If a Tree Falls in the Middle of the City…

If a tree falls in a forest and no one is around to hear it, does it make a sound?”

This question has served as an icebreaker for generations of introductory philosophy courses — highlighting the sometimes murky nature of reality and the role observation plays in determining it.

But what happens if a tree falls in the middle of a city? Whether or not it makes a sound is the least of concerns to the unfortunate city dwellers below.

Trees within an urban community provide aesthetic, social, ecological and economic benefits. Recent research even points to measurable cognitive and psychological benefits by introducing more nature to the daily regimes of urban workers. Despite these benefits, urban trees remain large physical structures in close proximity to people and property, and their failure can cause damage to individuals and infrastructure. Recognizing this, researchers at the Forest Products Laboratory (FPL) devised several methods to evaluate a tree’s condition and detect decay and defects that could jeopardize public safety.

These nondestructive evaluation techniques (NDE) allow researchers to investigate and verify the condition of a tree without causing it any damage. This way, experts avoid damaging healthy trees while they weed out the bad ones. NDE techniques vary, but can range from visual inspection regimes to cutting-edge electronic scanning technologies.


Decay detection in red oak trees using a combination of visual inspection, acoustic testing, and resistance microdrilling.

In FPL’s home, Madison, Wisconsin, a tree stability survey was conducted on 153 trees surrounding the Capitol using NDE methods. Researchers from the laboratory first conducted visual inspections of all directly and indirectly observable defects, and then followed up with acoustic testing. Acoustic testing uses sound waves to determine the structural soundness of a material — in the case of trees, even beneath the bark.

Thanks to lab benchmarks, researchers know how sound should travel through healthy wood. If the sound waves move slower through a tree undergoing acoustic testing, there might be decay beneath the surface. In the case of the Capitol’s trees, if the sound waves had more than a 25% reduction in speed, they failed the test.

Finally, some trees received resistograph testing. The resistograph is a fine-tipped drill that detects decay by measuring the resistance where the drill bit meets the wood (or more specifically, the electrical power consumption in the motor driving the bit). Since low density, decayed wood requires less power and torque to drill through, if the resistograph drill is consuming less power, there’s a good chance that it’s chewing through a decayed part of the tree.

A 300-page report was submitted to park managers detailing the cases of internal decay and defects revealed using these combined nondestructive testing methods. Select tree removal and pruning were followed by arborists based on the report’s conclusions. Sadly, even a 100-year old red oak was given a poor bill of health and had to be removed.

Just weeks before the last two trees were removed, a violent storm with unusually high velocity winds passed through the park. Of the remaining 143 trees in the park, the only ones to fail were two that had been identified as high risk using the NDE techniques. Luckily, they caused no harm to people or buildings, but their loss did verify the effectiveness of the researcher’s work.

Whether or not they made a sound however, is still up for debate.

For more information, please see FPL’s publication Nondestructive Evaluation of Wood: Second Edition.