(Editor’s Note: We posted one of our Virtual Safaris a few weeks ago in which we featured a tree that had been hit by a bolt of lightning.
This post was archived at the time but describes that same event…)
A particularly large electric storm hit Londolozi recently. Luckily it was during the middle of the night so no vehicles were out on game drive at the time. The next morning a few of the guides who weren’t driving guests left camp early to see how much the Sand River had risen after over 100mm of rain in under 24 hours and on their way to the causeway they came across something not seen too often – a tall tree on fire!
Unfortunately, I was on leave and not able to witness the spectacle first hand but after being sent a short clip of the burning Knobthorn (Acacia nigrescens) I was fascinated. Flames flickered forth from the top of the tree trunk and you could clearly see a charred scar all the way down the trunk where the electricity had exited the tree.
Such obvious visual signs of a lightning strike made me wonder why we don’t see this more often which spurred me on to do some research. Growing up I was warned – as many of you probably were – to not stand under a tree in search of protection during an electric storm. This is wise advice as trees are relatively good conductors of electricity. There are couple of reasons for this. One of them is that tall objects are more likely to be struck by lightning. This is due to the physics of electricity always taking the path of least resistance. Essentially this means that any tall structure such as a tree, is more likely to be struck than another nearby smaller object, as electrons jump from the negatively charged cloud to the positively charged ground. Electricity can side-jump from the tree or even flow through the wet soil. This is why it is recommended not to seek shelter under a tree in a thunderstorm. But it doesn’t explain why we don’t see more signs of trees having been hit by lightning.
I kept on reading until I came across something that made sense. It turns out trees are struck by lightning quite regularly but the effects of a lightning strike comes down to the texture of the bark. A tree with relatively smooth bark such as a Sausage tree (Kigelia africana) allows rain water from a thunderstorm to run down the surface of its trunk in a rather continuous film. This allows electricity to travel down the exterior of the trunk as water is a better conductor of electricity than wood. This effect results in minimal damage to the tree as it effectively earths itself. However it is different for a tree with extremely rough bark such as a Knobthorn. The wrinkles of a Knobthorn cause water to cascade down the trunk in a series of interrupted mini-waterfalls. This means the electricity cannot transfer to the ground along the exterior of the tree, but chooses the next path of least resistance which is the damp cambium layer (outer growth layer beneath the bark). The speed at which the electricity runs through the tree causes the sapwood to explode, leaving a visible scar. Sometimes if the tree is already dying, either from a fungus attack or being ring-barked by an elephant, the rotting heartwood in the centre of the trunk catches alight and this is when we see the tree on fire.
In theory this means that many more trees are struck by lightning than I initially thought. It just depends on their bark as to how severely they are affected by the blow. I will be keeping an eye out after the next storm to see any more signs of trees getting struck and what species they are to confirm or scrap this hypothesis. Either way, this rather peculiar sighting made me appreciate the power of lightning even more so than before.