John K Jordan
That IS sort of like cutting down a tree!
How about a hand saw of some sort guided by an angled wooden frame. I first thought of a bow saw but I don't think I've seen one that will cut 16". Could build a larger tensioned frame from 2x4s for a bow saw blade.
A standard crosscut hand saw might give a better cut if you can find a large one.
I have an antique 2-man crosscut saw somewhere that might work if cleaned up and sharpened.
Reed Gray built a chain saw chop saw that could keep the angle right: