Ken,
I have not done a tremedous amount of root pruning on seedlings. I have pinched alot during their first 2-4 months of age though.
Kadie has cut the tap root off and left the fine feeder roots before with good success.
I feel that this is best done young, although after saying this. The Thai growers and others will lop off the bottom (all roots) of a 6 month old seedling. Stick it to a disc and replant. Then many roots form around the circumference of the caudex.
This tree would have had the bottom cut off when younger and grown substantially for what you see in this photo.
I am not completely convinced that cutting the tap root necessarily makes the caudex get any larger than what it would be predisposed to have? It might? I think that the roots grow out the side rather than underneath giving it the appearance of being wider when lifted.
Rick
