Could someone explain what is really going on when you air-layer a tree?

Dennis Bangham wrote:Dr. Redhawk had a discussion on how he did air layering.  He did not cut a band all around but cut small triangles that allowed some of the cambium to remain attached.  

Robert Ray wrote:You could easily take cuttings and root those. The advantage for me is that with air layering I can get a much larger secondary plant. Successful rooting of large cuttings  is something that has eluded me. You've nailed what is happening. There is no reason you couldn't take cuttings and root them without them being attached. With air layering I can have a three foot plant instead of my 6-10 inch cutting. I go through my garden in the spring and there are always plants that I need to prune that I'm not happy with what I did with winter pruning. The cuttings go into a bucket with water, no need to waste a potential plant.[/quote

Thank you Robert,

All of the good advice and comments has made me want to air-layer even more.  Your comments made me realize that my father-in-law is out there just about every day lopping off long, beautiful suckers off of his apple trees.  Just think of the potential that I've been missing.

Eino Kenttä wrote:If I've got this right, the reason to strip away the bark is to make the auxin "pool up" above the cut, and maybe also cut the flow of cytokinins. Auxin is produced mainly in the growing tip of the shoot, and cytokinins mainly in the roots. Auxin travels downward throughout the plant, and cytokinins upward. A high auxin:cytokinin ratio promotes root formation, a low ratio promotes bursting of dormant buds and shoot growth. This is part of the plant's responses for dealing with loss of plant parts - loss of branch tips leads to reduced auxin production, which lowers the auxin:cytokinin ratio and makes dormant buds in the bark burst to replace the lost branches, while loss of roots reduces the cytokinin production and thus raises the a:c ratio and promotes root growth.

So, when you cut away a strip of bark on your air-layering branch, the auxin will be unable to pass the cut, and will instead gather above. The flow of cytokinins from the roots will also be cut off (Maybe? Not sure what layer of tissue it travels through...) and the combination of these two things will make the a:c ratio go way up just above the cut and lead to root formation.

As to why air-layering works better than cuttings, I'd guess it's because the mother plant still supplies the daughter with water (and possibly other things, like some minerals) through the xylem. Also, the cut cross-section area is smaller, so maybe the risk of fungal and bacterial infection is lower. When a cutting dies before forming roots, it's generally either because it dried or because a fungus or bacterium invaded it. So maybe air-layering reduces the risk for both of these?