Radar Imaging for Tree Risk Management in HOAs

Published in the ECHO Journal, May 2012

As you are traveling home one evening, you notice a consistent dull pain in your lower back. As the weeks pass the pain intensifies and you become increasingly concerned. You visit your family physician, and after an examination he explains to you your options to determine the source of your pain. He describes how exploratory surgery could provide him with some answers. But he also describes other options including non-invasive ones such as ultra sound, MRI (electromagnetic imaging) or an x ray. Knowing that complications can develop using exploratory surgery, the physician doesn’t take any chances and orders an MRI scan on your lower back.

Trees are somewhat like people; at some time during their life they too will develop internal problems that are not always able to be seen just by an exterior examination.

Arborists have used all sorts of methods to determine the internal structural condition of a tree and its potential risk to a property. Some of these methods were very invasive, such as taking core samples, drilling or even cutting away decay within a cavity. Those were the standard methods used at one time, and some continue to be used today. There’s reason to believe that the term “Tree Surgeon” may have been coined as a result of these invasive procedures.

The Value of using Non-Invasive Testing Methods for Tree Risk Management

What benefit is there in using non-invasive diagnostic procedures with trees? Trees can be adversely affected by the methods used to evaluate their health especially using invasive diagnostic procedures that penetrate the outer bark.

The establishment of decay[1] in living trees is affected by urban environmental stresses that range from a general weakening of a tree’s natural defense system to injuries that allows wood-rotting agents to gain entry through wounds. Trees have an internal protection system that uses a series of four internal walls, all beautifully designed to block the spread of disease causing pathogens within the tree. It’s referred to as CODIT, an acronym for Compartmentalization of Decay in Trees.  

However when invasive testing methods such as drilling are used, these four protective walls can be pierced by the drill bit, allowing decay pathogens that at one time may have been localized or contained to spread throughout the tree. One concern many people have always had with these testing methods is the many holes being drilled into a tree that one is trying to save.

When performing tree risk assessments, knowing the internal structural condition of a tree is only one part, but a very vital part, of the process. If this internal data could be collected without drilling or using other invasive methods, and you could just walk away with your data as if you were never there, then it would be a win-win situation for the tree under evaluation.

Use of Radar Technology

Today technology has advanced to the point that this non-invasive type of data collection is possible. It is being accomplished by using ground penetrating radar that does not harm the tree being evaluated. The use of radar imaging on trees creates the same type of high- resolution, non-invasive image that a medical professional would utilize in his diagnosis with his patient using MRI imagery. This is the very latest method to safely evaluate the internal structure of a tree without invasion of the tree itself. Having the ability to create and see an internal image of a tree and then to identify any hidden internal problems fills a critical gap in tree preservation.

How Does Radar Work?

Ground-Penetrating Radar (GPR) is an established technique that has been used worldwide for over 40 years. Radar is an object-detection system that uses electromagnetic waves—specifically radio waves—to identify the range, altitude, direction, or speed of both moving and fixed objects. Its uses today seem endless. When you look at the weather report, you are looking at Doppler weather radar that tells you where the heaviest amounts of rain will fall in your area. The radar, as it passes through the clouds, measures the density of the moisture in them and the speed they are traveling so you can know approximately when it will start raining and how much rain will fall. Radar is used in aviation, automobiles, law enforcement, and locating objects below ground.

When an electromagnetic wave[2] emitted from a small surface transmit antenna encounters a boundary between objects with different electromagnetic properties, it will reflect, refract, and or diffract from the boundary in a predictable manner. Radar waves or signals are reflected especially well by materials of considerable electrical conductivity. The radar signals that are reflected back towards the transmitter are the desirable ones that make radar work. An air-filled tree trunk (decayed hollow) or partially air-filled incipient decay zone (early stage) inside a cell wall of a tree are excellent reflectors for detection by GPS systems. Use of GPR instrumentation for internal tree trunk decay detection is one of its latest uses in the field of tree risk assessment.

How does radar imaging distinguish between decayed and healthy wood?

Wood decay fungi[3] decompose lignified cell walls in living wood tissue by using enzymatic and non-enzymatic systems. This decay creates a microscopically detectable hollow or void within the cell walls of the wood, thereby reducing normal wood strength. Radar imaging can identify these small changes (voids) in the wood composition. It is the loss of the wood’s mechanical strength caused by these organisms that is inherently linked to hazardous situations, often resulting in significant damage to property or injuries.

Radar imaging is one of the latest forensic technologies used for wood decay analyses; its sensitivity to hollows, internal cracks or voids enables it to detect and create an image of these small internal changes in wood density and composition. Incipient or early-stage decay is the very beginning of the biodegradation process of living wood tissue by decay causing pathogens; radar imaging can detect these early changes.

Using the trunk inspection diagram in Figure 1, you can see the radar antenna is slowly moved around the circumference of the tree. As it travels it sends out radar waves every two-tenths of an inch; with no obstructions these waves penetrate to the center of the tree as the antenna is moved around the trunk.

The radar waves in turn are reflected back to the antenna or receiver if decay is encountered anywhere within the tree. The presence of sometimes hundreds of these reflected radar waves creates an internal image of the compromised area that was found inside the tree, measuring the density of the wood and indicating the level of severity of the newly discovered decay.

In the process the remaining solid healthy wood is identified and displayed to the technician and measured in inches. The trained arborist, knowing the diameter of the whole tree section being scanned, can use this resulting image data to determine if the internal decay is sufficient to warrant further concern or action.

Today arborists have multiple options when making tree risk assessments, but most importantly there are tools available that will not harm the tree under inspection. While radar imaging does not create a picture of the inside of a tree, but it does yield an image or likeness that allows for reasonable decisions to be made regarding the future health of the tree.

What does all this mean for homeowner associations and planned communities that may have hundreds of trees on their common areas without knowing which ones have the highest risk of failure? It means that trees that are very valuable to your community and irreplaceable can be safely evaluated for the risk they may pose, without compromising their health in the process. This equates to tree preservation for your community and the retention of real property values.    

The use of ground penetrating radar has opened up other doors recently in the field of arboriculture. One that has also become valuable in the area of tree protection is below ground root mapping. How many trees are irreparably damaged every year because irrigation trenches are cut across their roots unknowingly? Is that tree root causing that damage to an association’s sidewalks or driveway?

You didn’t know in the past unless you excavated! Now ground penetrating radar is being used to locate and map tree roots below ground non-invasively, without removing the concrete or digging up those roots just to see if they are the nasty ones really damaging the property. Watch for a future article about locating and mapping below ground roots, using ground penetrating radar.

Robert Booty as a consultant at Arborist OnSite® Horticultural Consulting Inc. He is a Registered Consulting Arborist and an ISA Certified Arborist. You can reach him at Robert@arboristonsite.com.