Stationary Rope System (SRS): A Basic Understanding of Basal Anchors

In the vast world of tree climbing techniques, SRS, formerly known as Stationary or Single Rope Technique (SRT), is considered by many as a total game-changer. Many would even go as far to say that base anchors are at the heart of an effective SRS setup, but to each their own - I am a canopy or crown anchor fan. This anchoring method now plays a crucial role in setting up efficient, versatile, and rescue ready basal anchors. In this article, we will explore the basics to setting up an efficient base anchor, their types, and their myriad benefits for a professional and for rec climbers alike.

What is a Basal Anchor?

A basal anchor is created when the arborist installs their rope through a suitable branch union and anchors the working end of the climbing rope back to the tree in an SRS setup. Unlike moving rope systems (MRS), where the climbing rope moves through the branch union or a friction saver at the anchor point, SRS keeps the rope stationary while the tree climbing arborist moves up or down on the rope.

However, there is much more to basal anchors than simply throwing a rope over a branch union and calling it “good”. For instance, you must be disciplined in the art of the throw line, even though basal anchors don’t require true branch union isolation. The rope's path must still be “clean” – meaning free from obstructing the rope’s direct path up to the branch union and down the intended climbing path; a crucial step when setting up pre-directs from the ground. When the cleaning step is neglected, the climber may experience an unexpected fall due to limb failure along the path. The climber has therefore shock-loaded his entire system and may warrant retiring their climbing equipment, for our climbing gear is meant for suspension and not shock load absorption.

It's about the angle too!

In a traditional basal anchor configuration, the climber installs the climbing rope and ties it off at the base of the tree with the legs of rope parallel to one another or at what we will call a “zero-degree angle”. As the climber loads onto one side of the rope, a single unit of tension (climber weight) is input to that leg of rope. That single unit of tension travels up the rope, around the branch union and down the opposite leg of rope – which now sees one single unit of tension as well. Finally, the branch union or primary suspension point now sees two units of tension, meaning the primary suspension point is theoretically seeing twice the load– same principle as mechanical advantage. This twice the load theory truly applies when there is a friction-free configuration but since the rope makes contact with the tree and introduces friction, the load at the anchor is slightly less than double.

If and when the arborist determines a single suspension point is not suitable for the climber, you may see the arborist pass the rope through another branch union and rely on multiple anchor points. There is the need for a basic understanding of angles and vectors, and how this can drastically impact the strength of your anchor selection. If and when it is opportune, opening the angle to the following can reduce the amount of force seen at the anchor(s):

• 45-degree angle = 1.9x the load at the anchor
  

• 90-degree angle = 1.4x the load at the anchor
  

• 120-degree angle = 1.0x the load at the anchor
  

These figures apply when the load is at rest; when the load is in motion, the force at the anchor fluctuates higher. When the correct anchor points are chosen, again in theory, the tree limb will be loaded into compression as force is drawn down the limb rather than flexing and creating a lever arm effect. I think it’s worth saying, the climber’s weight never changes; what changes is the force applied at the anchor point(s).

Types of Basal Anchors

1. Simple Anchors. A straightforward setup where the rope is tied off around the base of the tree utilizing a Running Bowline backed with a Yosemite finish or an appropriate stopper knot. Reminder: the climber will be loading and unloading the Running Bowline constantly which can cause the Bowline to back itself out (when not backed up), and can lead to a disastrous outcome. An Alpine Butterfly, Bi-directional Bowline, and a Cow Hitch are alternate knots that can be used to anchor to the tree but proceed with caution when applying the Cow Hitch in this configuration. Don’t forget to tie an Alpine Butterfly above the terminating knot at arm’s reach of the ground crew member – this is done for rescue purposes.
   
   

2. Mechanical Anchor. This configuration requires a Petzl Rig, ISC D4, Skylotec Sirius descender. The incorporation of tools as such often provides a higher level of control, adaptability, and ease of rescue, making them a popular choice amongst professional arborists.

Benefits of Stationary Rope System Basal Anchors

1. Easy Access and Retrieval
   

• Establishing an anchor can be quicker without needing to truly isolate around the stem as an MRS climbing system requires.
  

• Retrieval is made simple without the need for the intricacies a canopy/crown anchor may require. Simply untie the basal anchor and begin pulling down the rope and voilà, c’est magnifique! Effortless, even with multiple redirects.
  

2. Efficiency in Rope Management
   

• Advancing the system higher is made easy, simply lanyard in and pass the climbing system through the desired branch union. Dropping climbing equipment is also removed from the equation.
  

3. Safety
   

• Ground-based emergency rescue systems can be established, and a TRAINED ground crew member has the ability to lower the climber in a controlled system at the basal anchor. By doing so, this reduces the potential for causing harm to a second climber. It is important to remember that if a climber has a lanyard in place, targets underneath, and/or has chosen poor redirects, a ground rescue may not be feasible.
  

4. Load Distribution
   

• Anchor point failure can be greatly reduced when the arborist opens the angle between anchor points, thus “reinforcing” them.
  
  

A basal anchor is an indispensable component of modern SRT tree climbing. By understanding the different types of basal anchors and their benefits, arborists can optimize their setups for a wide range of tasks. But do be aware, not every tree is a Basal Anchor candidate and an understanding of how forces will change throughout the climb is important! I don’t personally believe an entry level climber should immediately begin with basal anchors until a basic understanding of tree biology and forces have been met. Trees are ever changing; we do our best to “guesstimate” and create the best anchors for both climbing and rigging based off principles and experience. It’s not perfect but we make it work! 

Watch our video on Basal Anchors!