Arboriculture & Urban Forestry 46(6): November 2020 Power Line Clearance Standards and Reduction Pruning Treatments Two years after the main-stem reduction, in all season treatments, the number of problematic epicormic branches in contact with the virtual 2.5-m wire corri- dor zone was significantly higher in the lower inten- sity of reduction pruning treatments compared with higher intensity (Figure 5; F1,7 = 12.44, p = 0.0096). By contrast, no significant difference in the volume of problematic epicormic branches existed between intensity treatments (Figure 5; F1,7 = 0.05, p = 0.8288). = 9.23, p = 0.0028) of problematic epi- cormic branches compared with other reduction prun- ing seasons, except that the number of epicormic branches during summer reduction pruning had inter- mediate values between the late-summer and winter treatments. At both intensities, reduction pruning during winter increased the number (F2,14 volume (F2,14 = 4.04, p = 0.0412) and = 1.80, p = 0.2210), but was higher when reduction pruning was performed during the summer (2016, F2,14 = 7.00, p = 0.0078; 2017, F2,14 Reduction Pruning Treatment and Wound Compartmentalization In 2016 and 2017, 1 and 2 years after reduction prun- ing of the main stem, the pruning wound-closure rate followed the same significant pattern among treat- ments (Figure 6). The closure rate was similar between intensities (2016, F1,7 2017, F1,7 = 0.01, p = 0.9091; = 14.44, p = 0.0004). Conversely, the discolored area of the wound was significantly higher with higher pruning intensity after 2 growing seasons (F1,7 was not influenced by pruning season (F2,14 p = 0.9717). = 51.98, p = 0.0002), but = 0.03, DISCUSSION AND CONCLUSIONS Intensity and Timing of Reduction Pruning on Epicormic Branch Development The results from our study show that trees can vigor- ously respond by epicormic branches after a main- stem reduction pruning (Figure 2). The fact that a higher pruning reduction intensity resulted in an increased number and volume of epicormic branches, and that the resulting epicormic branches were taller than those produced after lower-intensity pruning reductions, confirmed that reduction pruning inten- sity largely controls the epicormic branch response. 441 However, the intensity was not the sole factor con- trolling the emergence of epicormic branches, as epi- cormic branches were also present in control trees. Colin et al. (2010) previously reported that epicormic branches can occur with an increase in light availabil- ity after stand thinning. This could explain the pro- duction of epicormic branches in our control trees after reduction of the main stem of adjacent trees. Still, the lack of, or very low, epicormic branching found on control trees compared with those in other reduction pruning treatments indicates that reduction pruning intensity was a major driver of the epicormic branch response. Although intensity has been reported as the primary factor causing epicormic branching with total removal of the main stem following har- vesting (Kays and Canham 1991; Babeux and Mauf- fette 1994) or primary branch order following pruning (O’Hara et al. 2008; DesRochers et al. 2015), this is the first study to our knowledge linking pruning intensity to epicormic branch response when only the main stem of the tree is reduced. Therefore, our study provides key knowledge related to our overall under- standing of the physiological response of the main stem with reduction pruning. However, to achieve a global perspective of the understanding of the physi- ological tree response to reduction pruning, a similar study should be undertaken at the branch scale. The timing of main-stem reduction pruning during the year, corresponding to the leaf-on or leaf-off period, is a significant factor in the development of epicormic branches, although to a lesser extent than reduction pruning intensity (Figure 2). O’Hara et al. (2008) and DesRochers et al. (2015) previously demonstrated this with the removal of lower primary branch order of the living crown for silvicultural pur- poses. However, because winter pruning was per- formed before summer pruning in those studies, a delay equivalent to half a growing season for the ini- tiation and development of the epicormic branch arose on trees pruned in summer, which could have significantly impacted the results (O’Hara et al. 2008). In our study, summer reduction pruning was applied before winter reduction pruning, and despite a decrease of density and mean height of epicormic branches on trees pruned in summer compared with those pruned in winter, the differences were not large enough to be statistically significant. Nonetheless, because at the end of the 2017 growing season, summer- pruned trees have more than half of a growing season compared to winter-pruned trees to restore the energy ©2020 International Society of Arboriculture
November 2020
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