Top roping in a gym is a well established sport with a very good safety record. I don't know the exact safety level of this activity. Rock climbing has many factors which are hard to quantify - the chance of rockfall, anchor failure, ropes cutting, etc. Gym climbing is much more controlled, therefore safer. Because I have little experience with top roped tree climbing as I do it, and have found few who have done it before, I can't quantify the safety of this version of climbing. My guess is that it falls somewhere between gym climbing and leading on rock. Use the same skills and cautions needed in other climbing situations, such as redundancy, equipment inspection, dynamic ropes, etc. If you suspect the tree will fall, don't climb on it.
If you are a beginner climber and don't understand the terms I have been using, this is not the place to learn. Read the climbing books by John Long, and join a local climbing club. Learning from experienced climbers is not only fun, it will keep you alive.
WHERE DID THIS IDEA COME FROM?
I live in Columbus, OH, a place not known for it's climbing areas. In fact since Springfield was closed, the nearest outdoor climbing is about 100 miles away. When a large Silver Maple in my backyard died, I needed to cut it from the top down, it was surrounded by buildings and could not be dropped. The only information I could find to help me was from rock climbers (arborists are a secretive lot, I could find no books)
After I joined the local AYH climbing club and learned the basics, I knew enough rope technique to help me cut down the top of the tree. After I had pruned the tree down to the point where only the main trunk and a large branch were left, inspiration struck (or maybe fatigue became overwhelming). The remaining trunk is about 3 ft. in diameter at the base, the top forks into two main trunks each about a foot across, at a height of 15 ft. One main branch angles out at 65 degrees, starting at a foot and a half dia. and is 8 in another 15 feet up. The main trunk has space for four independent lines at the cardinal points. The west line follows the underside of the branch, allowing a longer overhang TR than the local gym!
WHY SHOULD YOU CLIMB DEAD TREES?
First, don't try this with living trees, unless you want it to quickly become a dead tree. Placing anchors and holds would rip up the bark, not recommended. Dead trees are solid until they are many years old, using them conserves resources (why kill more trees for plywood ?). I have read that dead trees are good for the ecology, they are home for a number of bugs and birds (more on the woodpeckers later).
The economic factor is major. A climbing wall of this size made with standard construction techniques would have cost thousands. I have spent only about $45, plus labor that is a fraction of that needed to build a wall this tall. The low investment helps when you must make the decision to end the life of the tree. As soon as you don't trust it any more, cut it down, and until then save your money up for the real wall.
WHICH TREES ARE SUITABLE?
I have read about some people who "boulder" on trees. Short, wide trees like apples can be fun, but they are not what I want to cover here. Tall, straight trees are what you need for top rope tree climbing.
I have had some conversations with experienced tree guys, and the best general rule I can come up with is that the tree should be at least two feet diameter at the base, and not be dead for more than a few years. Dead trees often rot the most just under the ground level, so doing some excavation at the base might tell you the state of decay. You might be able to set up a cable and a come-along from the base of another tree to the high point of your dead tree and pull it to see the amount of flex you get ( just have a plan to get out of the way if it comes down). My tree has no flex at all when I am at the high point of the main trunk and throw all my weight into shaking it