Arboriculture & Urban Forestry 33(1): January 2007 17 0.15, and 0.05 live HWA/linear inch) for 2003, 2004, and 2005, respectively. In the treatments, we observed a decrease in HWA pressure over time. The live HWA per linear cen- timeter for control samples were 1.92, 2.16, and 2.76 (4.8, 5.4, and 6.9 live HWA/linear inch) for 2003, 2004, and 2005, respectively. In the controls, we observed increasing HWA pressure over the 3 year study. A Student’s t-test determined significant differences between IMA-jet and control samples in 2004 and 2005. The results for 2004 and 2005 indicate that one can expect a difference of 1.84 to 2.37 and 2.23 to 3.26 live HWA/linear centimeter (4.60 to 5.93 and 5.58 to 8.15 live HWA/linear inch) between treatments and controls, re- spectively, at a 95% CI. The 2003 sample populations for live HWA/linear centimeter on IMA-jet and control trees was not significant with P 0.066. New Linear Shoot Growth Each year, new hemlock shoot growth was determined for each sample. The current-year shoot growth is best correlated with the previous year’s treatment. The mean length of new growth for IMA-jet-treated hemlock was 6.03, 5.79, and 6.26 cm (2.41, 2.32, and 2.50 in) for 2003, 2004, and 2005, re- spectively. The mean length of new growth for control samples was 5.71, 4.78, and 5.48 cm (2.28, 1.91, and 2.19 in) for the same years, respectively. A Student’s t-test indicated there are significant differences between IMA-jet and control samples for the years 2004 and 2005 (P 0.000 and P 0.025). The results in the two most recent evaluations reflect from 0.53 to 1.48 cm (0.21 and 0.59 in) and 0.10 to 1.47 cm (0.04 to 0.59 in) more growth in the treatments, respectively, at the 95% CI. The distributions of hemlock shoot growth with <4 total, and 4 total or >, and <2 live, 2 live or > HWA/linear cm are plotted in Figures 4 and 5. Both followed a normal distribu- tion. These data reflect the entire sample population (1277 samples), including IMA-jet-treated and control samples. Figure 4 presents the difference (P 0.000) between the distribution of shoot growth in samples with less than four total HWA/linear centimeter shoot growth (6.02 cm [2.41 in]) versus four or greater total HWA/linear centimeter shoot growth (4.41 cm [1.76 in]). That is to say, we can expect from 2.25 to 2.97 cm (0.90 to 1.19 in) more growth when the total HWA/linear centimeter is less than 4 at a 95% CI. Figure 5 presents a statistically significant difference (P0.000) be- tween distribution of hemlock shoot growth in samples with less than two live HWA/linear centimeter (5.96 cm [2.38 in]) versus two or greater live HWA/linear cm (3.92 cm [1.57 in]). We observe 1.66 to 2.42 cm (0.66 to 0.97 in) greater shoot growth in hemlock when live HWA/linear centimeter is less than two at the 95% CI. Here we were interested in quantifying the live HWA pressure on hemlock growth. We used 4.0 total HWA/linear centimeter shoot growth as the number to negatively impact hemlock. This is analogous to Figure 4. Shoot growth distributions in hemlock compar- ing less than 4 and 4 or greater total hemlock woolly adelgid (HWA)/linear centimeter. Both IMA-jet-treated and control samples were used for this analysis of HWA density and hemlock shoot growth. 2.0 live HWA/linear centimeter shoot growth when percent HWA mortality in the control trees is 50 or less. In this study, we observed from 1.92 to 2.76 live HWA/linear centimeter shoot growth in the controls throughout; these trees remained stressed. DISCUSSION Eastern hemlock is sensitive to HWA infestation. This sen- sitivity may be expressed as an inverse relationship between HWA pressures and shoot growth. McClure (1991) estab- lished four total HWA/20 mm2 of branch as the level to serious affect growth in forest stands. This is consistent with Figure 5. Shoot growth distributions in hemlock compar- ing less than 2 and 2 or greater live hemlock woolly ad- elgid (HWA)/linear centimeter. Both IMA-jet-treated and control samples were used for this analysis of HWA den- sity and hemlock shoot growth. ©2007 International Society of Arboriculture
January 2007
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