Great presentation.
On slide #21 the author wrote "Remove cambium to prevent reestablishment of phloem." Does this mean that he scrapes the cambium layer completely off the branch, so there is no flow of anything in either direction?
John
Sorry for the late answer.
Once you remove cambium and bark, practically you create a "one way" branch on your tree. Water and mineral can flow from roots to the branch (through the wood) while sugar and elaborated substances cannot travel back to the roots (roots in fact can starve to death if cambium removal is made in a wrong way). Of course the branch, after the cambium removal, starts to be filled with sugars produced by the leaf which in turn are used to fuel the growth of the air layer's roots, if the air layer has been done properly.
If you don't eliminate the cambium, you basically keep alive the tissue that produces both the wood and the phloem. While the production of wood isn't an issue, the phloem would make the plant capable of sending sugars back to roots, rendcing the likeliness of success. Pplants can air layer even if you forget to remove the cambium - I know this first hand from a lychee air layer - but plants without cambium will air layer quicker and better - they haven't any other choice anyway: if they won't root they will eventually die when the old wood will become unable to feed them with water and nutrients.