Forest Riddles - Why does lightning strike a tree?

By Naturalist Eric D’Aleo




Last month, I was walking through the forest at Squam Lakes Natural Science Center near the Piper homestead on Mt. Fayal while leading a program when I came across an unexpected surprise: A large pine tree struck by lightning about 80 feet from the foundation of the old homestead. Ancient cultures long feared lightning believing it was a weapon of the gods. The Greeks thought Zeus hurled lightning, while the Vikings believed Thor produced lightning as his hammer struck an anvil while riding his chariot across the clouds. Indian tribes in North America believed lightning was due to the flashing feathers of a mystical bird whose flapping wings produced the sound of thunder. Why was this tree singled out for such wrath?

I had never come across a relatively recent lightening strike before and immediately began searching around the tree for clues and evidence to provide information about what had happened. By observing the color of the scar was blond and not gray, I knew lightning struck the tree within a year. Looking closer, I also saw some pitch had flowed out from the edges of the bark by the scar.

As I looked around, I noticed other equally tall pine trees were unscathed and wondered why lightning had struck this particular tree. There appears to be no pattern or “norm” that determines which trees lightning strikes. The height, geographic location, and species of tree, as well as proximity to building structures are factors affecting the likelihood of whether or not a tree is struck. Trees that stand-alone, rise above other trees, or are close to water are the most likely candidates. Some tree species are more likely to receive a lightning strike than other species, including oaks, elms, pines, spruce, poplars, maples, and ash, while beech, birch, and horse chestnut receive fewer strikes. Trees that are deep-rooted, decayed, and dying are more prone to being struck by lightning than shallow-rooted and healthy trees. A tree closer to a building is more likely to be hit than another tree ten feet further away, assuming that they are the same species and height.

Why does lightning strike a tree in the first place? Trees act as lightning rods because electricity seeks the path of least resistance when conducting electrical charges from the clouds to the ground. Although wood is a poor conductor, moisture from sap and water inside a tree conducts electricity much better than air, thereby completing the circuit from ground to sky. Large, tall trees that carry a high volume of water during the summer when electrical storms are common are most likely to be victims of a lightning hit.

Typically, lightning strikes a tree somewhere near the top of the canopy, rather than half way down the trunk, then it travels down the trunk into the ground. I looked up at the tree and noticed it was struck near the highest branches; the blond scar traveled down the trunk in a somewhat spiral shape. Often as trees grow, they “twist” in either a clockwise or a counterclockwise direction. If a tree grows slowly, the wood under the bark exhibits a pronounced spiral pattern, however, if the tree grows quickly the spiral in the underlying wood is less pronounced. The scar of this tree exhibited a slight, counterclockwise spiral. Knowing the history of the property, the diameter of the trunk (two feet), and the curvature of the lightning scar on its surface, I estimated the tree to be less than 100 years old and that it had grown quite rapidly. Pieces of bark and wood were scattered around the base of the trunk and up to thirty feet away. Some pieces were over eight feet long and up to four inches wide. One three-foot shard was actually driven a few inches into the ground. When lightning strikes a well-hydrated tree, the intense heat causes the bark and wood to be blown off the tree from steam generated by the sap boiling and cells exploding in the wood. Had the tree bark been rain soaked when it was struck, it would have shown very little damage, as the lightning would have followed the outside of the bark.

I thought about another pine tree hit by lightning several years ago near the Science Center parking lot, which is still alive, and wondered if this new casualty would survive. As it turns out, this is a difficult question to answer. Some trees die immediately with little sign of external damage, while others may live for a number of years. The loss of protective bark and the exposure of wood from the lightning scar increase the tree’s susceptibility to insects and disease. The intense heat from a strike also takes a lot of energy from the tree. To overcome the stress the tree needs to absorb additional water and nutrients from the soil.

As I headed back, down the slope to the Webster Building I didn’t think the tree would survive long due to the complications from the lightning strike and the competition and crowding of other healthy trees a few feet away. Who knew that even today the force of lightning could still fill us with as much awe and wonder as it did for ancient cultures?