When growing bonsai, one of the main things people wonder about is how to stimulate a nice shallow root system, with matching Nebari.
One of the ways to assist trees is by offering them no room for downward growth. Like with the development of Nebari for seedlings, you can plant more mature plants in shallow, wide containers. When planting the trees you need to take care that you spread all the roots out nice and evenly. They should not overlap, and should radiate smoothly from the centre. Also, you trim back big roots that are out of proportion to the rest of the roots. Cut back to a side-root. This facilitates healing of the cut, ensuring the survival of said root.
If you repeat this process of spreading out every time you repot during canopy development, and you systematically cut back big roots you will create a fine system of roots running just below the soil surface, without any major roots going down.