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Looking Beyond the Bark
By Robert Booty, Registered Consulting Arborist



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Ground penetrating radar (GPR) is one of the latest non-invasive forensic technologies used for wood decay analyses. Its sensitivity to hollows, internal cracks or voids enables it to detect and create an image of internal molecular changes in wood composition. Today, this technology has advanced to the point that GPR is becoming more and more in demand, in large part because radar is non-invasive and doesn't harm the tree under evaluation. Although radar images or any other imaging devices will not create an actual picture of the inside of a tree, it will provide the arborist with a likeness or image, allowing for reasonable recommendations for future care. In the photo, an Arborist OnSite technician uses radar imaging to determine the internal structural condition of an oak tree during a level 3 inspection. Photo: Arborist OnSite, Inc.


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 are increasingly concerned.

You visit your family physician, and after an examination he explains your options: exploratory surgery is one, but he also describes non-invasive alternatives such as ultrasound, MRI (electromagnetic imaging) or x-rays.

Knowing that complications can develop with exploratory surgery, your physician doesn't take any chances and orders an MRI scan.

Trees are somewhat like people. At some time during their lives, they too will develop internal problems that are not always identifiable by an external examination alone.

Arborists have used all sorts of methods and calculations to determine the internal condition of a tree and its potential risk. Some of these methods were very invasive, such as removing core samples, drilling or even cutting away decay within a cavity.

Those were the standard methods used at one time. Some of these are still in use today. There's reason to believe that the term "tree surgeon" may have been coined as a result of these invasive procedures.



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This is how GPR works: electromagnetic waves are emitted from a small surface transmitter and antenna, and the device is slowly moved around the circumference of a tree. Radar waves are sent out every two-tenths of an inch. When the signals encounter boundaries between areas with different electromagnetic properties, they reflect, refract, and/or diffract in a predictable manner. No waves bounce back when they encounter healthy wood tissue. Through reflected radar waves, an internal image of the compromised area within the tree is identified. By measuring changes in wood composition, the level of severity of a newly discovered defect can be evaluated.


Benefit Using Non-Invasive Procedures
Like people, trees can be adversely affected by the methods used to evaluate their health, particularly procedures that penetrate the bark. The establishment of decay 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 allow 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 or resist the spread of disease-causing pathogens that invade them. It's referred to as CODIT (compartmentalization of decay in trees.)

When invasive testing methods are used, these protective walls can be pierced, allowing decay pathogens, which at one time may have been localized or contained, to continue their spread within the tree.

When performing level three risk assessments, understanding the internal structural condition of a tree is a vital part of that process. If this internal data could be collected without drilling or using other invasive methods, if you could just collect your data and walk away as if you were never there, then it truly would be a win-win situation for the tree being evaluated.

Use of Radar Technology
This is the latest method to safely evaluate the internal structure and condition of a tree without causing physical harm. Having the ability to create and see an internal image of a tree and then identify hidden internal problems fills a critical gap in risk assessment and tree preservation.

How does it 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 depth and speed of both moving and fixed objects. Its uses today seem endless. As a simple illustration, when you look at the weather report, you are looking at Doppler weather radar, which tells you where and when 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 will 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.



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This valley oak (quercus lobata) is about 200 years old and sat between two homes. The homeowners on either side of the tree were concerned about its structural integrity. GPR scans were taken at four different elevations, beginning at the bottom. The examination showed advanced decay within the tree, the result of large, old pruning wounds, with average remaining solid wood of just seven inches (17.78 centimeters). The trunk diameter was 73 inches. In the photo above, Robert Booty (left) performs a level 3 inspection on the valley oak tree, while David Cárdenas uses a field computer to record the data. Photo: Arborist OnSite, Inc.


But what exactly is it that makes radar work?
Radar signals that are reflected back to the transmitter are the desirable ones that make radar work. An air-filled tree trunk (as in a decayed hollow area) or partially air-filled in an incipient (early stage) decay zone inside a cell wall of a tree are excellent reflectors for detection by GPR 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 distinguish between decayed and healthy wood?
Wood decay fungi decompose lignified cell walls within living wood tissue by using enzymatic and non-enzymatic systems. This creates, in the beginning, a microscopically detectable hollow or void within the cell walls of the wood, thereby reducing normal wood strength. If you were using a fish finder to locate fish, the sonar waves would bounce off the "air-bladder" within the fish giving you its location and depth. Ground penetrating radar does the same thing as it identifies changes in wood composition and strength, (except that) radio waves are used. Radar imaging can identify these changes within the wood cell composition. It's the loss of the wood's mechanical strength caused by these organisms that is inherently linked to hazardous situations, which can result in significant property damage or personal injury.

Incipient or early-stage decay is the very beginning of the biodegradation process of living wood tissue by decay-causing pathogens. Ground penetrating radar can detect these minute early changes in wood composition and strength loss.

With no obstructions, these waves are calibrated to penetrate to the center of the tree as the antenna is moved around the trunk or branch. The radar waves, in turn, are reflected back to the antenna or receiver when changes to normal wood composition are encountered anywhere within the scanning area.

The trained arborist, knowing the diameter of the whole tree section being scanned, can use the resulting data to determine if the internal deficiencies, if present, are sufficient to warrant further action.

Other Uses for Ground Penetrating Radar
The use of ground penetrating radar has opened up other doors recently in the field of arboriculture, including below ground root mapping. How many leaning trees are removed just because it's thought there is a lack of roots on the opposite side of the lean to support the tree?



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Right Before the tree was removed, the GPR imaging data was confirmed using invasive testing methods. Photo: Arborist OnSite, Inc.


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The finalized imaging data is shown here. Note that the gaps in the plot indicate bumpy sections of the trunk where the antenna was not making good contact. Analyst notes: There appears to be advance decay throughout the entire sector at this elevation, with an average remaining solid wood of approximately seven inches. The gray area is where the antenna was over a depression.


Trees are damaged every year because irrigation trenches are cut across their root systems unknowingly. Is that tree root causing damage to your sidewalk or driveway?

How many roots can we safely remove to install our root barrier? You won't know unless you excavate!

Ground penetrating radar is successfully being used to non-invasively locate and map tree roots below ground, without removing the concrete or digging up those roots just to see if they are the nasty ones really damaging the property.

Where does one obtain the equipment?
Dr. Anthony Mucciardi at TreeRadar Inc., 512 Ashford Road, Silver Spring, Md., 20910. More details at his website: www.treeradar.com.

About the Author
Robert Booty is a consultant at Arborist OnSite Horticultural Consulting, Inc. He is a registered consulting arborist (RCA 487) and an ISA qualified tree risk assessor. He has been involved in the practice of arboriculture and the care and study of trees for 49 years, and has been using ground-penetrating radar (GPR) in his practice for 12 years. His website: www.arboristonsite.com.



As seen in LC/DBM magazine, February 2017.






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