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Needle Electrode Driver for Tree Vigor Determinations

Reduced tree vigor can occur from either maninduced 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, 1 975). 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.