Welcome back! Today we are going to focus on how to anchor your slackline to forks. If you have no idea what kind of forks I am talking about, here is a picture:
Now, while all forks might look the same to the uneducated slackliner, it is important to understand that there is a significant difference in the physiology and strength of different types of forks.
Most forks out there are really just branches that grow out of the trunk. Those are not very problematic for slackliners, since the connection between the branches and the trunk is strong and fairly reliable. To identify such a connection, you can look at the angle the two arms form. If it is significantly wider than 25° and has a U-shape, it is a real branch-trunk type of fork. Another good indication is the „flow“ of the bark. The wood of the trunk grows around the branch and embeds it to further strengthen the connection. This results in a beard-like structure (the fork is the chin) where the bark overlaps. It starts at the top of the connection and grows diagonally towards the trunk.
A solid branch is a good anchor point for your rodeolines. Whenever you think about using a branch as an anchor, keep in mind to load it in the direction it is naturally loaded by it’s own weight. It is quite easy to break the connection between the branch and the tree, if you apply a load in the opposite direction by pulling a branch upwards. Don’t do that.
Unfortunately, some forks aren’t of the branch-trunk type. They are of the trunk-trunk type. In that case, the trunk of a tree splits into two or more seperate trunks that all continue to grow upwards. Those forks can be identified by their more acute angle (around 25° and less) and their V-shape. The beard-shaped overlapping of the bark is symmetrical and doesn’t grow towards either of the trunks. It can form into ear-shaped protrusions, that are orthogonal to the axis of the connection. The problem with these forks is that each trunk is independent and to sustain its growth, it becomes wider and wider. By doing so, the trunks push each other away putting enormous pressure on the connection between them. Sometimes that pressure becomes so great, that the tree literally breaks itself apart as seen here.
A single trunk of a V-shaped fork is therefore not a suitable anchor as any additional strain might lead to more damage or even total failure of the trunk-trunk connection. The only way to anchor to such a tree is by slinging around both trunks in an 8-shape to fix both trunks in their relative position to one another. This is not an equalising anchoring method. Each one of the single trunks needs to fulfill the minimum diameter requirement of 30cm, since they aren’t sharing the load of the slackline. Your direction of pull has to be in line with the fork. Don’t pull the trunks sideways, as you can shear them off their connection (like a door on its hinges).
It is not always practical to rig in such a way, especially if you cannot adjust the length of your slings. A girth hitch, as pictured below is probably the easiest solution in that case. The trunk in front needs the tree protection, as the sling moves with the slackline. The trunk in the back is holding all the tension, so make sure to use wide slings and spread them out.
If you can’t rig in line with the fork, better not use the tree at all! Be mindful and cautious.
This awesome solution and the explanation of the entire tree-science behind it were given to us by Bern’s most famous tree specialist Urs.
In his honor, this technique shall henceforth be known as Ursian 8.