Trunk thickening is essential to develop a trunk which has proper taper meaning it goes from thick at the bottom to thin at the top.
How do you thicken the trunk of a bonsai tree?
To achieve the proper taper of the trunk which gives the tree proper proportion and gives an older look to your bonsai tree you will need to strategically cut the trunk. The more weight the trunk supports the thicker the trunk will grow. Trunk thickening is in a direct relation with old cells laid down in the cambium of the tree. The cambium grows from outside outward and allows the tree to cover up minor wounds and wall off rotted wood. More light and foliage result in more food production which leads to mores cells being laid down in the cambium and a thicker trunk as a result. It is generally accepted that to build an impressive bonsai tree you must create a good root structure, then a thick strong trunk and finally the branch structure.
This can be a long process requiring patience, at times it can impair the look of your tree temporarily until the desired effect is achieved. For instance on this Ficus Retusa, the aerial roots give a very unique style which I love. However by trimming the aerial roots the trunk will thicken giving it the desired taper effect. This will temporarily take away from the style of the tree however even more roots will grow back long term and you will achieve a strong thick trunk. We can find allot more information on this discussion forum. Garden Web Discussions Thickening the Trunk