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The summer storm season is here and as usual we see an increase in lightning strikes to trees and therefore have an increase in the number of calls dealing with this issue.
Just recently I was at the Chickasaw Gardens Park assessing a large Water Oak that sustained a direct hit, and the morning that I wrote this article I observed two Sweet Gums with damage at an apartment complex.
Lightning is Hard on Trees.
Thousands of trees are struck by lightning each day. Trees are usually the tallest object in the landscape and their deep roots make them nature’s lightning rods, able to easily pass electric current from the air down to the ground. Taller trees are more likely to be hit, not only because of their height but also because they are more likely to suffer root or stem decay. As a result, the plant tissue is wetter and makes a better conductor. On the other hand, trees that are wet from a heavy rain have been known to be hit by lightning with little or no damage because the wet bark attracted the strike and channeled the heat and energy away from the vascular tissue of the tree. In a sense, lightning is nature’s way of eliminating old and sick trees. Lightning damage may be minor or sever. Often damaged trees become victims of further damage from insects, diseases, or wind.
Signs of Lightning Injury
Signs of lightning injury to trees can vary from no noticeable damage to total destruction of the tree. In many cases, minimal damage may be evident on the trunk, such as, cracking or peeling of bark, while the roots have suffered considerable damage. In such cases, the ground around the tree may show cracks that follow the roots of that tree. Small plants near the base (trunk) may be killed. Leaves may wilt immediately and die due to heat from the lightning bolt. At other times, branches may be sheared off, trunks may split down the middle, or the entire tree may explode or burn.
Lightning damage to trees depends on a variety of factors. The anatomy and physiology of the tree seems to have a direct influence on lightning effects. For example, lightning often follows the grain of the wood. The vascular tissue of pine and apple is arranged in a spiral fashion and on such trees when lightning strikes, branches die in a spiral pattern up the tree. Elm and oak, with conducting tissue aligned vertically, may show branch damage on only one side of the tree. It has also been noted by some observers that trees with smooth bark seem to deflect lightning bolts better than rough-barked specimens. In some cases the lightning discharge follows the line of least resistance, example, the cambium layer, burning a small channel down the trunk which often results in the formation of a ridge on the bark.
The nature and action of the lightning bolt itself has a direct influence on resulting damage to trees. Obviously, the more current a bolt carries, the hotter it is and the more destructive potential it has. Sometimes lightning discharge may disperse so as to cause no visible injury to the tree, but portions of the cambium (cell tissue just under the bark layer) may be killed, resulting in girdling and eventual death of the tree.
Lightning Resistant Trees
Location of a tree and the surrounding environmental conditions may also influence susceptibility of trees to lightning strikes. The tallest tree in a group; ones near lakes, ponds or streams; and solitary trees seem to be more likely to be struck by lightning.
Trees with high resin content make better conductors of electricity than those with low resin content. Therefore, trees such as pines, spruce, hemlock and fir are more susceptible to internal heating and explosion. The same is also true for trees with high starch content. That is why oak, maple, ash, poplar, and tulip trees act as good conductors of electricity. On the other hand, beech and birch are less affected by lightning because they have high oil content, and oil is a poor conductor of electricity.
What to do After a Lightning Strike
If an unprotected tree in your yard is hit by lightning often it can