“Nobody roots for Goliath." ― Wilt Chamberlain
Maybe Goliath should have tried elms. Elms will root for almost anybody. There may be no easier species to air layer. They root from cuttings easily, too. I've stuck elms into the dirt and forgotten about them, only to find strong young trees a couple of months later.
About a year ago, I found an unwanted elm growing in a patch of other trees. I had permission to take it, but it was totally unacceptable as bonsai the way it sat. It was a couple of inches thick, but 25 feet tall, with no foliage whatsoever on the lower ten feet. So I chopped it about four feet above the soil and waited. Within a couple of months, it had a flourish of new growth near the new top, and I collected it. I let it grow out for about six months, and in May, I decided to air layer the top.
The first step is to find the spot on the tree where I want to pull the roots, and start removing the bark and get into the sapwood, or secondary xylem, which is the layer that causes the water from the roots to make their way up the tree. We want water to go up, but we want to scrape the phloem, which is outside the sapwood and carries nutrients down the trunk to the bark and roots. This causes the tree to want to produce new roots at the cut. I cut about an inch all the way around and then scrape away the soft white phloem. If you leave the phloem, the tree will just callous over the cut. If you remove the sapwood, the top will simply dry up and die.
The next step is to pack the cut with wet sphagnum moss. There are two popular ways to do this. One is to tie plastic around the bottom of the cut, stuff the plastic with wet moss and tie it all up in a ball. However, if I have branches above the cut and can hang a pot from them, I prefer to use a pot and to grow the roots into bonsai soil. I cut a hole around the bottom of the pot, the same size as the diameter of the tree, and I slit the side of the pot to fit it around the trunk, with the bottom just an inch or so below the cut. I then tape the slit up with duct tape and secure the pot with wire.
Once it is secure, I pack the bottom of the pot with wet sphagnum moss, and put good bonsai soil above it, covering the layer of exposed sapwood. I then top the soil with wet moss, to retain moisture.
When done, it looks like this. I water it daily, like any other tree. After two months, I pulled the tape away to see if roots had developed. There were many roots sticking out (I didn't get a good photo of the roots because I was being careful not to disturb them or expose them to air), so I sawed the top off just below the bottom of the pot and put the whole thing into a larger pot.
Inside the larger pot (in this case, a 6 inch orchid pot), I put a layer of bonsai soil and set the air layer on top of it. Then I cut away the air layer pot and removed it, packing the rest of the open space with bonsai soil. In two months, I went from a four foot tall, taperless elm, to this, a future broom-style elm bonsai.