At the stage where the stems of young pine seedlings intended for use as bonsai material (especially JBP) are turning from green to orange/brown, experienced growers will remove the roots and treat the seedlings as cuttings. The reason for this treatment is to A) it ensures the roots all develop at the same height, from the base of the cutting, and radiate away from the trunk, and B) it ensures the first nodes are very low on the trunk, which is a major asset to those who wish to build visual power and weight into the pines, even if grown as shohin.

If you skip that step, the plants should be potted individually at the same point in time (trunk turning from green to orange/brown).

More here.

Al