node

The roots provide the water, but the energy is prodiced in the leaves. Now if the connection between the foliage and the roots is cut, water can still travel upwards through the wood, but the carbohydrates produced in the leaves can't travel downwards to the roots anymore.

The immediate consequence is a swelling right above where the tree has been girdled, ie. all the carbohydrates end up there because the can't travel further down the trunk.

Now if treated right, and rooting hormone is pretty much an essential here, at the point above the girdling where the swelling is, new roots will appear also.

As it is the foliage above the layer that will provide the plant with the energy to produce new roots, this also determines the best time for most layers. Namely, in early summer when the leaves have hardenend off, and the plant has suffucient energy to actually produce new roots.
hope this helps...

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