Tony

Naturally dwarfed trees in the wild will usually have their roots restricted in some way. This could be from growing in a small pocket of soil on a rock slab or growing in mostly gravel or growing very poor unfertile soil. Other things can play a part too, like harsh weather or animals eating the foliage keeping it constantly pruned thereby keeping the tree short in stature and slow growing.

You may be overlooking an important point though. Most trees placed in these extreme conditions will die. Mimicking harsh conditons really isn't the best way to grow potential bonsai unless you have alot of growing space and time to wait and are prepared to lose bunches in the process.

When you look at those pictures of nice collected trees that have been made into bonsai you have to realize that you are seeing the few that managed to live long enough to be spotted by a bonsai grower then managed to live through the collection process. Most of them will never make it to this point of development.

For growing bonsai from seeds or young trees you would do better to keep them healthy and use some of the bonsai techniques that Andy describes to give them the attributes of naturally dwarfed trees growing in harsh conditions.

Tony