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Lightning Injury to Trees

Storms can be accompanied by high wind, hail, and heavy rain; but it's the thunder and lightning we seem to remember the most. In addition to being beautiful and scientifically fascinating, lightning can be destructive to building structures and electrical systems critical to daily life; and it can be lethal to people, animals and plants. We are all aware of stories of lightning striking people (professional golfer Lee Trevino has been struck twice while on the golf course) or causing forest fires. Less often, but occasionally in the news, are incidents of lightning strikes involving trees in the landscape.

A lightning strike to a tree in one's landscape can be a traumatic experience for both the tree and its caretaker. If your tree is struck by lightning, the immediate concern should be your personal safety. If the lightning damage has created broken limbs hanging high in the tree or unsupported branches hanging over buildings or sidewalks, these should be taken care of quickly, preferably by a certified professional arborist. If the tree does not exhibit obvious safety concerns (structural or mechanical) and seems to be generally intact, the next step is likely to wait until the end of summer or even until the following spring to evaluate the tree's ability to recover by producing functioning leaves. That's not always the answer a homeowner wants to hear but a valid assessment of the tree's damage and ability to recover is difficult-to-impossible immediately after a lightning strike.

Generally, two classes of damage can and often do occur in a wide variety of combinations when lightning strikes a tree. First, the mechanical and structural damage to a tree may be very slight to the point of being almost unnoticeable, or it may be extensive as though a bomb had exploded from within the tree (the damage is believed to be caused by the electrical discharge of the lightning—up to 100 million volts at thousands of amperes, which vaporizes the water inside the tree, creating superheated steam which explodes when it exceeds the structural strength of the wood). A very common physical indicator of a lightning event is the classic vertical stripping where bark, and sometimes the wood beneath, is torn from the trunk or major scaffold limbs. This stripping may skip around or it may be continuous most of the way up the tree. In addition, it may rise straight up vertically or it may spiral around the trunk like a candy cane. With some lightning events, bark can be violently blown off the tree in circumferential sections partially or completely around the trunk or limb(s). This stripping physically interrupts the vascular tissues that conduct fluids up and down in the tree's living cambial structures under the bark.

The second kind of lightning damage is systemic, and it may not be easily or immediately observable. This is the functional interruption of the tree's vascular system due to burning and traumatization of root hairs and conductive tissues. Once again, this damage may be very slight or extensive to the extreme of complete vascular shutdown. Trees experiencing vascular shutdown would quickly brown out and die.

Recognize that mechanical and structural damage and systemic damage can combine in a tree struck by lightning in any number of ways. In other words, there is no definitive pattern or "norm" to be expected from the effects of lightning when it strikes individual trees.

Lightning injury to trees is difficult to predict, but it appears to be governed by the tree's position in the landscape, the moisture content of the part struck and the species of tree involved. Tall trees, those growing alone in open areas, trees with roots in moist soils or those growing along bodies of water are most likely to be struck.

Since water or sap is a better conductor than wood, lightning damage is often related to the concentration of moisture in and around a tree. For instance, if the moisture is concentrated in the phloem between the bark and the wood, then the lightning strike will follow this channel and create an explosive separation of the bark. If there is more moisture in the center of the tree, the explosion from within may blow the tree apart. Rain soaked bark often shows little damage because the lightning may follow the outside of the bark and flow into the ground.

Although no species of tree is totally immune to lightning, some are definitely more resistant to lightning strikes than others. Birch, for example, is rarely struck, whereas elm, maple, oak and most conifers are commonly hit. The reason for the wide variation in susceptibility between species is not clear. In some cases it is because some trees simply tower over others in the landscape, in other cases it is because some trees have a greater ability to collect water on their leaf surfaces (conifers) or to shed water on their bark surfaces (oak). Some authorities attribute the variation among species to the composition of the trees. Trees high in oils (birch and beech) are poor conductors of electricity, whereas trees high in starch content (oak, maple, ash, and poplar) are good conductors. Conifers (pine, spruce, hemlock, and fir) have high resin content. They conduct more electricity than trees with low resin content and are more susceptible to explosion and internal heating.

It's a myth that lightning never strikes twice in the same place. Trees, because of their height, are natural lightning rods. Lightning seeks the path of least resistance to the ground through the best available conductor in the area. If the best conductor happens to be a tall, isolated cottonwood, it can be struck many times during separate storm events. Multiple lightning scars on the trunks of trees are testimony to this phenomenon. In most cases it is best to wait 6 months or more before doing major (expensive) corrective work on a lightning struck tree. If, during this waiting period, the tree shows no obvious signs of decline, then it may be worth the expense to hire a certified professional arborist to treat the tree.

In many cases, it will become obvious at some point during the waiting period that the tree will not recover and that removal is the best option.

As you monitor the tree during the waiting period, remember that a tree struck by lightning has been stressed severely. The intense heat of the electrical discharge takes a great deal of energy from the tree. To deal with stress, trees may need additional nutrients. Studies have indicated that additional water after a lightning strike may assist the tree in absorbing nutrients from the soil. Supplemental fertilization with a moderate rate of nitrogen may also help the tree produce callus tissue to compartmentalize wounds. Preventing insect and disease problems may help reduce future stress, but painting the exposed wood with a pruning paint or wound dressing may do more harm than good since these products often provide a conducive environment for harboring insects and disease-causing organisms.

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