120 Ballard and Nowak: Timing of Cut-Stump Herbicide Applications Table 1. Herbicide treatment mixtures and quantity of herbicide mix applied. Herbicide treatment Herbicide mix Kow Accord + Arsenal 40% Accord (16.6% a.i. glyphosate) and 2% Arsenal (0.6% a.i. imazapyr) mixed with water Garlon 4 + Stalker 20% Garlon 4 (12.3% a.i. triclopyr) and 1% Stalker (0.3% a.i. imazapyr) mixed with Hy-grade EC basal oil Pathway A “ready-to-use” herbicide (20.9% a.i. 2,4-D and 5.4% a.i. picloram) zAhrens (1994). yU.S. EPA (1998). application date was represented by 28 to 30 trees with 10 trees from each of three blocks, except a few that could not be found the second year. After treatment, all treated trees in the plots were counted to determine tree density, and sample trees were located. We then measured the diameter of the cut surface of each sample tree. Two diameters were measured for each tree and averaged. At the end of the first (2001) and second (2002) growing season, each sample tree was exam- ined to determine mortality or survival based on green cam- bium or foliage. Because we observed changes in individual tree status between 1 and 2 years after treatment, with some trees perceived to be dead 1 year after treatment but alive the second year, and vice versa, we report only second-year mor- tality. The amount of herbicide applied in each plot was deter- mined by weighing spray bottles before and after treatment to ensure that normal, adequate amounts of herbicide were ap- plied. Volumes for the April and June treatments were cal- culated using specific gravities of each herbicide mix, derived from manufacturer’s Materials Safety Data Sheets. Novem- ber treatments were measured volumetrically in the field. The quantity of herbicide applied was calculated on per-area and per-tree bases. Statistical Methods Analysis of variance (ANOVA) with a 3 × 3 factorial experi- ment in a randomized complete block design (plot as the experimental unit, n27) was used to test for herbicide mix and application date effects on percentage mortality and amount of herbicide used. The SAS program (SAS Institute 2002) was used to conduct all statistical analyses. An -level of 0.10 was used as the critical value to determine statistical ©2006 International Society of Arboriculture significance, though P values up to 0.20 were considered as indicating potentially meaningful results. If a significant in- teraction was observed (0.15) between herbicide mix and date of application, we tested simple effects. ANOVA was also used to verify that treatment plots had comparable den- sities of treated trees. Since there was a marginally significant difference in tree density by date of application (P 0.19) but not for herbi- cide mix (P0.99), and the amount of herbicide applied per hectare was strongly correlated with the number of trees treated (r 0.60, P < 0.01, n 27), we report results for herbicide use calculated on a per-tree basis. These results were comparable to ANOVA with herbicide use on a per- hectare basis. Pearson product-moment correlations were calculated be- tween the amount of herbicide applied per hectare and the number of trees treated, between percentage mortality and average stump diameter, between percentage mortality and herbicide applied per tree, and between stump diameter and amount of herbicide applied per tree. We were initially con- cerned that tree size could have an influence on the efficacy of the treatments. However, because correlation between stump diameter and percentage mortality was low (r0.25, P0.20, n27), we used ANOVA rather than analysis of covariance. RESULTS Percentage Mortality Mortality of cut-stump–treated trees was affected by both herbicide mix and application date (Figure 1). Differences in percentage mortality among herbicide mixes were significant but differed by treatment date (significant herbicide mix by z Glyphosate: 0.0006–0.0017 Imazapyr: 1.3 Triclopyr: 0.11–2.64 Imazapyr: 1.3 2,4-D: 2.81y Picloram: 1.4–83.2 Target zone for application Cambium area Quantity applied 10.2 L/ha (4.4 qt/ac) Cut surface, plus bark of stump and exposed root collar and roots, to the point of runoff Cambium area 17.4 L/ha (7.4 qt/ac) Tree density treated 1,620 stems/ha (660 stems/ac) 1,610 stems/ha (650 stems/ac) Average tree diameter 8.1 cm (3.24 in) 8.7 cm (3.48 in) 10.4 L/ha (4.4 qt/ac) 1,650 stems/ha (670 stems/ac) 8.6 cm (3.44 in)
May 2006
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