Trees and Bad Weather


Managing a challenge course, adventure park, or zip line tour that uses trees to support various structures and elements is a complicated matter. There is much to consider, including the species of trees, your location, the climate zone/ecosystem, whether your site is heavily developed, and if your course is open year-round. Increasingly unpredictable and extreme weather further complicates the equation. Understanding the nuances of your environment, knowing what to look for, and regularly keeping track of the condition of your trees will ease the aftermath of bad weather and help ensure safe operations. 

To that end, I have often said that it’s best to think of your course property as its own ecosystem. This approach encourages your staff to think about everything that grows on the property as part of their management task. 

A Connected System

Why this approach? Trees are now, based on recent and ongoing research, known to be part of a cooperative system that makes a forest function. Other important parts of a forest system may include understory and herbaceous plants, water source, and the presence of invasive species. Much like shortages faced by humankind, competition for resources such as water and oxygen among these various parts of a forest is occurring behind the scenes. It is important to realize that a forest is more of a cooperative with connections between plant species. 

Ripple effect. How we build in a forest changes the ecosystem by altering the physical environment either above ground or below. When we remove trees that are growing next to others, we change how wind and weather hits the exposed tree. 

When we dig near trees to alter the grade, add a trail, or place a utility, we physically remove roots and organisms that the trees have relationships with that may be helping the trees gain access to nutrients, water, and air. This can cause stress and change their vigor and ability to respond to weather conditions over time.

When we drill into a tree or wrap it, we induce physical responses by the tree to seal a wound or develop new sprouts. Sometimes this response may not be successful enough to prevent the development of decay. In addition, these elements and the people using them load the trees in ways they are not adapted to. 

All these factors can affect how a forest or individual tree responds during a weather event. Good design and management will help managers anticipate any issues and support a long lifespan for the course—and increase the richness of the user’s experience.

The Weather Factor

Since bad weather is inevitable, you should have a plan in place for inspection of your entire property after any severe weather event. Strong winds, snow or ice, and heavy rains can all affect the stability of trees, which, in a forest, act as a damping mechanism, shedding and sharing the loads generated by wind and other forces both individually and collectively. Trees adapt their growth to cope with the loads that are normally present in the environment. 

Unusually high loading events like a strong snow or ice storm with winds exceeding 35 mph will likely cause changes in trees, including partial failure. 

Partial failure means that a tree has partially uprooted or that woody parts have been pushed beyond their elastic limits and wood fiber has been damaged but not fractured. Wood pushed beyond its elastic limit will not rebound to its original shape.

Different types of weather events are likely to have subtly different impacts on your trees, so it’s important to understand the differences. 

Ice and snow. By far the most worrisome event would be an ice storm, which is devastating for trees that are not already adapted to an alpine environment where snow and ice loads are more common. Trees in alpine environments are adapted to snow and ice loads, making them more resilient to the effects. But trees in areas where snow and ice storms are less common will not fare as well when the frozen stuff falls from the sky.

Ice can build up tremendous loads on trees, leading to whole-tree failure or breakage of large parts. The damage is not always easy to spot, though. I have seen failures occurring weeks after an ice event when cracked and damaged limbs finally let go. 

Heavy, wet snow that sits on trees or a warming event where snow buildup gains moisture and thus weight in a thaw can have similar results. 

To minimize potential damage, some course operators in winter climates remove cables from their trees for the winter, remove decking from bridges and other areas where snow build-up will occur, or hire people to ride the course, clear the cables, and shovel to keep snow loads under control.

High wind events that fall outside of a region’s “normal storms” can also cause damage to trees, either by breakage or uprooting. If the weather has been wet and soils are saturated, uprooting can be more likely. 

Over my years of inspecting challenge courses and zip lines, I’ve realized that connecting trees together with cables and guy systems causes significant changes in the dynamic movement of the forest. During the stormy season, when the canopy really starts to move, an oscillation of two trees connected with a cable can occur in a manner that causes a sudden heavy load when they move in opposite directions. Some arborists call this the “karate effect” when a support cable causes the same thing to happen. This is one of the reasons for removing cables during the stormy season, a practice that I recommend.

Extreme heat. Another weather event that is becoming more common: heat waves. When abnormally hot temperatures persist, many things can happen to trees in a forest. For example, a phenomenon called “sudden limb drop,” which is still poorly understood, often occurs during hot weather. 

We saw this during the deadly 2021 heat dome event in Washington, when temperatures were on average 30 degrees Fahrenheit hotter than normal—and reached a peak of 60 degrees above normal for the time of year. The event caused quite a few unexpected huge branch failures. Many in the science community think that the trees were so dry that a lack of water in the wood structure resulted in weakening of microscopic bonds in the wood, causing the breakage. 

This new phenomenon is something to consider during and after extreme heat events. Pay attention to noises in the woods because trees might make unusual sounds leading up to a failure. For example, one of our clients reported hearing a cracking and popping noise for a couple of days before a large branch tore out of one of their trees during the heat wave.

Regular monitoring with a plumb line attached to the low side will reveal whether this large elm is subsiding or failing after a storm that caused the tree next to it to fail; The crack in the soil was caused by a tree rocking in the wind, and serves as an indication that the tree is not stable.

