186 Aryal et al: Street Tree Spacing Effects on Crown Dimensions and Canopy Cover 8 neighbourhoods on the Halifax Peninsula. This was done in order to have a sufficient street tree popula- tion and variability of spacings to conduct a sufficient analysis. Halifax is situated in the Acadian Forest Region, where dominant tree species in residual and surrounding forests include red spruce (Picea rubens), white pine (Pinus strobus), balsam fir (Abies bal- samea), and red maple (Acer rubrum)(Loo and Ives 2003). Dominant tree species within urban areas of Halifax include Norway maple (Acer platanoides), American elm (Ulmus americana), and littleleaf lin- den (Tilia cordata)(Foster and Duinker 2017). Hali- fax is in the humid continental climate region, with an average annual precipitation in the last 3 decades used for normals calculations (1981–2010) of 1,468 mm, ranging from 96 mm in August to 151 mm in Novem- ber (ECCC 2021). Halifax is located in the 6a Cana- dian plant hardiness zone, with temperatures ranging between −4 and 19 °C (ECCC 2021). Data Collection We measured street trees on sampled street segments in the 11 residential neighbourhoods. Sampled street segments had to have sidewalks with a tree lawn (i.e., grass strip between curb and sidewalk) and a length of over 100 m between street intersections. Shorter segments were ignored, as they often had only 1 or 2 trees, making calculation of average spacing rela- tively meaningless. A complete street segment inven- tory for the 11 neighbourhoods was compiled using a geographic information system (GIS), and each street segment was demarcated with a street segment num- ber and total length, which was measured using aerial imagery. Altogether, 457 street segments (399 in the Halifax Peninsula and 58 in Dartmouth) were identi- fied. The total area of the 8 delineated neighbour- hoods in the Halifax Peninsula is considerably larger than that of the 3 in Central Dartmouth, so one-third of the measured segments in the Halifax Peninsula and half of those in Dartmouth were chosen randomly using a random number generator and the unique identification number of each street segment, giving a total sample of 188 street segments (Table 1). At least 10 segments were sampled in each neighbourhood. Trees in the 188 street segments were measured between April and August of 2016. Each tree was measured for its species, diameter at breast height (DBH), crown diameter perpendicular to the row of trees (i.e., street), and crown diameter parallel to the row of trees. Crown diameter summaries across DBH classes are shown in Figure 2. Street tree DBH was measured using a diameter tape, and crown diameter was measured using a 25-m measuring tape for 3 of the 4 radii of the crown, followed by summation to achieve the 2 crown diameter measurements. For measuring the perpendicular crown diameter, permis- sion from hundreds of residential property owners would have been necessary. Consequently, the per- pendicular crown radius away from adjacent proper- ties was measured and doubled to get diameter values. Table 1. Number of street segments and street trees measured in old residential neighbourhoods in the Halifax Peninsula (H) and Dartmouth (D). Neighbourhood H1 H2 H3 H4 H5 H6 H7 H8 D1 D2 D3 Total Total street segments 37 30 77 53 33 74 51 44 17 22 19 457 Total sampled segments 13 14 34 26 12 25 18 15 10 11 10 188 Total measured trees 156 155 418 452 129 304 148 154 71 97 78 2,162 Total segments length (m) 6,987 5,242 12,532 11,553 7,014 13,800 6,235 7,994 2,849 4,543 3,890 82,639 Measured segments length (m) 2,239 2,507 5,711 5,888 2,476 4,638 2,190 2,533 1,806 1,995 3,485 35,468 ©2021 International Society of Arboriculture
September 2021
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