Take a good look at a large tree. Its size suggests permanence, strength, and invulnerability. Yet the truth is that above ground a tree has a very thin layer between bark and core wood and this thin layer performs the life-sustaining processes that maintain the tree's health and growth.

Go to a shrub and dig into it with your thumbnail and you will see how very thin is this vital cambium layer, within which a tissue called xylem moves water around, and phloem stores and distributes food. Remove this thin layer from a tree, as in girdling, and it will die.

Before it dies it will make every effort to survive. Tissue bridges are networked if there is any cambium layer to work with. If you have ever girdled a nuisance tree in an effort to dispose of it you will realise that it is not as easy as advertised.

This wounding or girdling of part of a tree is the basis, ironically, of propagation. Every tree and shrub has growth points, or nodes, that below ground turn into roots and above ground produce branches. If a tree or shrub is girdled below a growth node and soil is applied to the site, then roots . . .

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