On the night of September 7th, 2019, over 1250 lightning strikes were recorded over Western Washington in a span of three and a half hours. Up until this point, I had never responded to a lightning struck tree. This storm presented me with the opportunity to assess four sites with lightning struck trees and two different types of lightning damage. This article will focus on the two most interesting.
The first tree I assessed was a blue atlas cedar (Cedrus atlantica ’Glauca’). This tree was 49 inches in diameter and was roughly 85 feet tall. This tree looked like what I thought a typical lightning-struck tree would look like. About a third of the upper canopy and trunk was blown apart, and there was a twisting line of missing bark down the length of the trunk showing the path the lightning took to the ground.
After assessing the tree, I determined the damaged limbs and upper trunk needed to be removed. The undamaged and lower portions of the tree could safely remain standing. This seemed a pretty straightforward prescription, but the next lightning-struck trees I was called to assess were not as easy.
The second location of lightning-struck trees was in the backyard of a residence. At this site, eight mature Douglas-firs (Pseudotsuga menziesii) showed signs of lightning damage. Each tree showed longitudinal spirals of missing bark along their trunks with tiny cracks in the exposed wood. Some trees had several streaks while some had only one. In two of the trees, the lightning appeared to go down one tree, then jumped to a different tree before contacting the ground. My assignment was to assess the extent of damage and determine if the trees needed to be removed.
To do this, I first used a resistance drill to look for internal decay to make sure the lightning did not travel down an internal decay column and damage the roots. Not finding any internal decay, I hired my brother to climb the trees to assess the depth of the cracks. I handed my brother a bunch of small pieces of aluminum to insert into the cracks as a gauge of their depths and he cautiously ascended the tree. My guess was that the easiest way for lightning to reach the ground was to travel down the water found between the bark and sapwood along a tree’s trunk. I assumed that if we didn’t find cracks deeper than an inch or so, the trees were not severely impacted and would likely recover.
Upon his return to earth, my brother informed me that he didn’t find any cracks deeper than an inch. He also informed me that he didn’t think this was the first time this grove of trees had been hit by lightning. Interesting.
We re-inspected the trees and discovered that four of them had similar crack depths and strike patterns, but had mold growing under the bark around the lightning cracks. We guessed these trees had been stuck at least a year ago.
These four trees were also responding very well, growing new wood to seal the damage. Seeing this lent confidence to recommending that the newly struck trees could be retained. I believe the trees were structurally stable. If the lightning damage results in dieback or decline in the future, then the trees can be re-evaluated then.
I don’t know how common it is to see trees struck by lightning more than once, but of the four calls of lightning-damaged trees I received after this storm, three were Douglas firs that had been hit before. Another thing that surprised me about this storm was the number of calls I got from other arborists looking for advice on assessing lightning-struck trees. I couldn’t offer a lot of help as I was still working through my own process, but I now feel somewhat confident and hope this article helps you with your assessments of lightning-damaged trees in the future.