rockm

I think this can also depend on species and time of year. Japanese maples can stand root pruning in early summer/late spring after their leaves have hardened off. I've repotted and root pruned Japanese maples in June, reducing the root ball by half and had no ill effects. However, if you root prune them after leaves have just emerged, but before they have obtained leather hard texture, the tree will suffer.

Additionally, I've found that I can collect American hornbeam after they have pushed new leaves by trunk chopping and then collecting them with a bare bones root structure (by bare bones, I mean clipping roots six inches from the trunk on 2-5" trees and six inches or so down--much in the way Bald Cypress are collected). I've collected Hornbeam very late in the season this way with alot of success. The trick is to remove all top growth to minimize the load on the roots. You are right about top growth not resuming until roots become better established.

The key is to think of the relationship between roots and top growth as symbiotic. One can't live without the other, but the roots are the initiator of the whole process.