"Eliminating
a root-bound condition is not necessary for the continued existence of the
container grown sequoias but it is necessary to encourage significant new
growth. The lack of space for new roots will create a condition where the plant
must abort roots and then replace them, as they decompose, with new feeder
roots."

And the same is true for any tree grown in a container long term. Without sufficient ability for unobstructed root spread, container culture is a natural dwarfing mechanism. Think bonsai without the constant pruning and training :-)

"That such mammoth trees have such
shallow root depth is astonishing."

Not as astonishing as one would think. Most trees have very shallow but wide spreading root systems. Even so called 'tap-rooted' trees tend to lose their deep roots as they mature and develop a very lateral root system. It is estimated that 85% of ANY tree's roots are located within the top 18" of the soil profile with the feeder roots located just under the soil surface.

I could not agree more with Gal's points above, especially regarding the typical architecture of tree root systems. If one thinks back to some very early-and hugely erroneous-pictorial representations of tree roots, the were often depicted as a more or less mirror image of the above-ground structure. This is wholely inaccurate, and the existence of a high water table and/or a heavier soil only accentuates the tendency for tree roots to spread out far and wide. Not at all sequoia-only thing, but common to most tree types.

+oM