root pruning western white pine

I have about three of these in the ground that im growing for bonsai. i am planning on leaving them alone for a few years, but, i also have one in a pot i collected as a start. its kind of my "experimental" pine. its about three years old now and im wondering how much of the roots can be trimmed away when repotting. ive heard they are pretty finicky and so i dont want to kill it, but i need to repot as its been a few years and is probably getting pretty root bound.