Growing trees in the ground or larger pots is a way to increase the trunk size quickly but as the trunk is growing, so are the roots underneath. At some stage the roots will need to fit into a bonsai pot so how can you tame those wayward roots?
Nebari is the term for the junction of trunk and roots, including the visible roots that radiate out away from the trunk before they disappear into the soil. A strong nebari is highly valued, particularly in maples. Good nebari is more than just a few strong roots sticking out from the trunk. Like the branches above, roots should radiate out from the trunk and divide and ramify before they disappear into the soil so that the bonsai looks old and stable. In order to produce such well ramified surface roots you need to root prune regularly and fearlessly during the development phase.
Through progressive trials I have found that trees actually need very few roots to survive transplant as shown in these photos of field grown trident maples as they are dug from the Shibui Bonsai grow beds.
I have also noted that, after root pruning, the vast majority of new roots form from the cut ends of the roots.
That means that there is very little point leaving roots long during the initial transplant. At some stage in the future they will need to be cut short so you can fit your bonsai into a suitable bonsai pot. This radical root shortening is better done earlier than later.
Most other deciduous species that I grow as bonsai are treated in a similar manner.
The Japanese maple shown above had a trunk diameter of 12 cm and recovered well after being dug in 2011.
Not all species can tolerate such radical reduction of roots. I generally leave more roots on evergreen species such as pines, junipers and most Australian natives. Ficus rubiginosa is one native species that can tolerate extreme root reduction provided it is done during the warmer months.