I'm very interested in learning to air layer, and have been reading everything I can get my hands on about the subject, both in print and on the Web. I understand the basic concept, but there are a couple of things I'm not clear on regarding the wounding.

As I understand it, there are three basic methods of wounding. The first is to wound by cutting a slice lengthwise up the branch 1/3 to 1/2 way through, then propping it open either with a stick or some long-fiber spaghum moss. The second method is to completly remove a ring of bark (along with the green layer just underneath)--girdling. The third is to remove a partial ring of bark, leaving a small bridge or series of bridges of bark (I've linked to an illustration of this below).

Some writers claim that if you don't remove the entire ring of bark, or if you leave any of the green layer beneath the bark, that roots will not form. If that's true, then it strikes me that neither of the other two methods would ever work.

According to the URL below, the purpose of leaving a bridge or bridges is for slow-rooting species (unless I'm reading it wrong), so that the branches and leaves above the wound can continue to receive water and nutrients and continue to grow while the roots are forming. However, as I understand it, the purpose of girdling or wounding is to stop or slow the flow of sap and auxins DOWNWARD. If I'm understanding what I'm reading correctly, girdling or wounding doesn't stop or impede the flow of water and nutrients upward, even when you remove the entire ring of bark. (If completely removing the ring of bark stopped the flow of water and nutrients upward, it strikes me that girdling wouldn't work, or at least would be the same as making a softwood or semi-hardwood cutting.) Given this, it's not clear to me what the purpose is of leaving a bridge.

I'm hoping someone with a background in botany can shed some light on wounding, girdling, and bridges. Thanks in advance.

George

Here is a link that might be useful: Bridge air layer