Arboriculture & Urban Forestry 46(6): November 2020 balance between the above- and belowground sys- tems, it appears safe to presume that summer or late-summer reduction pruning should result in epi- cormic branch densities and heights less than those obtained with winter reduction pruning, especially because the volume and the tallest epicormic branch were lower on trees pruned in the summer (Figure 2). These last results corroborate previous findings by Kays and Canham (1991) and Perrette et al. (2014) on deciduous broadleaved trees 3 years after total main-stem harvesting. According to Kays and Can- ham (1991), divergence in epicormic branch devel- opment between seasons is related to a phenological gradient in carbohydrate reserves. In fact, pruning during the leaf-on season, when stored reserves are low (Barbaroux and Bréda 2002; Furze et al. 2018), limits the potential for epicormic branch develop- ment. Conversely, epicormic branch development is higher when pruning occurs during the leaf-off sea- son, when stored reserves are highest. Epicormic Branch Cohort Recovery Dynamics By examining individual epicormic branch cohorts generated after applying reduction pruning to the main stem, our study was able to show contrasting dynamics of density and volume over time (Figures 3 and 4). The first epicormic branch cohort was imme- diately initiated in the second half of the year of growth following both main-stem reduction intensi- ties in the summer (Figure 3a and d). However, the initiation of a new cohort in the second growing sea- son of summer reduction pruning that was denser than the first one showed that the contribution of the first cohort was not enough to restore the energy bal- ance between the above- and belowground systems. Nevertheless, because the volume of the first cohort at the end of the third growing season was higher than the volume of the second cohort at both reduction pruning intensities, this finding emphasizes the pre- dominance of the first cohort initiated in the process of recovery on a tree pruned in summer (Figure 4a and d). A similar finding was observed with both late-summer and winter reduction pruning intensities after the 2 growing seasons, as epicormic branch den- sity and volume were primarily composed of the cohort initiated during the first growing season (Figures 3 and 4b, c, e, and f). On one hand, this result suggests an incapacity of trees pruned in late summer to 443 instantly initiate the restoration process in the year of pruning. This could be related to the short length of the remaining growing season (Figures 3 and 4b and e). On the other hand, this once again highlights the dynamics and primary role of carbohydrate storage levels for epicormic branch development, as a lower volume of epicormic branches with a similar density were produced in late summer compared with winter reduction pruning at the end of both growing seasons (Figure 4b, c, e, and f). Considering that reduction pruning in late summer was performed at the time of maximal carbohydrate storage (Furze et al. 2018), late-summer pruning appears to have circumvented the buildup of carbohydrates for optimal epicormic branch development in the following growing season. The minor establishment of a third cohort in the summer reduction pruning treatment and a second cohort in both the late-summer and winter reduction pruning treatments indicates that the entire system was equilibrated after 1.5 growing seasons for summer and only 1 growing season for late-summer and win- ter reduction pruning (Figure 3). Thus, the epicormic branch density dynamics in the time after reduction of the main stem and between the leaf-on and leaf-off periods are in agreement with previous studies, such as Perrette et al. (2014) following total harvesting, and DesRochers et al. (2015) after crown-raising of the main stem. This indicates that the epicormic branch dynamics initiated to rebuild the loss of leaf area is independent from the intensity of the opera- tions completed on different parts of the tree. Line Clearance and Problematic Epicormic Branches Our study examined the number and volume of epi- cormic branches that should be removed in according to clearance standards 2 years after reduction pruning of the main stem. Unexpectedly, a lower pruning inten- sity increased the number of problematic epicormic branches when compared with the higher pruning intensity (Figure 5). Several authors reported that removing less than 30% (Collier and Turnblom 2001; O’Hara et al. 2008; Maurin and DesRochers 2013) or 20% (Grabosky and Gilman 2007; Dujesiefken et al. 2016) of the biomass limited epicormic branch devel- opment. In our study, a low pruning intensity removed 35% to 52% of the biomass, because the trees were in contact with a virtual power distribution network located 7 m above the ground. As a result, the ©2020 International Society of Arboriculture
November 2020
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