Taper
From BonsaiWIKI
Taper is the progressive narrowing in diameter or caliper of the trunk of a bonsai when measured at the soil surface vs. points above. Taper may be gradual, radical or non-existent. Under certain conditions reverse taper can occur.
Trees that develop slowly are generally subject to more significant taper than those growing rapidly. The taper of a tree satisfies a form follows function principle of biophysics: the lowest portions of the tree bear the weight of the plant above, and are consequently thicker. At its top the tree is more slender because the environmental forces of gravity and wind are correspondingly weaker.
Ideally, taper begins below the soil line, is visible in the buttressing visible roots and lower trunk and progresses throughout the tree to its apex
One way to create taper is to continously replace the leader when its growth becomes coarse. Another is to have multiple sacrifice branches that are allowed to grow freely to expand the lower trunk.
