Aerial adventure operators should take care of all trees utilized in their operations. That includes any tree located where staff and guests frequent—not just trees that support structures. It is important to recognize the risk each incorporated tree presents, and be able to identify damage and decline.
There is inherent risk in using a tree as a structural support for recreational activity. Trees, like all living things, have a lifecycle. They respond to changes in their environment, such as construction damage or stress, recreational usage, extreme weather and other natural events, and react biologically to wounds, damage, and other stressors.
These stresses can increase the risk associated with a tree. By recognizing damage and decline early, and employing mitigation strategies when possible, you can reduce risk and extend the useful life of the trees in your park.
BASIC TREE BIOLOGY
Knowing the basics of a tree’s structure, and the functions of each part, makes it easier to identify damage and decay. From bottom to top, a tree’s structure is made up of a relatively shallow yet broad root system, the root flare, the trunk, and the canopy. Each of these parts has multiple functions in tree structure and biology.
Roots support the tree. There are larger buttress roots and small, fine roots. Roots spread out around the base of the tree and are relatively shallow. The majority of tree roots are found in the first few inches of soil. Trees need soil that is aerated and loose so that oxygen, water, and nutrients can move freely through the soil, making it easy for the tree to access them.
The root flare is the transition between the roots and the trunk where the tree tapers outward as it goes below the soil’s surface.
The trunk starts above the root flare. The basic composition of the trunk from outside to inside is: bark, cambium, sapwood, and heartwood.
The canopy consists of the limbs, branches, twigs, and leaves.
Each of these parts provides both structural and biological processes to keep the tree alive and standing tall. Trees grow from the tips and outsides, so new growth on the branches is from the tips of the twigs—aka the terminal bud—and the circumference of all parts is added on the outside, just under the bark (the outermost growth rings are the newest). The inside of mature trees is mainly non-living heartwood. This is very important to consider as we begin talking about damage and decline.
Trees limit the effects of damage through compartmentalization (called CODIT, shorthand for compartmentalization of decay in trees). For example, if a live branch is ripped off at its base, CODIT immediately begins to wall off the damage at the site of the wound in four directions: one is vertical, two is inward, three is lateral, and four is the new wood added annually after the injury.
However, CODIT is not perfect. Over time, a cavity will often form inside a damaged tree within wall four. The result can be a structural weakness, which in some cases (but certainly not all) will increase risk.
Let’s work from the roots up and discuss common causes of damage and mitigation strategies.
Tree damage in aerial adventure operations most commonly occurs during the construction process and by human activity. Roots and the bark are typically the most impacted. Limbs can also be damaged. It is important to recognize that construction design—how the construction of a course is planned and executed—is a potential cause of damage. Proper construction design will minimize the impact of building the facility.
Tree roots are most commonly damaged in two ways: soil compaction and cutting. Adding too much soil or wood chips can also damage roots. It is important to consider all of these factors when building and/or operating an adventure park or challenge course on or near trees. Take all available steps to protect the roots!
The Issue: Soil Compaction
Compacted soil severely restricts the movement of oxygen, water, and nutrients through the soil, and thus makes it difficult for trees to access what they need to survive.
Causes: Foot traffic and vehicles/machinery during the construction process.
Mitigation: Make trails narrower to reduce the surface area where people walk; restrict where vehicles can drive, and use pads for construction vehicles to drive on; aerate the soil using hand tools or specialized machines; and build elevated walkways for pedestrian traffic.
The Issue: Cutting Roots
The roots of a mature tree can extend past the spread of the branches, and most roots are quite shallow. Cutting one large root on a tree can kill 15 to 25 percent of its roots. This will limit a tree’s ability to take up nutrients and support itself structurally.
Causes: Roots are most often cut, of course, when digging. At an aerial adventure operation, this can happen while trenching a utility or building a structure.
Mitigation: Pretty simple—if you have to dig, try to do it away from trees to avoid cutting roots. Obviously, DO NOT cut the roots of trees that support activities or structures. You can use air-powered tools to dig trenches without cutting roots.
The Issue: Adding Soil or Wood Chips
Too much added soil or wood chips can inhibit the tree’s ability to uptake oxygen, water, and nutrients. Plus, adding too much soil or wood chips directly to the base or trunk of a tree can introduce disease and decay.
Causes: Adding an excessive amount of wood chips or soil to the area around a tree.
Mitigation: Mulch under trees should be six inches deep or less. Rake back wood chips or soil that has been added too thickly or too close to the trunk. If deeper mulch is needed for padding below, say, auto belay landing areas or zip landings, make this deeper area as narrow as possible to minimize the affected area.
The bark on trees serves to protect the sapwood and heartwood from the elements, as well as fend off organisms that feed off of wood.
The Issue: Damaged or Removed Bark
When bark is damaged or removed, it exposes the inside of the tree and the tree becomes more susceptible to disease and decay.
