Gday Jamie & Gerard!Jamie wrote:...but surely the transport of nutrients from the bottom of the layer to the top is minute. but the if that was the case why do we botther with layers if basically we were just doing one big cutting?
Good question you pose here, Jamie, and it is one which leads me to explain to the best of my ability the theory behind layering techniques...
The short answer to your question is this: cuttings (generally thin cross-sections) transport water and nutrients to the foliage only by means of capillary action, in which the leaves transpire (lose moisture) which causes water to be drawn upwards. Assuming that conditions are right, this capillary action will be maintained until such time as the cutting will begin to form roots. In the case of layering, it is the intact xylem (sapwood) that actively supports the layer above (ie: the roots of the parent tree are 'pumping' water and nutrients to the layer as if the tree was intact and un-injured).
At this point, a diagram or two might be useful to illustrate the vascular workings of (most) woody tree species: Now, it appears that I've spent too long on that diagram, so I don't have the time to fully explain the theory. However, I can refer you to a number of articles which explain in relatively simple terms the inner workings of a tree, as well as the way in which layering works:
Layers Of A Tree (This is a great resource for understanding the function of each layer of tissue in a living tree.)
What Is Air Layering?
Layering Techniques For Bonsai
I think the best explanation comes from Andy Walsh (evergreengardenworks.com):
I hope this all makes sense, and will try to answer any further questions regarding the air layering process.evergreengardenworks.com wrote:The removal of the bark, cambium, and phloem, but not the xylem, prevents carbohydrates and photosynthates from flowing down the trunk past the girdling site but still allows water and mineral nutrients to flow upward to the leaves. This keeps the leafy portions of the shoot from drying out and maintains them with an adequate supply of nutrients. The removal of the actively growing cambium layer prevents the regeneration of phloem and healing over of the wound. Because of this the carbohydrates and photosynthates flowing down the trunk collect at the girdling site. The presence of these excesses of carbohydrates and photosynthates (esp. auxin) at the girdling site, plus the presence of the water in the sphagnum moss, causes dormant adventitious buds in the area to grow into roots. When there are enough roots to sustain the shoot independently the shoot is cut off of the tree and then planted or potted.
Thanks.
Fly.
taking the top half of trees of since 2005! 
