190 Aryal et al: Street Tree Spacing Effects on Crown Dimensions and Canopy Cover Figure 4. Average canopy cover (m2 ) for street trees in each of the 8 DBH classes in each of 3 categories of crown interaction with neighbouring trees (no interaction, one-sided interaction, or two-sided interaction). Table 5. Two-way ANOVA results for both the crown diameter ratio and crown projection area (m2 Variable Crown diameter ratio DBH Crown interaction Crown projection DBH Crown interaction df 7 2 7 2 F 7.95 49.80 1,422.93 13.43 ) variables. P-value < 0.001 < 0.001 < 0.001 < 0.001 DISCUSSION AND CONCLUSION The findings of this study are two-fold. First, we found that the effect of closer tree spacing and increased crown interactions in rows of street trees in Halifax, Canada, translated to altered crown dimen- sions, with crowns extending away from the direction of interaction. This effect was far more pronounced in trees of medium size (i.e., 20 to 40 cm DBH) and not larger trees, the reason for which is uncertain and requires further research. Street trees typically started having crown interactions at maturity with a spacing between 10 and 15 m, while the crown diameter ratio tended to increase noticeably (i.e., elongation of the crown) below 10-m spacing. Second, among larger trees, these differing crown dimensions were also associated with increases in crown projection area ©2021 International Society of Arboriculture and canopy cover. However, some important limita- tions of this study are that it did not explicitly con- sider or quantify the effects of management on street tree crown architecture, in particular pruning, and it did not consider other drivers of urban forest struc- ture that may influence crown architecture, such as the surrounding built environment (e.g., impervious surfaces), other land uses, and citizen stewardship of urban trees (Vogt et al. 2015; Steenberg et al. 2019). These should be included in future research. More- over, future research might also address species and light availability (e.g., using aspect) effects of shad- ing on crown interactions, inclusive of trees that are in close proximity but not physically interacting. Other limitations include the restriction of data collection to older residential neighbourhoods, which introduces potential bias given the social processes across different neighbourhood types that can influence urban tree health and structure (Vogt et al. 2015; Steenberg et al. 2019). There are several factors that might explain these findings. In addition to genetics, environmental factors influence tree crown dimensions, such as light avail- ability, growing space, soil, nutrients, and water. Indeed, many studies have shown that trees tend not to have fixed crown shapes at maturity, even within the same species (Brisson 2001; Getzin and Wiegand 2007; Purves et al. 2007; Schröter et al. 2012; Olivier et al. 2016). This flexibility in crown shape, along with
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