Bonsai as a Japanese art requires or involve having some specifics features. I like to consider it as any other art trend. Impressionists painters will all paint some common features etc..
Well for bonsai, a particular feature is having a great nebari. Nebari is Japanese for exposed root base. Having a strong exposed roots system give the impression of a well anchored mature tree. Nebari development is easier if planned from the beginning of growing. With a new seedling, a cuttings or an airlayer, development of the nebari needs to be done from the start.
Let’s start with a seedling. A tree grown from seed will develop a tap root. A tap root is the principal root pointing and growing down in the soil. A first technique of developing a nebari is to cut the tap root of the seedling as soon as the little tree stem start to harden. Then you can replant it and lateral roots will grow.
I don’t prefer this method. Growing some lateral roots can be done naturally, if the seedling is given all the space and all the sun it needs. Growing your seedlings in the ground will give them all the space they need and the maximum exposure of light will help the tree develop branching. The roots are usually a mimic of the branching, so a well develop foliage by exposition to light will result in a nice root system composed of lateral roots and not just a tap root.
For cuttings and air layer propagation of tree’s, is to wait for new roots to form and at the first re potting you need to comb the roots all around the new trunk. With cuttings and air layer there is no tap root growing, so you already have some lateral roots forming, which can result in a nice nebari very fast.
Nebari, or exposed roots is one of the elements of traditional Japanese bonsai, other elements of a Japanese bonsai tree are the development of a triangle form between visual points. The number three is pretty fundamental in bonsai. Even numbers are considered imperfect.