Does a ground fault compromise the whole circuit?
I assume the neutral wire is still unbroken after the tree is across it, in this case nothing would happen.  The tree is already at ground potential, as is the neutral wire, as is the person.  If the neutral conductor is broken then there will likely be an introduced earth fault impedance (either tree or person) and the load would see a voltage drop. If the person becomes the lowest impedance to earth then he will likely be zapped.