Reduced tree vigor can occur from either man-induced impacts, such as construction and trampling, or by naturally occurring disease and insect attack. Various techniques have been developed to measure these losses of vigor, and one of the most recent is the Shigometer.
The Shigometer (Shigo and Shigo, 1974) is a battery operated, field instrument which assesses tree vigor by determining the electrical resistance of bark, cambium, and outer xylem. Needle electrodes are inserted through the bark into the sapwood, and resistance to a pulsed electric current is measured.
Vigorous trees exhibit characteristic readings to a pulsed electric current that differ from stressed or damaged trees. Healthy, dominant trees have lower resistance readings than intermediates, which in turn have lower resistance than those which are suppressed. Also, trees with full, dense crowns have lower resistance readings than those with sparse crowns (Wargo and Skutt, 1975).
Resistance patterns are also present when decay develops in wounded or pathogen infected trees (Tattar and Blanchard, 1977). As wood tissues decay, concentrations of potassium, calcium, manganese, and magnesium cations increase, and as these cation concentrations increase resistance to a pulsed current decreases. The magnitude of the decrease in resistance indicates the degree of tissue deterioration (Shigo and Shigo, 1974).
The electrodes used in resistance measurements are two stainless steel needles embedded in a nylon insulated handle. The needles are inserted by an oscillating action which frequently results in the bending and ultimate breakage of needles.
In order to overcome this problem, a handle driver has been developed to insert and extract the needles. This paper describes construction and operation of the driver. The driver decreases measuring time and eliminates needle breakage.
The Driver
The driver has an overall length of 12.5 inches (31.25 cm) and can be used with 1 or 1.5-inch (2.5 and 3.75 cm) standard needles (Figure 1). Its total weight is 33 ounces (924 grams) with the hammer alone weighing 14.5 ounces (406 grams). The hammer is a sliding weight which is pulled back, then slammed forward, driving the needles into the wood with no bending stress. Figure 2 illustrates driver components, and Table 1 identifies each component part. Figures 3 through 6 illustrate Parts B, C, D, E, and F. To control rust all metal parts should be zinc plated.
The driver can be constructed for approximately $350.00.
Needle electrode driver.
Components of the needle electrode driver.
Sliding hammer bar (Part B).
A study of the driver’s performance found that measuring time and needle bending were reduced and needle breakage eliminated. Insertion methods were compared by conducting penetration tests of 40 hardwood and conifer species. For each species ten tests were made using the conventional insertion method, and ten with the driver. Each test utilized a pair of needles and was made at breast height (135 cm). Needle bending, which was determined by the perpendicular displacement of needles from the face of the probe, was assessed after each test.
Table 2 lists tested species and a comparison of mean time and percent of needle bending for conventional and driver insertion methods. Measuring time was found to have been reduced 51 percent for hardwoods, and by 40 percent for conifers. Utilizing conventional insertion techniques, the needle bending rate was 52 percent for hardwoods and 31 percent for conifers, and the rate of breakage was 1 percent for hardwoods and no breakage occurred for conifers. In driver tests no needles were broken, and the rate of bending was 5 percent for both hardwoods and conifers. No charge in measurement accuracy was found when the driver was used.
Sliding hammer (Part C).
Nylon insulator (Part D).
Electrodes crew (Part E) and the electrode holder nut (Part F).
- © 1986, International Society of Arboriculture. All rights reserved.
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