Arboriculture & Urban Forestry 45(5): September 2019 Arboriculture & Urban Forestry 2019. 45(5):167–200 URBAN FORESTRY ARBORICULTURE Scientific Journal of the International Society of Arboriculture & Urban Tree Mortality: A Literature Review By Deborah R. Hilbert, Lara A. Roman, Andrew K. Koeser, Jess Vogt, and Natalie S. van Doorn Abstract. Tree survival is a performance metric for urban forestry initiatives, and an understanding of the factors that influence mortality can help managers target resources and enhance survival. Furthermore, urban tree planting investments depend on tree survival to maximize eco- system services. In this literature review, we categorized factors commonly associated with urban tree mortality and summarized mortality rates published in 56 studies, focusing on studies of trees along streets, in yards, and in landscaped parks. Study designs included quantitative field monitoring of uneven-aged tree populations and tracking planting cohorts of even-aged trees, as well as qualitative analyses. Annual mortality rates ranged from 0.6 to 68.5% for cohort studies and 0 to 30% for repeated inventories of uneven-aged trees. The 1st, 2nd, and 3rd quartiles of annual mortality were 2.8 to 3.8%, 4.4 to 6.5%, and 7.1 to 9.3% for planting cohorts, and 1.6%, 2.3 to 2.6%, and 3.0 to 3.3% for repeated inven- tories of uneven-aged trees (ranges reflect studies that reported a range for the time period or mortality rate). For cohort studies, annual mortal- ity tended to be highest during the first five years after planting. The most commonly cited biophysical factors associated with mortality were taxa (15 articles), tree size/age (13 articles), and site characteristics (12 articles). The most commonly cited human-related factors were stew- ardship, maintenance, and vandalism (15 articles). More long-term studies are needed to investigate how site characteristics influence mortal- ity, including rarely examined soil and microclimate characteristics. Future research should also examine institutional structures related to mortality outcomes, as well as parcel-level sociodemographic factors and resident behaviors. Key Words. Ecological Monitoring; Street Tree; Tree Death; Tree Demography; Tree Population; Tree Survival; Urban Park; Yard Tree. INTRODUCTION In urban forestry, substantial resources are invested in the planting and maintenance of trees. For instance, 81% of municipalities in the United States allocate public dollars to tree planting and care, which amounts to an estimated $37.50 annually per public tree (street trees, park trees, and trees in other public places; Hauer and Peterson 2016). In total, 45% of municipal tree budgets are spent on planting and care-related activities, and another 23% is spent on removal (Hauer and Peterson 2016). These efforts aim to enhance the functional lifespan of trees, maximizing the many environmental, economic, and societal benefits pro- vided by urban woody vegetation (Pataki et al. 2011; Roy et al. 2012). The success of these planting and maintenance efforts can be measured by tree survival (Roman et al. 2013; Roman et al. 2016), as survival is essential to achieve the intended ecosystem services associated with tree maturity (Ko et al. 2015b; Widney et al. 2016). Continuing research on factors that increase or decrease survival provides insights into the basic demographic processes of urban forest population dynamics. Indeed, the population dynam- ics and growing conditions of planted trees in land- scaped and heavily built-up urban areas are quite dissimilar from trees in natural forests (i.e., trees in rural, wildland settings) (Urban 2008; Roman et al. 2016). Translating urban tree mortality research into practice can ultimately strengthen management of individual trees and planting projects as well as the urban forest system as a whole. Research on tree mortality from natural forests suggests that trees often die as a result of many differ- ent additive and interacting factors. As trees age, the impact of chronic and acute stressors accumulates, which ultimately leads to tree death (Franklin et al. 1987). This interpretation of the mortality process, supported by patterns in long-term growth (Das et al. 2007), was described by Manion (1981) as the “dis- ease decline spiral” and later modified by Franklin et al. (1987) as the “mortality spiral.” In Manion’s (1981) classic book, “urban environment” was listed as a predisposing factor, yet the “urban environment” encompasses a wide range of biophysical and ©2019 International Society of Arboriculture 167
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