Tracking Your Trees

Tracking the condition of your trees will help you to quickly recognize changes that may need to be addressed by a professional.

Site map for trees. To facilitate post-event inspection, I recommend that you first create and maintain/update a map of your site with as much detail as possible regarding trees being utilized in and around operations. All those with fittings attached to them should be noted, as well as every tree within range of your course and associated facilities. Trees that are within a distance of 1 to 1.5 times their height to any facility should be considered in range. 

An arborist and your builder can help you gather these details during their initial inventory and subsequent inspections. Generally, I recommend that an independent consulting arborist inspect the trees at least annually and should update the map during the inspections. Additionally, at least one employee should have extra training in visual tree assessment and be assigned to conduct regular inspections prior to daily course opening and after weather events. 

If you have trees that are already of concern, it’s a good idea to have up-to-date measurements of their height, diameter of the main trunk, diameter of parts that might strike a target, and angle of lean if present. 

On a side note: There are two commonly used ways of recording the angle of lean. One is to hang a plumb bob on the low side of the tree and use a piece of steel or a wooden stake to mark the plumb line. The second uses two aluminum nails set in the high side of the tree and a digital level is used to measure the angle. Regular monitoring of tree angles is easy to do and will ease any concerns or alert you to a problem, especially following a weather event.

Photographic evidence. It’s also helpful to take photographs of areas that you’re monitoring. Set up spots where you take the same picture on a regular basis. Photos should be taken at minimum in the dormant and active seasons, and each time your arborist assesses the course. You may need to take photos of certain trees more frequently, though, especially if there is a condition being monitored, such as a tree with a trunk wound—photos at frequent intervals will help to assess how well the tree is producing reaction wood or how quickly it is developing decay.  

Data and understanding. Create a file for each tree or area. If you have the means and know someone who can help you, the use of photogrammetric data (obtaining information about physical objects and the environment through the process of recording) can be extremely helpful. Having lots of information will help you and your arborist understand and assess risk over time and notice if anything has changed after heavy weather. 

Tracking your trees will help you and your arborist to identify symptoms that indicate a decline in health and structure like yellowing or dying foliage, development of cracks in the stem, growth of fungal decay conks or other organisms that may affect the tree, changes in trunk anatomy that may indicate there are internal defects, and ground level changes that may indicate movement in the root plate. This, of course, applies to both long-term monitoring as well as in the aftermath of bad weather. Again, your consulting arborist can help train staff on what to look for. 

Further reading. I recommend two books by Dr. Claus Mattheck as resources for understanding tree anatomy and failure modality: “The Face of Failure in Nature and Engineering”; and “Updated Guide for Visual Tree Assessment.” These books will help you to understand the structural anatomy of trees by observing them.

Ins and Outs of Inspecting Trees

When it comes to tree health, the four different forces that park operators should pay attention to are compression, tension, sheer, and torsion. All trees and combinations of trees experience loads that will increase any of these. 

As trees adapt their growth according to the loading and the need of the tree to resist, it’s possible to learn to look at a tree and see where the forces are active. 

Take the long view. Establish a protocol for any inspection or procedure that you repeat. To this end, when learning to observe trees, start by taking a long view of the area you will be inspecting. This may involve choosing several observation points with which you are familiar and use repeatedly along the course or tour. Again, the use of historical photographs from these observational aspects can be very helpful in understanding if there have been changes to the canopy after an extreme weather event.

All about that base. Next, visit the bases of all the course trees, focusing on any that may have monitoring points such as a plumb line installed. You are looking for any sign of movement at the root plate of the tree. For this, I find a steel soil probe to be very helpful. Use it where you see potential soil movement by pushing it into the soil profile—if there are voids, you will find them with the probe. 

Trees that are loose at the roots should be considered a failure in progress. There may be exceptions to this rule in certain soils and with certain forests. I’m reminded of a zip line in Alaska built on glacial till, where an astounding amount of movement in the trees is normal, especially during wet weather.

If ground anchors are in use, they should also be inspected for any signs of movement. You should also be looking at trees with basal codominant stems (one tree with two stems growing directly adjacent to and in contact with one another or emerging from one trunk) or trees growing adjacent to each other to see if any soil cracking, trunk cracks, or other signs of stress from a load are present.

Make the rounds. Next, using your map, visit all the trees that are within range of course elements. This should put you further away from the course trees and allow the use of binoculars to scan the canopy. Look carefully for broken but still attached branches, hanging branches, and changes in codominant unions. 

Inspect at course height. Only after completing all the aforementioned inspections should you access the course and begin an inspection at course height. For this assessment, some knowledge of how the course is constructed is helpful. An accredited ACCT inspector should look at the course if you suspect any problems with hardware, cable attachments, etc.

You get a very different perspective looking down at tree canopies from height, and you may notice things that you can’t easily see from the ground. Use photographic records of course inspection points to compare with current conditions. The more accurate this data is, the more certain you can be that things are OK—or not.

Understanding the condition of your trees when the weather is calm will help identify issues that result from weather extremes—and keep your trees healthy and your property safe for staff and visitors to enjoy. 


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