[nuttyarb]

Hiyah Frimsley. The intact cambium is probably the right answer- but only if the stem is fluted so that the cambium can be protected from grazing animals.
The idea behind the root grafting is that water- and therefore nutrients- can still be transported in the xylem and symplast. The tree will be able to support a crown as food can still be produced and stored in the branches and twigs.
The problem for the tree is that food cannot be transported down to the roots, due to the loss of the phloem. This is where the root grafting comes in- the tree can get food from other trees root systems to feed its own roots.
As long as water and nutrients can travel up to the crown, a tree may be able to survive in this way.

Hey Dan- it may be a sycamore, but it is an interesting conundrum nonetheless!