Billy M. Rhodes wrote:
I am not familiar with the species "Star Magnolia."

Magnolia stellata. Of all the magnolias, it's the only one you might
possibly want to try as a bonsai, but I doubt it will cooperate about
branch placement. Original poster: is this really about needing one
branch, or is the trunk too long? You might need to "top off" the
plant to fix this, only I don't know what a magnolia will do if you top
it off. Plants that top off well have an abundant supply of bud
primordia in the trunk, just waiting for something to trigger their
growth. Maple is an enthusiastic producer of bud primordia; that's why
it's so easy to top one off.

The idea is that stopping the flow of sap up the cambium
layer at this point will cause a branch to pop just below.

Actually, the idea is to cut off the supply of the phytohormone auxin
coming down from the apical meristem. Lateral branching is inhibited
by auxin. So the farther from an apical meristem a lateral bud is, the
less auxin it is exposed to, until finally the bud breaks and a new
lateral branch forms. Cutting the cambium above a bud severs the
phloem, and reduces the amount of auxin flowing downward. However,
there needs to be a lateral bud. Doing this on a trunk without bud
primordia won't have any effect. SOme trees have lots of bud primordia
(maple) others have little or none (fig). Notice that "topping off" a
tree is a more radical way to reduce auxin flow to bud primordia: you
are removing all the apical meristems.

When I was at Cornell, I had access to a book called "The anatomy of
economically important plants"; it would tell you about bud production,
root anatomy, branching, etc., on a bunch of plants, mostly crop
plants. I learned an amazing amount about grapevines; the reason you
have to prune them a certain very specific way in order to get grapes
is that they don't produce an infinite amount of buds.