ArticleTrees are commonly utilized as support systems for zip lines, tree houses, and other structural applications. These arrangements can be more complex than traditional construction methods because they introduce a living organism into the system that is susceptible to disease, death, and decay. In this article, Board-Certified Master Arborist, Mark Webber discusses the use of trees as structural support members and highlights several issues relevant to their failure.
The experts at Robson Forensic are frequently retained to investigate injuries involving the design, maintenance, and operations of zip line courses. Experts involved in these matters may include recreational programming specialists, structural engineers, materials scientists, or master arborists. Contact us directly to discuss your case and determine which expert is best qualified to assist.
Zip Line Hardware in Trees
Any installation of hardware into a tree should be carefully considered in order to reduce the likelihood of future failure. Considerations should include the selection of appropriate tree species, installation location on the tree, tree size, and hardware selection and installation techniques.
Drilling any hole into a tree creates a wound that makes the tree more susceptible to infection and decay. Proper planning and craftsmanship is required to limit wounding, promote tree health, and the long term integrity of the system. Trees, as living systems, are unique amongst construction materials, because they have the ability to grow, heal, and die. Proper care, planning, and inspection will provide the best possible chances for long term structural safety.
An arborist should be consulted in the construction planning process to evaluate the viability of proposed trees and periodically after construction to perform follow-up inspections of the system.
Trees have the ability to compartmentalize damage and limit the spread of decay when they are wounded. Research shows that the discoloration and decay that frequently results from the installation of dead-end and through-hardware is limited if the tree is in good health. Several factors influence the degree of discoloration and decay, and the tree’s ability to compartmentalize the wound: tree genetics and vitality; condition of wood penetrated by drill hole; time of year of installation; and the size, depth, position, and number of holes drilled all affect the tree’s wound response.
The image above is a European beech showing discoloration associated with drilling a hole for through-hardware installation (29 years since installation). A more recent drill hole is evident at the bottom of the photograph: faint discoloration with the same shape as the dark drill hole discoloration.
Some trees have a more robust wound response than others; they can compartmentalize invaded tissues more effectively, thus limiting the spread of decay and damage. Trees that are good compartmentalizers are better candidates to have hardware installed verse poor compartmentalizers. Wood discoloration and decay associated with hardware installation varies with species, ability to compartmentalize can be different in individual trees depending on the condition of the tree.
Tree Size Matters
It most applications, the trunk is the most likely position for zip line anchors. Trunk diameter and wood strength should both be considered when selecting trees for anchor installation.
Methods and Installation of Hardware
When proper hardware is installed, the hole is typically drilled through the entire trunk, a bolt is inserted, and a washer and nut on the opposite end of the bolt secure it in place. For through-hardware, installation holes should be drilled no more than 1/8 inches larger than the diameter of the hardware to be installed. Holes should not be drilled closer together than the diameter of the trunk being drilled or 12 inches, whichever is less. The installation site must be inspected to assure that hardware is installed into sound wood only. Furthermore, the standards of arboriculture state that when installing through-hardware, heavy-duty or heat-treated round steel washers should be between the bark of the tree and the nut. The washer should not be countersunk into the wood.
The image above shows a longitudinal view of discoloration and decay extending from drill hole for hardware installation (note countersinking of washers into wood)
Wood poles, whether treated or not, can decay as readily as living trees. Wood poles used as part of any zip line system should be inspected for any type of incipient decays.
It is critical that a detailed inspection be conducted on the tree before and after any hardware is installed. If the tree and its associated parts are in poor health or structure, the hardware installation in most cases should not occur, or alternate options should be considered. The adverse physiological consequences of installing hardware by drilling holes in trees cannot be overlooked because any wound requires the tree to reallocate resources to compartmentalize the wound.
Trees with hardware installed require detailed inspections and assessments by a qualified arborist. Those inspections should involve a full ground-level assessment of the tree’s root flare, trunk, and canopy. The arborist should recognize and remove tree branches with preexisting defects that may break or fail onto zip lines or may sway into the pathway of the zip line corridor.
More so, the inspecting arborist should perform an aerial inspection of the areas of the tree where the hardware has or will be installed.
Forensic Tree Investigations
Among our tree experts you will find a Board-Certified Master Arborist, a Professor of Forestry, and a group of facility operators who specialize in the maintenance of commercial, industrial, and recreational properties.
Within the context of a zip line investigation, Robson Forensic is positioned to provide a thorough and comprehensive investigation by addressing every aspect of the case, from structural engineering and materials failures, to recreational programming and tree forensics.
For more information submit an inquiry through our website, or contact the author of this article.
Board-Certified Master Arborist
Mark Webber is a Board-Certified Master Arborist with nearly forty years of relevant professional experience. He provides investigations, reports, and testimony in matters related to tree maintenance and removal; management of nursery and landscaping operations; and an assortment of horticultural issues.
Mark earned a Bachelor of Science degree in Horticultural Science with a minor in Agricultural Business from The Ohio State University. He is designated as a Board-Certified Master Arborist by the International Society of Arboriculture, and as a Master Nursery Technician by the Ohio Nursery & Landscape Association.