There are a few ways to add mechanical advantage to a tree climber's SRT (single rope technique) climbing system, which can help to reduce the physical effort required to ascend the tree.
One common method is to use a "rope wrench" or similar device, which creates a friction hitch on the climbing line that can be released with a small tug. This allows the climber to use their body weight to tension the line and climb higher with less effort.
Another approach is to use a "split-tail" system, which involves attaching two separate lengths of rope to the climber's harness and tying them both to the climbing line using a friction hitch. By pulling on one of the ropes while standing on the other, the climber can create a mechanical advantage that makes it easier to ascend the tree.
Finally, some climbers choose to use a mechanical ascender such as a Petzl Zigzag, Notch Rope Runner Pro, a Rope Wrench, or Rock Exotica Akimbo in combination with their SRT climbing system. These devices can be attached to the climbing line and allow the climber to "step up" the rope using a pulley system, reducing the amount of effort required to ascend the tree.
It's important to note that adding mechanical advantage to a climbing system can increase the complexity and potential for error, so climbers should ensure that they are properly trained and experienced before attempting to use these techniques.