The art of bonsai attempts to grow full scale trees in miniature. One not only needs to make the size of the overall tree and branches smaller, but the leaves as well. Full scale leaves on an otherwise miniaturized tree tends to ruin the perspective. People who practice the art of bonsai use a process called defoliation as one of the techniques to produce smaller leaves on their bonsai trees.
It has been a longstanding belief in the bonsai community that defoliation was used as a primary method to reduce leaf size, but this is not the case. Defoliation tends to cause ramification, branching, which in turn has the side effect of producing smaller leaves. The tree needs to maintain more branches and leaves than before so the same energy is distributed over the larger "network". Therefore, less energy goes into the production of any one leaf and the tree has more leaves to collect energy from. Smaller sized leaves come about as a result.
The primary reason bonsai hobbyists defoliate is to increase the branching out of the tree which is known as ramification. Secondly, defoliating is believed to produce better fall colors in deciduous trees. Additionally, removing larger leaves allows light to penetrate areas that were shaded before. Lastly, if done correctly, it can be used as a technique to balance out the bonsai tree. For example, if one defoliates an area of denser growth it can cause less developed areas of the tree to respond and fill out more.
There are some cases where defoliation can be harmful to a bonsai tree. One should never defoliate an unhealthy tree. As a general rule, bonsai trees that produce fruit should not be defoliated as they do not respond very well. After defoliating a tree its water needs will be lessened and watering should be reduced accordingly. Certain species such as ficus can exhibit branch dieback as a result of defoliation. Sometimes leaving some leaves on a branch reduces that risk. Lastly, one should be aware the defoliation process tends to temporary stop growth of a tree.
There are mainly two ways of removing the leaves from your tree. The first is to pinch off the leaves with ones fingertips and the other is to use bonsai shears for leaf removal. Some prefer to leave the petiole, leaf stalk, as it is believed less damage to the tree will result. If one partially defoliates a tree it is best to remove the large exterior leaves to expose the smaller ones inside. If one totally defoliates a tree it is also a good time to do any wiring they may want to do as well since the tree branches are exposed. The leaves should come back within about 4-5 weeks.