In article , John H Wood
writes
Can anyone explain why ring barking a tree will usually eventually
kill it, but cutting the stem either at head height as in pollarding
or at ground level as in coppicing will merely produce shoots from
where the cut was made.

The food from the roots travels up the tree in a thin zone just inside
the bark, so ring barking will prevent food getting to anywhere above
the ring. Water travels up much deeper in, so the death is quite slow
since the tree is still getting water to all parts.

Food obviously can still get to anywhere below the ring or the cut, so
those trees which throw new shoots readily (and not all do) will produce
new shoots from below the cut.

The two extremes are probably willow, which re-shoots very easily, and
therefore is often pollarded or cut to ground level, especially if
you're after young shoots for basket work or for their winter colour;
and conifers, which don't like throwing new shoots, so if cut back
either just grow the branches they already have or, if cut right back to
the ground, die.
--
Kay
"Do not insult the crocodile until you have crossed the river"