Re: decay detection for diff levels of risk asses
see arbnews feb 09 for DECAY DETECTION DEVICES
In his most recent case (December 2008), Detective Dendro wrestled with three fungal antagonists attacking the base of his client’s tree. Dendro’s inspection and assessment methods raised some persistent questions about devices deployed to detect the deep, dark decay within trees. This article will survey some of the devices used in the field, describing advantages and limitations of each. Our purpose is to see which devices can answer which questions, and lead to more comprehensive diagnoses and better management of trees at risk of succumbing to deadly disease.
Excavating soil that covers those portions of the tree that might be decayed, especially the buttress or “root crown”. Hand tools such as shovels and trowels are cheap but not fast or easy, and can damage bark. Water works well, but it makes a mess. Air tools readily clear away mulch and coarse soil, but they move dense clay very slowly unless it is premoistened. Carpenter ants around a tree indicate decay, and they can expand cavities. These and other insects fly without wings when compressed air is applied to decayed areas. Air tools will not break boundaries as they excavate cavities and allow inspection and measurement. Operators are advised to wear personal protective equipment (PPE), including respirators and ear protection.
Exposing the root crown is a diagnostic step and also a treatment, because infections can be rendered inactive when they are dried out. Once the outside of the tree is exposed, it can be inspected for signs and symptoms of decay. Bulges can indicate expanding decay inside. Sunken areas may be localized infections, or “cankers.” Any material that is loose or decayed can be removed in order to examine the limits of decay. Striking the trunk with a fiberglass or rubber mallet—the “tap test”--can indicate decay when there is a hollow sound. Electronic hammers are basic tools for measuring wood strength via sound waves using impact and sensor screws. Some kinds of decay are more audible than others, and air pockets inside thick bark and other characteristics give “false hollow” sounds. The arborist gets up close and personal with the tree during the tap test, making it a good first step toward detecting decay.
Examining decay in living trees can be done in many noninvasive ways. Binoculars can scan the scaffold structure and locate areas that may need aerial assessment. Open cavities can be inspected by using a long probe with a measuring tape attached. Even small holes can be wide enough to look in, using optical devices designed for mechanics and plumbers. Both the borescope and the See Snake (registered trademark) consist of a tube with an eyepiece on one end and a lens on the other. With attachments the views can be angled, magnified, and recorded with still images and video. Flexible tubes allow more complete views, but with less clarity. Even closed cavities can be viewed by drilling a small hole and inserting the device. If you save the shavings, you can send them in for a DNA assay of decay fungi, as described in the December issue. If the drill bit gets hot the DNA can be damaged, but keeping it sharp and frequently backing it out keeps the hole clear and the bit cool. By pushing a foam earplug onto the drill bit and backing out when the wood resistance goes down, you can measure the thickness of the wood outside the cavity.
Less tissue loss and more precision results from IML’s Resistograph (registered trademark) and the Sibtec digital microprobe. The Resistograph generates a paper printout with each test, and the data can also be analyzed by a computer program. The Sibtec’s output is digital, but an optional field printer can also deliver hard copy. The Sibtec measures changes in the rate of penetration at a steady pressure. The Resistograph measures changes in torque as the probe penetrates at a steady speed. Both microdrills can be deflected, giving misleading information. The Resistograph probe is stiffer and can snap more easily instead of being deflected. All drilling methods need a trained eye to first determine where the test should be done, and also to reliably analyze the data, starting by visually inspecting the shavings and the wood in the flutes of the drill bit. If the practitioner knows how much resistance the drill meets in a sound specimen of a given species, the difference in compromised wood may be very noticeable. The invasion may only be one millimeter in diameter, but in research on ash Fraxinus sp. trees this size channel was more infected than the five millimeter holes made by increment corers.
These highly portable, easily-assembled but shoulder-straining devices are traditional tools used by foresters to extract cylinders of wood so the growth rings can be viewed directly. The arborist gets a look at not only the tree’s growth but also the advancement of the fungus by direct observation discoloration and incipient decay in enzyme-altered tissues can be identified. The information gained may be worth the breaking of boundaries that occurs when the tool goes from sound to decayed wood. Aside from this lateral damage, the tool can also create longitudinal cracks when it is forced. As with all tools, keeping them clean and sharp is vital to getting reliable data. The cores can be preserved in plastic straws for later viewing. The force required to break or compress a core can also be measured with fractometers, to determine both the compressive and bending strength of the wood. Drilling and coring only provide information about the area around the hole.
