Emery Davis writes
Klara, if the stripped bark rings the tree so that the cambium is
broken, everything above that point will die. You might as well cut it
back now, and start pruning for basic shape next year. This is better
than starting again because your maple is already somewhat established,
which is the hard part. If 90 degrees or more of cambium remains at
any place my experience is that the tree might recover. Use a sharp
pruning knife to clean up the wounds, and then _do not_ at all costs
treat or fertilize. Do your best to keep the area dry. The only
reason to start over will be if the damage is below the original graft,
in which case you have just the root stock. (Which will be nice but
will also grow into a large tree!)

Thanks, Emery. I tried to find 90 degrees' worth of bark, but I doubt
there is that much there - and I'm pretty sure the crown is dying. I
can't work out what caused the bark to split ... I thought it might be
an animal, but if there is some illness that might have that effect,
then that looks more likely. Could all the rain this summer have
affected it?

That leaves us with 65cm of trunk (from what was about 1m, and then lots
of shoots below that level going right down to the ground - as if the
tree is trying to make up for the crown dying. I don't know where the
graft is - the shoots below that point all look the same at this stage
as those in the crown, and I don't know enough to know where I should be
looking. What rootstock would the sango kaku have been grafted on?

We would really like to keep what is left, for its sentimental value if
not anything else. There is just a little damage to the bark below 65cm
presumably we leave that alone?

Is it essential to cut the trunk now? (It's just really painful to do
that, and what if I'm wrong and it would recover?)

If we do cut it, presumably at an angle away from the top healthy shoot,
do we paint anything on the cut? Do we cut it now or when it's dormant?
Being a maple, will it react like a Vermont maple and lose its sap?

Finally, what shape can we aim for, being left now with a 65-cm trunk
and shoots going all the way down? (So sad, for three years we had such
a healthy, lovely little tree....)

Sorry about all the questions, but we know very little about trees and
nothing at all about Japanese maples....

--
Klara, Gatwick basin