Arboriculture & Urban Forestry 46(6): November 2020 between one-half and two-thirds of the scaffold branch to stimulate the recovery of the apical dominance by this scaffold branch (Gilman and Lilly 2002; see Fig- ure 1 in Grabosky and Gilman 2007). Nonetheless, a few years after reduction pruning, the space created within the internal tree structure is usually filled with epicormic branch recolonization (Goodfellow et al. 1987; Millet and Bouchard 2003; Follett et al. 2016). The epicormic branch initiation process, originating from proventitious or adventitious buds (Meier et al. 2012), occurs primarily to rebuild the leaf area loss of the crown (Deal et al. 2003) and restore the energy balance between both the above- and belowground systems following an injury (Valentine 1985). It is necessary to plan cyclical tree pruning to remove these epicormic branches entering the security corri- dor beneath the power lines (Millet and Bouchard 2003; Follett et al. 2016; Lecigne et al. 2018). Each year, more than 800 million dollars are spent for line clearance pruning in the United States (Good- fellow et al. 1987) compared with 60 million in the province of Québec, Canada (Millet 2012). These costs of tree maintenance depend on the length of the return interval, the time a tree is pruned, and the amount of biomass removed (Nowak 1990; Browning and Wiant 1997). In Montreal, the return time for tree maintenance can vary from 3 or more years (Millet and Bouchard 2003; Millet 2012; Lecigne et al. 2018), depending on the growth rate of the tallest epicormic branch (Follett et al. 2016), although 5 to 6 years is the optimum length of time based on economics (Browning and Wiant 1997). Therefore, as higher expenses are incurred with shorter intervals, a better understanding of epicormic branch growth rate is needed in order to increase the return time interval and optimize maintenance of the distribution network. On the other hand, pruning creates wounds and dys- functional wood at the cutting point and may provide an entry for microorganisms of decay that, over time, can induce cavity formation and alter the health, mechan- ical strength, and safety of the tree (Dujesiefken and Stobbe 2002; Dujesiefken et al. 2016). The wound compartmentalization process has been well defined ever since the CODIT (compartmentalization of decay in trees) model was established by Shigo and Marx (1977). Following an injury in functional sapwood, trees react by surrounding it with 4 walls laid down in the wood (Shigo and Marx 1977; Gilman 2011). Although walls 1 to 3 prevent the spread of discolor- ation and decay in the internal wood structure by 433 forming a reaction zone around the wound site, wall 4 closes the exposed wound area over time by form- ing a protective barrier zone. An increasing number of studies on the compartmentalization process that occurs when a branch is removed have been carried out (Dujesiefken and Stobbe 2002; Gilman and Gra- bosky 2006; Dănescu et al. 2015). However, few studies have focused on tree response to branch (Gra- bosky and Gilman 2007) or main-stem reduction (Gilman and Grabosky 2006). This study was undertaken to specifically investi- gate the predominant factors that control the growth- rate response of epicormic branches following a main-stem reduction and their influence on wound compartmentalization. Epicormic branch establish- ment and development have been extensively investi- gated in forestry management for stand regeneration after harvesting or for pruning of the lower primary branches in order to improve bole value (Meier et al. 2012). As it is well documented that higher stand basal area prior to harvesting (Kays and Canham 1991; Babeux and Mauffette 1994; Perrette et al. 2014) and higher pruning intensity (O’Hara et al. 2008; Des- Rochers et al. 2015) produce a greater number, length, and biomass of epicormic branches, our first objec- tive was to determine the magnitude of this effect at the tree main stem reduction scale. As the timing of silvicultural operations can also influence the epicormic branch response (Kays and Canham 1991; Babeux and Mauffette 1994; O’Hara et al. 2008; DesRochers et al. 2015), our second objective was to evaluate the benefits of main-stem reduction during the leaf-on season versus the leaf-off season. Our final objective was to investigate the influence of the intensity of reduc- tion pruning and time of year on the closure rate and the area of wood discoloration of the pruning wound. To avoid urban environmental conditions that could affect tree growth (Jutras et al. 2010), this study was carried out within a controlled nursery environment. MATERIALS AND METHODS Study Site The study was conducted 40 km northeast of Montréal at the Montréal Municipal Nursery in Assomption, Québec, Canada (45° 48’ N, 73° 25’ W). In this area, the climate is continental and humid, with hot summers and cold winters. The mean annual temperature is 5.3 °C, and the mean annual precipitation is 1018.7 mm, with a mean annual snow cover of 208.9 cm ©2020 International Society of Arboriculture
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