Arboriculture & Urban Forestry 42(3): May 2016 Electrical line-clearance pruning in Montréal typically involves pruning entire street blocks during a pruning cycle; as such, all the trees in a plot would have been pruned at the same time. For the streets sampled in this study, this ranged from three to five years prior to the sam- ple date. To estimate the pruning date, epicormic growth from heading cuts was used to back-date the cut. This assumed that the growth sampled occurred the year of pruning; however, late sum- mer or autumn pruning may not have induced growth during that year; as such, the pruning date recorded would have been the following year. Measuring Annual Branch Extension with a Laser Rangefinder To measure branch extension without climb- ing near the electrical corridor, a ground-based measurement system was developed and tested using a handheld laser rangefinder/clinometer (TruPulse® 360, Laser Technology Inc., Centen- nial, Colorado, U.S.). This allowed for rapid, rela- tively inexpensive data acquisition, while elimi- nating the need for aerial lift access or climbers certified to work in proximity to the electrical conductors. All linear measurements were taken from the ground using the laser rangefinder. The device measures distance to target, as well as in- clination angle and azimuth of the device. It can then calculate the distance between any two re- flective points (Figure 1). The device was used for all in-crown measurements, as well as distance to the nearest building and building height. The device resolution is 10 cm for linear lengths (rounding to nearest tenth) and is specified as accurate to ±0.25 degrees for inclination, ±1 degree for azimuth. The viewfinder/scope includes 7× magnification to allow for improved sighting accuracy. To assess the device’s realized accuracy for total branch length and internodal growth, re- searchers conducted field tests and verified laser measurements with in-tree linear measurements of 19 branch samples of F. pennsylvanica (Figure 2). For each tree included in the study, measure- ments of annual branch extension growth were collected for: a) branches that had regrown into the utility corridor following pruning in 2008 – 2010 (response growth), and b) control growth of unpruned branches growing on the periphery of 135 the crown. For control growth, exterior branches that were distant to the pruning site and visually assessed to be indicative of the average growth rate for the tree were selected. For response growth, the longest branches from each of the selected pruning response types that had regrown into the utility corridor were selected. This was based on a visual assessment during initial inspection of each tree, and followed up by routine measurement of the largest branches to select appropriate growth. Annual growth increments were measured as the distance between successive bud scars, or archi- tectural markers. In temperate areas, trees exhibit yearly rhythmic growth. When the bud scales that protect the terminal bud during overwintering are shed, they leave behind a prominent marking on the stem referred to as bud scars. As a new set of bud scales is developed for each overwintering bud, each year of growth is marked by the bud scar. Therefore, the bud scars allow for precise delineation of the length added each year, and for many species can be used to measure past growth for several years before present depend- ing on species characteristics (Canham 1988; Runkle 1992; King 2003). When the twig even- tually outgrows the bud scars, branch architec- ture can be used to delineate yearly growth for several more years (Millet et al. 1999); thus, it is possible to determine five to six years of yearly growth through visual analysis of branch features. Figure 1. Method for measuring branch length. The TruPulse 360 measures distance to target, inclination, and azimuth. It can then calculate the straight line distance between two targets. ©2016 International Society of Arboriculture
May 2016
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