In growing trees in small containers as they must be here it is necessary to prune and train the roots. In repotting I use a very nonclumping mix, featuring light organic things and sand and perlite, which will separate from a root system nicely when dry. Then roots are nipped or cut back or left alone depending on their dominance and position. Roughly speaking, the thicker the root tip and the lower down it stands in the system the more it is cut back. Some trees object to significant loss of feeding roots quite strenuously, even when put in the mildest of light. For the last year or so I have experimented with saving working root surface while breaking root dominance. This can be done in two ways. In a tree with a complex root system all side roots are permitted to go where their angle takes them, but dominant roots go as far as I intend them to go in the normal direction, and then I bend them in a U-shape, bringing the end of the root up almost to the surface, and ideally pointing straight up. In a seedling Pinus cembroides, which had one incredibly long root with a feathering of side roots I cut it back a bit and then p lanted it so that the main root went down, turned a corner, and then slowly ascended like a spiral staircase. This puts the newest part of the root at the very top. The trees put up with this sort of thing, too. Later repottings reveal nice branching coming from way back up the taproot, and often even unnipped root tips simply stop growing, but the more main roots continue to nourish the tree while a stocky new root system in produced. Wheen there is enough surface area those old once-dominant roots can be cut off with no shock to the tree. I wonder what interesting thaings other people have done with root systems, and how those things have directed the health and shape of trees. As I am restricted to bonsai the whole container thing is out of the question, which includes watching a tree respond to root training without interference. There just isn't the space.