Arboriculture & Urban Forestry 46(6): November 2020 an average rounded to the nearest millimeter. To obtain the total height and volume per epicormic branch, growth units of primary order lengths and growth unit volume of all branch orders were added. Growth unit volume (V) was calculated according to the formula: where L and dm 2 are the length and median diameter of the growth unit, and π is equal to 3.1416. An epi- cormic branch was considered above the reduction pruning cut when epicormic branch initiation or growth reached beyond the height of the reduction pruning cut. To be considered a problematic epicormic branch, part of the growth unit had to be in contact with the virtual wire corridor zone located 5.5 m above the ground. The mean number, volume, and tallest epicor- mic branch per reduction pruning treatment were obtained by averaging the number, sum, and length results of each tree, whereas mean height was obtained by averaging epicormic branch height per tree prior to averaging per reduction pruning treatment. Reduction Pruning and Wound-Closure Rate Immediately after reduction pruning of the main stem in 2015 and at the end of the growing season in 2016 and 2017, the vertical length (parallel to the retained scaffold branch of the main-stem reduction pruning) and horizontal width of the pruning wound, both crossing the pith, were measured with a caliper to the nearest centimeter to determine the rate of pruning- wound closure. Each year, the surface area of the wound (S) not fully closed by the callus tissue was calculated as an ellipse according to the formula: where L and l are vertical length and horizontal width, respectively. The pruning wound-closure rate was expressed as a percentage of the immediate surface wound area after pruning reduction. Discolored Wood Area Following Reduction Pruning At the time of harvest in 2017, a 1-m trunk section containing wound-reduction pruning treatments and two 0.5-m trunk sections containing both selected aspect ratios of controls were removed from trees with a chain saw. Lateral branches originating within these sections were removed close to the union with the trunk, with the exception of the retained scaffold 435 branch of the main-stem reduction pruning, where a length of 5 cm was preserved. All trunk sections were dissected with a sliding table saw along a radial lon- gitudinal plane of 30 cm, bisecting both centers of the reduction pruning wound and the scaffold branch. Dissected sections were progressively polished with up to 400-grit sandpaper and scanned at 2400 dpi. The area of discolored wood on each scan was delin- eated, and its surface area was calculated based on pixel counts using Adobe Photoshop CC 2018 (Adobe Systems, Inc., San Jose, CA, USA). All areas of dis- coloration were normalized by dividing by the length of the cross-sectional pruning cut area. The final area of discolored wood per reduction pruning wound was computed as the average of the 2 halves. Statistical Analysis Linear mixed effect models were used to predict epi- cormic branch (height, number, volume, tallest, and problematic) and wound (closure rate and discolored area) responses as a function of reduction pruning intensity and season. Sampling blocks were included in the models as a random effect. Differences between cultivars were first tested, and because they were found similar (Figure 1), models were rerun with both cultivars pooled. As no interaction between reduction pruning intensity and season was found in any model, these results are not presented. To examine the effects of reduction pruning treatment over time on the den- sity and volume of epicormic branches in the 2015, 2016, and 2017 cohorts, a multivariate analysis of variance (MANOVA) was performed. Main effects were treatments and years. All statistical analyses were conducted using JMP software, version 13.0.0 (SAS Institute, Cary, NC, USA). RESULTS Physiological Tree Response After Reduction Pruning Treatments In 2017, 2 years after reduction of the main stem, the dynamics of epicormic branch initiation and develop- ment through treatments above the pruning cut were similar to the epicormic branch dynamics of the whole tree (Figure 2). All reduction pruning treat- ments had greater effects on the epicormic branch ini- tiation and development than control trees (Figure 2; results not shown). For all season treatments, a higher intensity of reduction pruning of the main stem sig- nificantly increased the number (F1,7 = 106.71, ©2020 International Society of Arboriculture
November 2020
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