We have blogged in the past about the growing popularity of installing access lines and ascending into the tree on a single rope. The most common method used involves a single line passed through a fork and anchored at the base of the tree.
While this set-up offers distinct advantages, climbers need to be aware of the downsides, one being the doubling of the load on the top anchor.For example, an 80kg climber on the access line needs 80kg of downward force on the base-anchored line to counter-balance his weight allowing him to climb the access line. That means there is 160kg of load (reaction force) on the top anchor: 80kg on the access line and 80kg on the base-anchored line to counter-balance the effect.
Treemagineers, Mark Bridge and Chris Cowell wrote an article called 'Safer Ascent into Trees' for the Arborist News about this very subject… and plenty of others relating to ascent. It is the best article you will find on the subject.
You should download a copy now and read it a couple of times. You will need to read it a couple of times because it contains a wealth of information that requires some digesting (mentally speaking).
The 80kg figure is used to demonstrate a principle, just think of your own weight times two on the top anchor point as soon as you step off the ground. This is only one principle, there are many other forces to be accounted for in this system… read the Treemagineers article (twice).