434 Perrette et al: Optimizing Reduction Pruning of Trees Under Electrical Lines (Environment Canada 2018, Assomption weather station). The soil is clay and clay mixed with fine sand subsoil. Experimental Design and Reduction Pruning Treatments The experiment took place in 2015 in an existing plan- tation composed of 2 cultivars of Pennsylvania ash trees (Fraxinus pennsylvanica Marsh.) from field-grown seedlings propagated in 2004 and transplanted in 2009. A total of 21 and 35 trees from ‘Prairie Spire’ and ‘Patmore’ cultivars, respectively, devoid of stress were selected among 22 and 39 individuals, respec- tively (see explanation below for selection). Trees from ‘Prairie Spire’ were 6 m to 6.6 m in height and 7.7 cm to 9.4 cm in DBH, whereas the ‘Patmore’ attains a height of 5.6 m to 7.3 m and a DBH of 5.7 cm to 9.7 cm. The experiment consisted of 7 treatments, arranged in a random block design, with 3 and 5 blocks (repli- cates) for ‘Prairie Spire’ and ‘Patmore’ cultivars, respectively, and 7 trees per block. In addition to a control with no reduction pruning treatment, 2 main- stem reduction pruning treatments were performed between 2 m and 2.5 m, as well as between 3 m and 3.5 m above the ground (hereafter referred to as high and low intensity of reduction pruning, respectively) to simulate a prescribed corridor zone of 2.5 m around a fictitious power distribution network located 7 m above the ground during 3 distinctive season periods: early July, early September, and early December (hereafter referred to as summer, late summer, and winter, respectively). As the retained scaffold branch diameter relative to the parent axis diameter (aspect ratio) affects the surface area of decay after pruning (Eisner et al. 2002; Gilman and Grabosky 2006), we tried to keep the aspect ratio of the main-stem reduc- tion pruning across trees within a small range (from 0.38 to 0.46). Although control trees were not pruned, they had one similar aspect ratio between the trunk and a scaffold branch in each part located between 2 m and 2.5 m as well as between 3 m and 3.5 m above the ground. To obtain the range of aspect ratio between trunk and scaffold branch in pruned and control trees, similar unions were first selected on each tree for both intensities of reduction pruning and prior to assigning random block treatment. For each branch union selected, the trunk and scaffold branch diameters were measured 10 mm above the scaffold branch ©2020 International Society of Arboriculture bark ridge with a 2-m Lufkin tape measure to deter- mine the aspect ratio of the main-stem reduction pruning. Trees with no aspect ratio that ranged from 0.38 to 0.46 for both intensities of reduction pruning treatments were excluded from the study. Trees with aspect ratios for both intensities of reduction pruning treatments were conserved as controls and not pruned, whereas season treatments were randomly assigned to trees on which only one intensity of reduction prun- ing was applied. For each reduction pruning treat- ment of the main stem, only one reduction pruning cut was made using a hand saw, so as to comply with the American National Standards Institute (ANSI 2008). Pruning wound diameters ranged from 5 cm to 7.5 cm and from 4.2 cm to 6.6 cm for high and low intensity of reduction pruning treatments, respec- tively. The amount of biomass removed was visually estimated by 2 assessors and ranged from 60% to 72% for the high intensity of reduction pruning treat- ment and from 35% to 52% for the low intensity of reduction pruning treatment. Including the retained scaffold branch of the main-stem reduction pruning, 4 to 6 and 10 to 15 lateral branches remained on the trunk for high and low intensity of reduction pruning treatments, respectively. No reduction pruning treat- ment of the main stem was made on a scaffold branch with included bark or codominant aspect, and no heartwood was visually present on any reduction pruning cut. Data Collection Epicormic Branch Inventory Live epicormic branches from each tree were counted and measured during late summer from 2015 to 2017. As defined by Bégin and Filion (1999), all deferred or proleptic epicormic branches on the trunk and branches were counted. Additionally, all immediate or sylleptic epicormic branches on branches were counted (except in 2015) if their annual growth length was greater than the annual growth length of the retained scaffold branch of the main-stem reduction pruning. Each inventoried epicormic branch was first labeled using a tapener, measured for initiation height, and classi- fied relative to the year of its establishment, i.e., 2015, 2016, or 2017. All the growth units of each epicormic branch were classified per branching order (Barthe- lemy and Caraglio 2007). The length was recorded with a ruler, and the median diameter was recorded with calipers at the widest part and at right angles for
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