Tree Lightning Protection Systems in National Cemeteries

NPS arborists installed new tree lighting protection system in this tree at Fredericksburg National Cemetery in March 2023.

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Trees are an important feature in national cemetery landscapes. An Eastern red cedar stands full and green in a gray winter landscape, framing the symmetrical layout of a cemetery landscape. The broad canopy of a tulip poplar shades a walkway from bright summer sun and gives a sense of protection and tranquility to the surrounding burial area.

Lightning damage can shorten or end the life of a tree. Lightning can send a mega volt of electricity through a tree, converting tree sap to steam. This can cause the tree to explode, collapse, lose limbs, or (at a minimum) lose strips of bark. If trees aren’t killed outright, the damage can lead to decay, causing loss of vitality or mortality. Installing tree lightning protection systems is one way the NPS protects significant trees to extend their lifespan and maintain their benefits in the landscape for as long as possible.

Identifying Trees for Lightning Protection

Multiple factors influence the decision to install lighting protection in a tree. Trees can have significant value in a cultural landscape for a variety of reasons. For example, a “witness tree” was present at the time of a historic event. Others help to define the character of a historic landscape, reflecting how it appeared at another point in time.

Tree height, location in the landscape, relationship to buildings or paths, age, and historic cultural value can all be factors for deciding which trees to protect from lightning.

Additionally, certain tree species are more likely than others to be damaged from a lightning strike depending on internal moisture content and regional differences. For example, data collected in the northeast United States shows that locust, tulip poplar, oak, and ash trees are susceptible to lightning strike damage.

Understanding Lighting Behavior

Key Concepts

During storm events, electrical charges build up in clouds. Negative charges accumulate in the bottom of clouds, and they repel negative charges on the ground. The ground beneath thunderstorm clouds becomes positively charged.  Updraft causes the upper part of clouds to become positively charged. 

Opposite charges within a cloud and between the cloud and the ground cause a discharge of electricity from the cloud to the ground, known as a leader. Once the leader makes contact with the ground, a return stroke occurs, visible as the bright flash of lightning.   

Trees are vulnerable to lightning strikes as they provide a path of least resistance for an electrical leader traveling from a cloud to the ground.  Trees contain water in their conductive tissues just under the bark and, as a result, provide passage for electricity.  Lightning passes through the tree with incinerating and explosive heat.

When properly installed, a lighting protection system provides a path of least resistance between the tree canopy and the ground, allowing energy to be safely dissipated and protecting both the tree and its surroundings.

• A side flash can occur when energy from a direct lighting strike to an object, like a tree, jumps laterally to another object, such as a person standing nearby.

• Step voltage is a difference in potential energy between the ground and an object on the ground nearby, such as a person standing beneath a tree.

In both cases, electrical energy can injure people or objects beneath or near a tree. This is why it’s not advisable to stand beneath a tall tree for cover in a storm.  
 

Cable is coiled at the base of a tree during installation of a lightning protection system.

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An Overview of Tree Lightning Protection Systems

The Installation Process

Typically, an arborist with specialized training installs a tree lighting protection system. The arborist climbs or uses a lift to reach the top of the tree and attaches a conductor cable as high in the branches as possible. This cable is secured to the tree branches using specific fasteners.

Arborists and archeologists work together as cable for lightning protection system is buried in a trench at the base of the tree trunk in the national cemetery at Little Bighorn Battlefield National Monument.

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The conductor cable is then installed down the trunk. When it reaches the ground, it is buried in a shallow trench that extends from the base of the trunk out beyond the canopy (or a minimum of ten feet from the trunk). Here, the cable is attached to a grounding rod that is driven eight feet into the ground. In shallow soils where this depth may not be possible, adjustments can be made to disperse the lightening charge.

Arborists also perform frequent inspections of existing lighting protection systems and replace components that are damaged.

Industry Standards

The NPS follows ANSI standards (ANSI A300 (Part 4) - 2008 Lightning Protection Systems) for tree lightning protection systems, developed by the Tree Care Industry Association as a resource for stringent safety and performance standards for professional tree care operations. These standards describe the system components, materials, and methods.

Parts in a Typical Tree Lightning Protection System

• primary and branch conductors (14- or 32-strand 17 gauge copper cable)

• blunt air terminal

• copper nails

• fasteners (to secure the cable to the tree)

• branch connectors (for joining conductor cables)

• ground rod or ground plate

Protecting Multiple Resources

Since installing tree lighting protection systems involves digging a trench and installing conductor cable below the surface grade, archeological consultation and monitoring is a part of the installation process in national cemetery landscapes.This avoids impacts to archeological resources as a result of ground disturbance.

