The requirements mentioned above are based off DIN 79400 (the industry standard for slacklines) and are valid for slacklines rigged between trees where jumping off is feasible.
The strains that a slackline system puts on a tree can be broken down in three categories, in a simplified form.
The tension is the pull the slackline exerts on the tree. The tree needs to be able to withstand the loads pulling on it from slacklines unscathed. It is not easy to assess the maximum pull a tree can withstand. That is why we recommend a minimum diameter of 30 cm at the height of the anchor to be sure that we are not damaging the tree. For medium to high tensions larger diameters are recommended. If you want to evaluate the forces in your slackline system you can take a look at this flyer or use this calculator.
The pressure, transferred from the slackline to the tree is, among other things, dependent on the tension of the line, the diameter of the tree, the width of the slings wrapping the tree and the thickness and quality of treeprotection used. The larger the tree and the wider the slings, the greater the area of contact of the sling and the lesser the pressure on the bark. Trees with thin bark are more vulnerable when it comes to pressure compared to trees with thick bark. To protect the tree it is advisable to use slings of 5 and more cm in width (ideally spread out).
In order to protect the bark of the tree from the friction caused by the slings we use tree protection made from robust materials such as felt or carpet. This allows the sling to rub on the tree protection without damaging the bark and, with it, the tree. Tree protection that is 20 cm high and 2-3 meters has proven to be suitable.