Hmm good questions for which I should have provided the information already. I used sphagnum moss around the cut and enclosed that within Calidama. It was then covered in a slightly transluscent plastic and secured with electrian's cables. In the case of the elm, it was enclosed in an open-aired four inch plastic pot, which never fully dried but became drier than I would have liked.
Did you see my thread down in the Elms forum about air-layering a cork elm? I am about to air-layer several more elms, but don't typically take photos since they take some time to set up, upload, etc. But I will be willing to do so if you have more questions.
Have you taken a look at the air-layer since the tops died? Is there callous formed at the cut or any signs of roots that started? I am just curious as to why the top died and was wondering if any healing activity started.
I see this is active again and I would like to point out a huge (stupid) mistake I made not too long ago, wondering if you did the same thing.
I was had a grafted pine, otherwise junk based on the height of the graft but a nice cultivar none the less. I have several of them I am growing out as scion material but this one had some nice movement so I thought I would layer it. In my haste one afternoon I decided to do it while the growing season was young. I cut a ring around the trunk, peeled it and let the area dry out a bit. I applied a paste of a proper hormone, did the plastic bag pouch with moist chopped moss/akadama and loosely closed the top so I could add water as needed. All went well - or so I thought. Several days later I realized I had made a huge boo boo. I cut the ring below the lowest growth and left no branches below the cut to support the roots during the layer period. It was a slow stupid death.
In my haste I did not think about what was needed to support above and below the layer. I know better and it was not my first layer but I was rushing and simply goofed up. Oh well - another tree in the ever growing pile of dead ones.