• heavy equipment grazing the tree trunk
• a worker’s feet being in the same place for a long time can damage some tree species’ bark
• safety equipment, such as continuous belay shuttles, repeatedly traveling around a tree
• people simply pulling the bark off
• elements attached to trees
• fence off a tree during the construction process
• band the trunk with temporary boards
• attach boards to limit belay equipment wearing away bark
• rig construction materials properly to avoid violent swings that can result in material damaging the tree.
The Issue: Attached Structures That Constrict and/or Damage the Tree
Since trees add circumference to the trunk from the outside, and nutrient transfer takes place in the sapwood on the outer inches of the tree, interruption of these biological processes can be very damaging.
Causes: Wrapped cables or compression. Trees can partially incorporate, i.e., grow around, construction materials within a season. This is similar to CODIT.
In addition, the pressure points created from this type of construction can limit nutrient flow or create a dead spot where living tissue should be.
Another consequence of neglected wrapping is an enlarged area of tree above the attachment point. This is “response growth” and signifies an internal defect in the tree.
Mitigation: Monitor and adjust hardware on a very frequent schedule determined by each individual tree’s growth. Pay extra attention to species of trees that grow very rapidly.
Alternatively, use hardware that inserts into the tree, such as tree attachment bolts (TABs) or through bolts to support structures while providing space for tree growth.
Designing structures that rest away from the trunk, allowing for the tree to grow, will extend the longevity of the installation while minimizing ill effects to tree health. This is especially important when building in young or fast-growing tree species.
Of course, trees are subjected to environmental elements and naturally occurring organisms. These can cause damage or stress, resulting in an unhealthy tree or elevated risk. Compared to manmade issues, some natural impacts are more difficult to see, and often the mitigation of these impacts will require the help of a professional arborist or plant healthcare professional.
Wind can break tree limbs and leave them hanging above activities or congregation areas, increasing risk. Trees can also be uprooted during extreme wind events. You should remove the failed limb or tree, but remember, limbs and/or trees are very heavy and can be under extreme pressure. Hire a tree care professional anytime you feel out of your comfort zone.
Too much or too little water can negatively affect a tree’s health. Leaves will be the first indicator of an issue with water. Signals can include: wilting, color change, early dropping, etc. It is important to pay attention to rainfall amounts and how the trees are responding to variances.
If you are experiencing a drought and your trees appear to be suffering, a good watering may be necessary, if possible. Use a slow release system
if you can.
Too much water is difficult to mitigate. Often, the best you can do is limit stress—hold off on building, for example—during and immediately after a flooding event.
Nutrients, Insects, Fungi, and More
Trees can suffer from poor soil nutrition, insect infestations, fungi, and other diseases. All of these conditions are very common, but some can be hard to diagnose, and some may have no treatment options. It is most important to catch abnormalities early and seek the advice of an arborist or other plant healthcare professional.
Soil conditions can be determined via a soil test. Fertilize your trees only after conducting a soil test to determine what, if any, deficiencies exist.
Insect infestations are increasing with climate change and introduction of foreign species. Some of these impacts can be mitigated. For example, defoliating caterpillars can be trapped; it may take several seasons to eliminate them. Ash trees can be effectively treated against Emerald Ash Borer, though it’s costly and must be repeated every two years.
Fungi are present in many trees and are a major culprit of rot. Good arboriculture practices and proper pruning are your best defense against fungus. The signs of fungi are fruiting bodies (mushrooms) on or around the tree. Consult an arborist to determine the risk associated with fungi and all other infestations or disease.
Trees, like all organisms, decline and die over time. Their decline can be very rapid or take decades.
A tree in decline will begin to shrink in height and canopy spread. The upper out branches and limbs will die and fall off. Leaves will shrink in size, and the tree may hollow. It is important to notice the change and then begin planning to either mitigate the risk associated with a declining tree or remove the operation from the tree.
Mitigations for a tree in decline include pruning, cabling, and bracing. Dead branches and limbs should be pruned each spring. Other defects, such as a crack in a limb union or a hollow limb that exceeds the operator’s risk tolerance, may require cabling or bracing to limit possible risk to guests and staff. These techniques use either cable or threaded rods to support the tree.
An old gnarly tree can be a valuable aesthetic or educational piece of your operation. You do not need to remove a tree unless it poses a significant risk.
Ultimately, the park operator or owner needs to assess his or her tolerance for risk when it comes to tree defects. Consulting with a TRAQ certified arborist to help qualify the risk and outline possible mitigation techniques and strategies can aid that assessment.
Also, practicing good arboriculture and employing proper pruning techniques will increase the overall health of your trees (see “DIY Tree Care,” p. 70). Understand your trees’ biology, and work with that to keep them healthy. With proper care and handling, your trees can live and support you for a long time.
Timothy Slape is ISA Tree Risk Assessment Qualified and an ACCT Certified Challenge Course Inspector. Contact him at email@example.com.