Following on from the tractor pull idea, I have had surprising success
with a long rope tied to the highest but strong part of tree for
maximum leverage and a Volvo estate and tow bar. Of course you do need
space to pull in the right direction. Start with some gentle tugs and
dig up the ground on the opposite side of pull to expose roots. When
root is pulled up near the surface chop with axe. Proceed as each root
is exposed. No chain saw needed !!!