If a Tree Is About to Fall, Is It a Hazard?

2015 Sept 23

Turns out that “it ain’t necessarily so”!  Recently, I attended the 25th Western Colorado Community Forestry Conference in Montrose on Tree Risk Awareness.  Once again, it was a wonderful event. In the past, I thought any tree that looked like it might fall was a hazard tree. 

For several years, I’ve dreamed that the dead tree across the street that blocks our view of Blanca would be designated a hazard tree and the owners would be required to cut it down. This is not the case! Tree risk has two separate parts: risk assessment and risk management. Assessment is the likelihood of an event and management is based on the severity of the consequences.

Risk assessment has three parts: likelihood of failure, likelihood of target impact, and consequences of failure.  I think it is fairly likely that the tree will fall over in the next few years. Probably it will fall to the northeast (since our winds are normally from the southwest) and hit dirt and weeds. Therefore, the consequences of falling would be negligible.

Risk management also has three parts: target, species, and action. The target impact (where the tree will fall) for my sample tree would be considered a low usage area.   There’s a small probability that it could crash through a broken fence to the west and land on our very quiet street. If it fell in any other direction, it wouldn’t hurt anything. From the management viewpoint, the Siberian elm is considered a high risk species.  High risk species tend to be large trees with poor branching habits (limbs that are weakly joined to the trunk), and have a poor ability to compartmentalize defects (if a tree can’t wall-off injured portions decay is more common).

What action should be taken? Since it is not likely to harm anything, no action is considered necessary for safety.

This is a simple example. What about larger trees in Alamosa?  If you have a tree you’re concerned about on your property or adjacent property, please contact the Alamosa office of the Colorado State Forest at (719) 587-0915.  I’m not an expert, but learned some guiding principles at the conference.

Obvious hazards include large, dead branches that are 3 inches or more in diameter -- especially those that are leaning over a site where people tend to be such as a playground, well-used picnic table, or busy sidewalk. Please be aware that if a tree on your property falls over or drops branches causing damage to adjacent property, you are liable!

Sometimes you can’t tell if a large tree with green leaves is a hazard.  So what should you do?  On a field exercise during the conference, Colorado Forester Vince Urbina tapped on the trunk of a large tree with a large mallet.  You could tell the tree was solid by the sound – it didn’t sound hollow. That reminds me of a large cottonwood tree at Adams State than fell onto the business building many years ago.  Fortunately, no one was injured and there was only a bit of damage to the building. There was no outward indication that the trunk was hollow, but it was obvious after it fell. You may not need to hug your large trees, but you might consider thumping them with a mallet every year or so!

On our field walk, we also inspected the tree trunk.  There were no vertical cracks that descended all the way to the ground.  The tree wasn’t leaning and no roots were above ground. All signs of a healthy tree.

“When checking trees, don’t forget to look up,” Vince said.  People tend to look straight ahead or down at the ground.  Again, especially with large trees, look for any dead branches in the canopy.  If you see some, consider the consequences if they fell.  Trees are more likely to shred branches or fall over as a result of severe weather.  In Alamosa, high winds and prolonged drought seem to be our biggest tree threats.

One presenter described the situation in which a large tree in a public park was considered to be risky. The inspector suggested cutting it down to avoid hurting people who might walk or sit under it.  The powers that be didn’t want to cut down the tree, so they spent several thousand dollars to install a fence around the tree! I don’t think Alamosa has funds for this.  And really, might it be better to use the money to install new trees that future generations can sit and walk under safely?

A future column will discuss how to reduce long term risk by proper tree selection.

“Trees are not power poles.  They need water. They need care.” Scott Grimes of Colorado Tree Consultants