Lighting Protection in National Cemeteries

Arborists use a rope to climb to the top of a tall tree to install a cable, part of the lightning protection system. Note the fastener near the bottom of the photo.

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Tree lighting protection systems have recently been installed or replaced at Andersonville National Cemetery, Fort Donelson National Cemetery, Fredericksburg National Cemetery, and Little Bighorn National Cemetery. The trees were based on size, age, species, and contribution to the historic character of the cemetery landscape.

In addition to helping to protect the trees, the installation was an opportunity to train additional staff in lighting protection systems.

At Fredericksburg National Cemetery, the protected trees included two tulip poplars, a gingko, a bald cypress, and an Eastern red cedar, with heights ranging from 55 to 80 feet tall. The crew used a lift to reach into the heights of the canopies and climbed from a rope when necessary.

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Arborists in Action

Lighting Protection at Andersonville National Cemetery

In May 2023, NPS arborists installed a lightning protection system in an Oak tree at Andersonville National Cemetery. On the cemetery's relatively level terrain, the nearly 100-foot tree stands tall. The tree is also a significant focal point of the historic landscape, located near the rostrum where many of the cemetery's events and ceremonies occur.

Oak trees can have a long lifespan with proper care, so protecting this large and prominent tree from potential lightning damage is an essential part of landscape management.

System Overview

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Top to bottom overview of a lightning protection system installed in a tall oak tree, beginning with a look upward into the high leafy branches, moving down the tree where a metal cable follows the trunk, and backing up over turf where the cable is buried under ground. It ends on the top of the conductor rod, just visible at the ground surface. 

Duration:

21.68 seconds

A top to bottom look at a newly installed lightning protection system in a tall oak tree at Andersonville National Cemetery, following the cable from the top of the tree down the trunk to the grounding rod.

A View from the Top

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View from an arborist's perspective in the top branches of a tall oak tree, towering over a national cemetery landscape. A pair of gloved hands nail a cable to a thin tree branch. The terminal, a rounded node at the end of the cable, points upward in the top branches of the tree. [no audio]

Duration:

1 minute, 19 seconds

Adam, an NPS Arborist, uses climbing equipment to ascend the tree, reaching as high into the upper branches as safely possible. He nails the conductor cable into the tree branch. If a tree has more than one central leader, the system might require multiple terminals. Adam then begins the descent.

Attaching the Cable

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Point of view from an NPS arborist high in the branches of a leafy oak tree in a national cemetery landscape. Multiple ropes of different colors are used to climb through the branches and descend the tree. On the way down the trunk, a pair of gloved hands nails metal fasteners into the tree, using a hammer to bend the top over a wire cable to hold it in place. It switches to fast motion mid-way through the descent. [no audio]

Duration:

2 minutes, 42 seconds

Adam, the NPS arborist, navigates the tree on climbing ropes and equipment. Adam is attached to the tree in three ways. The red hook is an auxiliary attachment, serving as a work positioning aid. As he descends, Adam using fasteners to attach the cable to the trunk at regular intervals. The fasteners are solid enough to withstand pounding, but malleable so they can be shaped with a hammer to hold the cable in place. The fasteners hold the cable away from the surface of the tree to protect the bark.

Burying the Cable and Conductor Rod

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Three people dig a narrow trench that extends out from the trunk of a tall tree in a national cemetery landscape, in fast motion. A closer look walks us alongside the trench, where a wire cable is placed in the soil after descending the trunk of the tree. An NPS arborist kneels and hammers a metal fastener into the ground at the base of the tree, then nails the top to bend over the cable and hold it in place. Another NPS arborist carries a long, slender metal rod to the end of the trench. Next, we see the rod has been pushed deep into the soil, and the cable is attached to the top. The three people fill in the trench with soil, move sod to cover the dirt, and make a thumbs up sign when the task is complete. [no audio] 

Duration:

1 minute, 47 seconds

Now on the ground, the conductor cable is laid into a trench, extending out from the base of the tree. The cable is attached to an eight-foot-long grounding rod, pounded into the ground 35 feet away from the trunk. The crew fills the trench with dirt and covers it with sod.

The system is tested every five years, using both visual inspection and by connecting a multimeter to the grounding rod to confirm it is working.

The installation of tree lighting protection system is a testament to the value of trees in a landscape and the importance of skilled arborists.

About the National Cemetery Investment Initiative

The National Cemetery Investment Initiative is a multi-year plan to preserve and protect the 14 national cemeteries managed by the National Park Service through investments in repair and rehabilitation of facilities and grounds, training, personnel recruitment, youth partnerships, and specialized equipment acquisition.