184 Aryal et al: Street Tree Spacing Effects on Crown Dimensions and Canopy Cover Dahlhausen et al. 2016; Monteiro et al. 2016). How- ever, the studies do not specifically address the issue of competition and spacing in street plantings, nor provide subsequent recommendation on spacing stan- dards. Indeed, the focus is often on the space require- ments of trees based on crown dimensions (Pretzsch et al. 2015), not changes in crown dimensions at dif- ferent spacings. There is a plethora of municipal stan- dards and guidelines for street tree spacing. For example, Vancouver, British Columbia (Canada) and Visalia, California (USA) have spacing standards based on tree size, which are 6 m to 10 m for medium trees and 9 m to 14 m for large trees (City of Visalia 2005; City of Vancouver 2012). However, street trees are typically planted at considerable distances apart, much further than municipal standards might demand, which results in increased leaf area and reduced above- ground biomass of individual trees (Nowak et al. 2008), but lower overall leaf area and canopy cover on a given street. For example, Aryal (2017) found that the average street tree spacing in Halifax, Canada was over 15 m (in some neighbourhoods, over 20 m). The street tree density in several studied streetscapes in California was found to be lower than 50 trees/km and has declined nearly 30% since 1988 (McPherson et al. 2016). Arguably, there are two issues at play: (1) based on the very limited empirical evidence available, it seems that street tree spacing is typically exceeding recom- mended standards, leading to fewer overall trees and lower canopy and ecosystem services; and (2) there is no published evidence that any of these highly vari- able street tree spacing standards and guidelines are based on empirical research on street tree performance and costs, though our assumption around this issue is certainly a source of uncertainty in this study that warrants future research (e.g., interviews with munici- pal practitioners that have standards in place). Regarding the first issue, several factors might explain existing street tree spacing patterns. First, street trees are expen- sive to plant, as the balled-and-burlapped nursery stock costs over $400 to install in Halifax (J. Simmons, personal communication, 2018 February 2). Second, it is often necessary to space street trees at certain dis- tances away from existing infrastructure, such as drive- ways, hydrants, signage, underground pipes and wires, street lights, and utility poles (The City of Winnipeg 2009; Miller et al. 2015). Third and paramount, a main driver of street tree planting and management is ©2021 International Society of Arboriculture their aesthetic value, which is based on fully devel- oped crowns unaffected by competition with neigh- bouring trees (Miller et al. 2015; Aryal 2017). If spacing guidelines and practices are to consider street trees not only as amenities, but also as ecosys- tem service providers, then a reconsideration of spac- ing standards is necessary, as is compliance with these standards in municipal street tree plantings. Many of the ecosystem services provided by street trees are positively associated with the amount of tree foliage (Nowak et al. 2008). Over the long term, the amount of tree foliage per unit area of city street is highly dependent on the number of trees. Our theory is that increasing the density of street plantings has the poten- tial to achieve higher levels of ecosystem service sup- ply, especially when the trees are young. However, there is also the potential for adverse impacts due to competition and costs, which are discussed later in this paper. It stands to reason that, at a minimum, improving planting practice to conform to established street tree spacing guidelines is needed, while con- ducting research on spacing guidelines to optimize ecosystem service supply would be valuable. With regard to increasing street tree density to improve urban forest benefits, it is important to con- sider trade-offs in planting costs (i.e., cost versus size of the stock), but also trade-offs in reduced tree per- formance due to biological competition between trees. With regard to crown architecture, research has primarily focused on commercial forests and silvicul- ture, in particular on young trees, saplings, and seed- lings, to examine the competitive effects on individual tree growth and tree crown structure (Weiner 1982; Canham et al. 2004; Coates et al. 2009; Thorpe et al. 2010). To be applicable to street trees and optimal street tree spacing, there is a need for research that investi- gates competitive effects on trees growing only in linear rows, with competition occurring with no more than two neighbouring trees. In this study, we examine variability in crown shape and size for street trees in Halifax, Canada, to test for possible interaction effects of street tree spacing. Specific research objectives include: (1) to measure variability in crown diameters both parallel and perpendicular to street tree rows; (2) to test whether differences in crown dimensions can be explained by interaction effects with neighbouring trees; and (3) to measure if any existing differences in crown dimensions associ- ated with crown interactions lead to a reduction in
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