightly tree-ish, but as they get older develop into more tree-like beings, depending on which tree they chose to guard.
I can just imagine it: Over time. First a little dark patch, then a spot, then a lump, then a shoot, and after about 20 years of slow growing, a small brach.
edit: A sad thought just came into my mind. Maybe all ents are sufferers of a disease, like
this?