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21 Dec 2017

What happens when lightning strikes a tree?

Fast moving storms brought high winds and lightning to Sydney this week. While parts of the city reached into the 40s on Wednesday, there was a rapid cool change when several intense thunderstorms hit the area.  

The powerful flashes of lightning were captured across social media, with some remarkable images showing bright bolts of lighting up the harbour. One of those photos shows an electric purple bolt of lightning touch ground by the Sydney Opera House.

More precisely, this bolt struck a tree on the Bennelong Lawn in the Royal Botanic Garden Sydney.  It’s believed that this tree, Forest Red Gum, Eucalyptus tereticornis , predates European settlement in Australia and has been at the Gardens long before its inception. 

Trees, because of their height, are natural lightning rods

When a tree falls victim to a strike the damage can be minimal or quite literally explosive. Since water and sap is a better conductor than wood and bark, lightning damage is often related to the concentration of moisture in and around a tree.

Electricity seeks the path of least resistance, and the moisture inside a tree is a much better conductor than air. This means a tree provides the preferred path for lightning to reach the ground. Some trees escape completely unharmed by a direct hit, while others sustain moderate damage to total failure. 

Death of a tree from a lightning strike is not easily diagnosed

In most trees, the layer just under the bark contains moisture. Since water is a better electrical conductor than wood, lightning striking a tree tends to travel just underneath the bark. The explosive expansion of the lightning's return stroke will literally blast off the bark, and sometimes some of the wood, along the length of the lightning channel. The result is visible scarring along the trunk of the tree.

A tree struck by lightning has been stressed severely. The intense heat and energy of the strike compromises the trees internal infrastructure.
Dr Dale Dixon

Stressed trees are targets for insect and disease problems

Arborists say that there's not much that can be done for a tree that has lost a significant amount of bark.

“Once a tree is scarred it loses its protective bark and becomes susceptible to disease, extreme weather and parasites," says Dr Dale Dixon, Curator Manager at the Royal Botanic Garden Sydney. "It may survive for a few months or even a year but this takes a toll on the trees health because the bark can’t be grafted or grow back. Some trees die immediately from what looks like small external damage while others will live for a number of years.”

In trees that are already dying or rotting, the moisture may be concentrated deep in the trunk. When an already vulnerable tree is truck the lightning will travel through the core of the tree with significant results. The powerful strike will be explosive and blow the trunk apart, splitting the trunk in two, or totally obliterated to splinters.

If heavy rain has occurred prior to the electrical storm and the outside of the tree is soaked, a bolt may flash over the outside of the bark, leaving little or no damage in its path.

What happened to the tree that was struck by lightning in the Garden?

Once images of the storm surfaced online, our aborists were quick to identify which tree was hit by lightning. From there a full assessment was made and the portion of the tree that suffered damage was deemed structurally unsafe. Fortunately, only part of the historical tree needed to be removed.

The arborists will continue to monitor the situation making sure tree is healthy and remains safe for the public. The wood that has been removed will be given a second life by the Treecycle program. Through this program, reclaimed wood is transformed into artwork.


 

Fast Facts:

  • Lightning kills more people than tornadoes and hurricanes.

  • Most deaths occur in open fields near or under trees or around water.

  • Lightning strikes the earth somewhere 100 times every second.

  • The temperature of a lightning flash can be 30,000 degrees Celsius- five times hotter than the sun.

  • Peak currents can be 20,000 am

Category: Arboriculture
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