Marin Master Gardeners
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Marin IJ Articles

Knock, Knock—Who’s There?

June 16, 2008
Janice Austin

No doubt you’ve heard of the Horse Whisperer, someone with the intuitive ability to communicate with another species?  In the world of arborists and potentially unruly big trees, there are also the Tree Whisperers who have an uncanny ability to read what lies beneath the bark using hammer tapping and resistograph technology.

Though not empowered with X-ray vision like Superman, these high-tech arborists employ a special talent and expertise that permit them to “see” into the tree and read the possible hazards, such as decay or cavities, which are invisible to the naked, untrained eye. Combined with the hammer tapping, the resistograph gives tree care professionals a fast, accurate and reliable assessment with measurable data, which can let the home or business owner know to what extent a tree is at risk of failure.  The resistograph helps the tree professional detect decay without damaging the tree and can help prevent accidents before they occur.

The resistograph, developed by Frank Rinn of Heidelberg, Germany, rests on the user’s shoulder like a bazooka, and is invaluable in making a detailed diagnosis when a tree shows signs of poor health or weakness, which may be displayed as cracks, root decay or leaning.  Not every tree in your backyard landscape needs resistograph testing, but if you or your arborist find any visible defects in a feature tree prominently located in your landscape, this testing is the most current and reliable method to check for stability in the tree branches, trunk and root regions.
The resistograph instrument uses a 19-inch long, 1/8-inch diameter drill bit, which measures the resistance of the wood as the microdrill enters the tree.  Held steadily, the instrument records the variations in the rotation speed of the drill as it goes up to 18-inches into the tree; the resulting record looks like an EKG printout spiked with peaks and valleys, which the arborist then can closely examine to determine the tree’s health. Though minimally invasive, this testing does not harm or weaken the tree and the tiny drilling hole closes up without any serious damage to the tree after the testing is completed.
I recently watched an arborist use a resistograph to measure tree health in two more than 150-foot Douglas fir trees in Woodacre. He explained the process and how the resistograph record produced from the testing could be interpreted. First, he climbed up an orchard ladder and sharply tapped along the trunk and root areas, listening to the sound of the wood to find the areas he wanted to investigate with the resistograph.  His perfect concentration reminded me of the intensity piano tuners have when they are practicing their craft.
Then after unpacking the two metal cases and assembling the resistograph equipment, the arborist and his assistant cleared around the major buttress root flares of one of the huge trees and drilled into several of these to check for tree health at its base. Then, he moved up onto the ladder next to the tree trunk. It looked very much like the arborist was carefully listening to the tree trunk as he used the resistograph propped up on his shoulder, leaning into the tree as though they were having a heart-to-heart discussion, which actually they were, as he was accessing the hidden heartwood with the instrument.
Looking at the long, waxy paper record, it was possible to see into the tree, and just like a dendrologist reading tree rings, with his help, I could see the tree’s long history of wet and dry seasons in the spikes running across the narrow page, and begin to have a sense of the tree’s age. The arborist pointed out that this tree has 6-inch thick bark, which seemed like extraordinary armor for the silent giant.  He is also a trained dendrologist and can analyze both tree rings and the longevity of trees using the resistograph. As a gardener who generally focuses on the more diminutive landscape of flowers, vegetables and short shrubs, I found it awe-inspiring to look up and intimately know the giants in the forest through an arborist’s expertise and resistograph technology. We can now see into the trees with which we live, when interpreted by Tree Whisperers who know how to read them.

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