192 Aryal et al: Street Tree Spacing Effects on Crown Dimensions and Canopy Cover higher rates of ecosystem service supply (Nowak and Greenfield 2012). In addition to increased canopy cover and ecosys- tem services from street trees, there may be some possible co-benefits from closer street tree spacings. A decrease in built-surface and building-facade tem- peratures associated with urban trees has been docu- mented in previous research (Bajsanski et al. 2019). Trees in tighter spacings can utilize the available growing space more efficiently, with greater crown leaf area and biomass compared to trees in wider spacings (Ceulemans et al. 1990; Benomar et al. 2011; Benomar et al. 2012). Conversely, low-density forest conditions can lead to reduced soil moisture availabil- ity resulting from increased evaporation rates (Erkan and Aydin 2016). Further empirical research on microclimate and soil conditions among different street tree spacings to test whether these co-benefits are possible in urban conditions and linear planting arrangements would be valuable. It may also be possible that closer street tree spac- ing standards and practices would have negative out- comes. First, it is important to note that street trees face a myriad of stressors and disturbances associated with their location next to streets in heavily built-up and high-traffic areas. These include de-icing salt, water shortages, insufficient light, degraded soil con- ditions, pollution, and vandalism, among others (Lu et al. 2010; Steenberg et al. 2017). These translate to increased rates of tree mortality and reduced growth rates (Jutras et al. 2010). While mortality from com- petition (e.g., suppression and stem exclusion) typi- cally occurs at very close spacings in forest settings, the cumulative effect of any additional stress associ- ated with increased biological competition between street trees at closer spacings might exacerbate these adverse conditions and lead to even higher mortality rates and reduced growth rates. If closer spacing standards were to be adopted and implemented for street tree management, the chal- lenge associated with increased costs must be addressed. The costs of street tree planting will of course increase proportionally with closer spacings. The potential for increasingly constrained municipal urban forest management budgets is possible, given both the existing low planting densities of street trees (McPherson et al. 2016) and existing sparse budgets for many municipal urban forest programs (Kenney and Idziak 2000; Haaland and Konijnendijk 2015). ©2021 International Society of Arboriculture However, it is important to consider both trade-offs between costs and benefits and the temporal dynamics of these trade-offs. Our findings suggest that closer spacings will potentially achieve a given level of tree crown per unit area in a shorter period of time com- pared to wider spacings, if indeed canopy extent can be larger with crown interactions, which can offset some of the costs of increased tree planting and mainte- nance. Stock type and size constitute another possible mitigative factor for increased street tree planting costs, with lower costs associated with smaller nursery stock. While smaller stock may have higher rates of damage and mortality than larger stock (Roman and Scatena 2011), their considerably lower price could still improve the feasibility of closer spacing stan- dards in the long term. In other words, the increased price of more trees may be offset by the lower cost of smaller stock even with known challenges of planting smaller stock in urban settings. This study and further research in this area could be useful for municipal urban forest practitioners. Publicly owned street trees, while being a smaller proportion of the total number of trees in a given city, represent a large proportion of municipal manage- ment activities and investment. Street tree spacing standards and practices should be based on empirical research that addresses both the supply of ecosystem services and any adverse outcomes associated with competition and increased costs. This study does not provide evidence for how to design new street tree spacing standards, which will be needed in future research. However, its findings do suggest that closer spacing may not necessarily be a disadvantage for canopy cover of street trees. Urban forest ecosystems are a vital resource for North America’s increasingly urban population and the increasing exposure of urban landscapes to the changing climate. It is critical that urban forest management be firmly rooted in evi- dence and scientific research for supporting best practices. LITERATURE CITED Aryal B. 2017. Economic and biophysical implications of alter- native street-tree spacing in Halifax, Canada [master’s thesis]. Halifax (Nova Scotia, Canada): Dalhousie University. Bajsanski I, Stojaković V, Milošević D. 2019. Optimizing trees distances in urban streets for insolation mitigation. Geograph- ica Pannonica. 23(4):329-336. https://doi.org/10.5937/gp23 -24242 Benomar L, DesRochers A, Larocque GR. 2011. Changes in specific leaf area and photosynthetic nitrogen-use efficiency
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