Arboriculture & Urban Forestry 48(5): September 2022 Arboriculture & Urban Forestry 2022. 48(5):293–307 https://doi.org/10.48044/jauf.2022.022 URBAN FORESTRY ARBORICULTURE Scientific Journal of the International Society of Arboriculture & Long-Term Growth of Highway Rights-of-Way Trees By Allyson B. Salisbury, Jason W. Miesbauer, and Andrew K. Koeser Abstract. Background: Highway rights-of-ways (ROWs, or verges) contain multiple stressors which can influence tree growth, including com- pacted soils, soils with little topsoil, poor drainage, air and soil pollutants, construction activities, and de-icing salts in cold climates. Yet high- way ROWs often provide ample planting space for growing trees, which can contribute to the mitigation of negative environmental impacts associated with highways. Methods: For this study, we assessed the trunk diameter of 1,058 trees from 11-, 22-, and 31-year-old planting cohorts along a highway in the Chicago metropolitan region (Illinois, USA) to examine factors which could influence long-term growth. We analyzed the impact of location factors within the ROW (e.g., distance and elevation relative to highway, slope, and aspect) on trunk diameter at breast height (DBH), since these factors are relevant to the landscape design process. Using estimates from i-Tree, we compared carbon sequestration, carbon storage, runoff reduction, and air-pollution removal within and among the 3 cohorts. Results: Of the 6 site location char- acteristics we evaluated, no single characteristic consistently impacted DBH, though some characteristics were significant within a single cohort. DBH measurements of most species were smaller than model predictions based on existing urban tree models. Since all cohorts included large- and small-statured trees, and even within species DBH could be highly variable, the range in per-tree ecosystem services varied substantially within cohorts, especially the 31-year-old cohort. Conclusions: These findings highlight both the potential for and challenges of growing trees alongside highways. Keywords. Ecosystem Services; i-Tree; Roadside Woody Vegetation; Site Conditions; Urban Tree Growth. INTRODUCTION Urban forests are expected to provide a variety of benefits for their communities while minimizing costs and disservices (Turner-Skoff and Cavender 2019; Roman et al. 2021). However, a range of human and biophysical factors can increase tree mortality (Hil- bert et al. 2019) and compromise growth in urban landscapes (Vogt et al. 2015). Observing and model- ling the effects of exposure to disturbances and stress- ors on the structure and function of the urban forest is a necessary step to assess the overall vulnerability of urban forests and take actions to limit potential losses of ecosystem services (Steenberg et al. 2017). While there are multiple ways to evaluate the structure of urban forests (Leff 2016), tree size is a particularly valuable metric. Many urban-forest functions such as building energy-usage reduction, air-pollutant removal, and carbon sequestration directly scale with tree size (Nowak et al. 2008). Conditions in the built environment often, though not always, lead to reductions in tree growth and con- sequently the benefits provided by mature trees. Within urban environments, growth is often greater in park or garden settings compared to street trees grown in strips or planting cutouts in close proximity to nonarterial streets (De Lacy and Shackleton 2014; North et al. 2018). The presence of pavement can limit soil moisture, aeration, and nutrient availability (Hodge and Boswell 1993) and increase surface tem- peratures (Chen et al. 2017), reducing growth. In some cases, open growing conditions found in some urban planting sites favor greater aboveground growth compared to trees in forest settings (Rhoades and Stipes 1999; Smith et al. 2019). Smith et al. (2019) also observed that any potential benefits to carbon sequestration resulting from faster growth of urban trees was offset by shorter life spans. Growth responses to urban settings in the first several years after planting can also vary among species and be influenced by management decisions and practices such as planting season, initial caliper, irrigation, and mulching (Lawrence et al. 2012; Koeser et al. 2014; Vogt et al. 2015). While land-use type can also influ- ence growth (Lawrence et al. 2012), factors that ©2022 International Society of Arboriculture 293
September 2022
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