Lightning and Trees

Posted by David Whiting, Department of Horticulture & LA, Colorado State University

 

As reported in local media, last week a mature Honeylocust tree at the Annual Trial Gardens here on campus was hit by lightning and destroyed.  The large trunk was blown apart, with the tree in six large pieces.

 

Lightning hit this old Honeylocust tree at the Annual Trail Gardens on the CSU Campus.  Photo from The Coloradodan at http://www.coloradoan.com/article/20130705/NEWS01/307050051/Lightning-strike-demolishes-tree-damages-CSU-Trial-Gardens

  

An estimated 100 million lightning strikes occur per year in North America.  Colorado is one of the regions with extremely high occurrence of lightning.  Lightning is the greatest natural destroyer of property and kills about 80 people per year in the United States.  In addition, thousands of forest trees are struck each year, sometime starting fires. 

In risk management, people typically take cover in tornados and hurricanes reducing the risk potential.  However, people often don’t take cover in thunderstorms, leading to injury and deaths.  Personally, I love to watch a thunderstorm with lighting mover across the high plains, and have to tell myself to exercise some caution.  In a thunderstorm, straight-line wind microburst often exceed 55 mph.  Outflow winds from a thunderstorm can exceed 100 mph. 

Lightning can suddenly pop some 5 miles away from where the storm is focused.  Last summer, while walking my dog out on the high plains, I enjoyed watching a thunderstorm off in the distance.  Suddenly, without any warning, lightning hit in the open field next to me.  I was lucky and it really reminded that you don’t have to be "in the storm" to have lightning!

Lighting follows the path of least resistance.  When lightning strikes a tall unprotected tree, it may travel down the stem for a distance, then may leave the tree “jumping” to a more conductive tree structure, person, or animal.  Sideflashes can cause serious damage to structures, often starting fires or damaging electrical systems or appliances.  People or animals taking refuge beneath trees during a storm can be seriously injured or killed by sideflash.

As lightning exits an unprotected tree through the roots, it dissipates into the soil.  If people or animals are standing in the area, a potentially harmful or even deadly flow of electricity may go through their bodies, up one leg and down the other.

Thousands of unprotected trees are struck by lightning each year.  Most lightning-struck trees will not be seriously damaged; other will linger for years before succumbing to secondary pests.  Some will die within a short time.  Relatively few struck trees will be completely shattered or blown apart by a lightning strike.  In addition, when severe damage occurs parts of the tree can be thrown hundreds of yards, causing damage.

Several years ago, lightning hit my neighbor’s mature birch tree.  No visible damage was observable and normal growth suggested no injury.  A year later, lightning hit the same tree again.  I will always remember the extremely loud thunder boom!  Limb on the birch tree fell to the ground in a donut shape, as if they were cut off and just dropped to the ground.  However, the trunk was completely blown apart.  A few larger trunk pieces were the size of fireplace logs.  Most of the trunk was rendered to finger size to matchstick size.  Tiny fragments of the trunk were over 300 feet away.  Sideflash to the house damaged the home electrical system, and charred the light switched and electrical plugs on the tree side of the house.  Luckily, the house did not catch fire.

Lightning protection systems in trees are intended to safely conduct the lightning to the ground.  Properly installed and maintained, systems are reported be 98% effective at preventing serious tree damage. 

The National Fire Protection Associations’ Standard for the Installation of Lightning Protection Systems recommends installation of lightning protection systems in trees that are within 10 feet of a structure, that are taller than the structure, and have limb over the structure. 

The National Arborist Association recommends protecting trees of historic interest or high value; trees in recreational areas, parks, gold course, and other areas where people might be injured; and trees more prone to strike because of their location on isolated hills, open pastures, or near water.

Details on tree lightning protection systems are outlined in Best Management Practices: Tree Lightning Protection Systems and ANSI A300 Lightning Protection System Standards. Both are available from the International Society of Arboriculture at www.isa-arbor.com.

 

Information Source: Arborist News, April 2003.