Tree Type and Urban Growing Conditions Associated with Street Tree Stress: Lessons from Two US Cities

Abstract Background Trees provide crucial ecosystem services for urban areas, but the stress of the urban environment can influence tree health and ecosystem service provision. Street trees grow in particularly stressful conditions, but often receive care from some combination of municipal agencies, private businesses, nonprofits, and individuals. Methods In this study, we quantified tree stress using 4 canopy-based metrics (leaf discoloration, leaf defoliation, dieback, and overall crown stress) to see how stress varies with growing conditions and tree characteristics in 2 US cities: Chicago, IL, and Durham, NC. Using separate Bayesian models for each city, we examined the relationship between tree stress and tree characteristics (e.g., species), site-condition variables (e.g., land use) and cues to care (e.g., mulch). Results In both cities and for most tree stress metrics, the degree of tree stress was associated with species group and either site type and/or land use. Cues to care were not clearly associated with reduced stress in either city. Defoliation was better explained by the models than the other metrics of tree stress. Discoloration, defoliation, and dieback provided unique information on tree stress and therefore can be useful indices for tree health monitoring. Conclusions Consistent with arborist practices, species selection plays a large role in informing the degree of tree stress. Because the benefits of tree care were unclear, future work focusing on the context dependence of tree care effectiveness could clarify the conditions under which tree care (especially mulch) is most effective.