Before we begin with air layering we need to remind ourselves of the anatomy of a woody branch or trunk.
The innermost part is the heartwood. This is made up of old xylem vessels that are no longer functional as such but will help with the support of the tree.
The xylem vessels transport water, with dissolved minerals, and oxygen from the roots to the foliage for photosynthesis.
The cambium layer produces new xylem vessels and phloem vessels.
Phloem vessels carry the end products of photosynthesis to the roots supplying the living part of the tree with nutrients as it does so.
Bark is the redundant phloem vessels which acts as a waterproof cover and protects the tree from sudden temperature changes.
Reasons for air layering.
Propagation of a species. This might be due to its rarity or because you want consistency in the colour of the bark and foliage, especially with group plantings. Graft unions can be unsightly if you use that option.
To remove the existing root system, if it is too unsightly, or to shorten the trunk.
To give you a bonsai with a good trunk diameter and the beginnings of refinement in one year. Bonsai grown from seed will obviously take considerably longer. Most air layering is complete in three to six months. The trunk diameter of the tree to be air layered is mainly restricted by the material available and the ability of the person carrying out the layering process. Experience counts and practicing on other trees will help when it comes to the important ones.