Whether a single tree or a forest, bonsai require a thick, tapered base and a lot of branching to give the appearance of age. At this stage trees are normally planted in the ground and allowed to grow out naturally for 5-6 years before being considered for bonsai training. I have a Coast Live Oak that's about 25 years old, started from an acorn, and used directional pruning to achieve a unique shape and found that pruning the top results in just one new branch from the last node.