The technique shown in the pictures here and below, is usable for younger trees started as seedlings or cuttings. To develop a good nebari (surface roots) and trunk thickening at deciduous trees like Japanese maples or as shown here, European Beech (Facus sylvatica), a simple technique can be used.
The trick is to let the roots grow over an item, stopping them from growing directly downwards. this will enhance the root spread and lower trunk thickening. In this example a young piece of European Beech is used. A plastic screw cap from a bottle is used. It is fixed underneed the trunk, forcing roots to spread equally outwards. The growth outwards will enhance the root growth and trunk thickening at once, if correct pruning of the tree is done at the same time. This is dome by pruning back the trunk when in good growth, allowing a low placed branch grow freely for some seasons until the desired trunk thickness is achieved. The trunk will thicken below the strong growing branch, so be sure this is positioned low enough to secure a good tapering of the trunk upwards.
The plastic cap will be overgrown by the trunk and roots when time passes and will do no harm, so just let is stay.
First the tree is potted in a shallow pot, to force the strongest roots to grow downwards when they reach the pot. These will weaken then at this point, letting weaker inner roots grow and equalize the strength, forming a better root base. Later, after one or two years depending on development and growing speed, a wider flatter pot will be selected, so enhance the development further.
See the step-by-step photo guide below, and try it out at home After a few years the base for a good shohin is achieved with a little luck.