184 but such behaviors have been studied in the residen- tial ecosystems literature (Cook et al. 2012), and pre- sumably relate to tree maintenance and therefore mortality vulnerability. Other factors listed in Figure 6 which did not come up in our review are pests and diseases, which are widely acknowledged to cause major tree losses, both through death-in-place and preemptive removals based on management responses (Hauer 2012; Kovacs et al. 2014; Sadof et al. 2017). Contemporary and historical examples include Anoplophora glabripennis (Asian longhorned beetle, Faccoli and Gatto 2016), A. planipennis (EAB, VanNatta et al. 2012; Sadof et al. 2017), Phytoph- thora ramorum (sudden oak death, Rizzo and Garbe- lotto 2003), and Ophiostoma ulmi (Dutch elm disease, Cannon and Worley 1976; Ganley and Bulman 2016). However, this body of literature was not included in our review, therefore the mortality rates summarized in our review (Table 2) do not include catastrophic losses from pests and diseases. As urban forestry scholars move forward with new lines of tree mortality research, it will be valuable to understand the relative contributions to these various predisposing, inciting, and contributing factors to tree mortality in different regions and programs, and fur- thermore, to disentangle the processes and interactive effects linking factors together. Comparison of Factors Affecting Urban Tree Mortality to the Disease-Decline Model of Tree Death for Non-Urban Trees Overall, the prevalence of studies citing multiple sig- nificant factors for mortality supports the disease- decline model of tree mortality typically applied to non-urban trees (Manion 1981; Franklin 1987; Das et al. 2007). However, the literature we reviewed did not generally tease apart which factors should be con- sidered as pre-disposing, inciting, or contributing (Manion 1981; Franklin 1987), nor did the urban tree mortality literature discuss how causes of or factors associated with tree death and removal vary across age classes (Franklin et al. 1987). One exception is the Helama et al. (2012) publication, a dendrochro- nology study of trees in an urban park lawn which investigated the disease decline theory and the possi- ble role of competition as a predisposing factor and drought as an inciting factor. The second exception is Boyce’s (2010) study of the effect of stewardship on survival for all ages, new trees, and established trees. Additionally, the decline spiral for urban trees may be ©2019 International Society of Arboriculture Hilbert et al: Urban Tree Mortality: A Literature Review cut short by removal of live trees due to perceived or actual risk, construction, or human preference. LIMITATIONS The results of this literature review—and in particular the summarized mortality rates and the factors that influence mortality (Tables 2, 3, 4, Appendix Tables 1, 2, and 3)—are not without limitations. First, our review concentrated on published studies available through online searches. We did not include reports or unpublished data that might be gathered by urban forestry practitioners for internal use, such as repeat street tree inventories conducted for management purposes or monitoring of cohorts to track planted tree survival over time (see Roman et al. [2013] on urban tree monitoring data collection performed by practitioners). Greater insights into mortality trends and processes could be obtained by reaching out directly to urban forest managers for datasets. Second, this literature review has a relatively lim- ited set of geographies and climates. The bulk of our studies were from the United States (Table 1) and from warm temperate climates with hot or warm summers (Cfa, Csb, as categorized by Kottek et al. 2006) or snowy climates (Dfa; see also Figure 2). It is possible that a larger sample size from many different regions or climates could reveal trends in mortality rates and factors not found in this review. Third, we are limited in making conclusions about trees planted on various site types. Street trees were the best represented site type in the studies we reviewed. Residential yard tree planting and distribu- tion programs are fairly new (Nguyen et al. 2017), and monitoring yard trees is logistically complicated and time-consuming, as it requires cooperation from numerous private residents. Urban tree mortality research could benefit from further studies of trees in residential yards, other private properties, and land- scaped parks; or random plot-based studies could bet- ter differentiate between planting site types in addition to land use categories. Fourth, our review was limited because we assumed a constant rate of mortality when calculating mortal- ity and survivorship percentages from cumulative survival rate data presented in cohort studies, which may not be appropriate for all scenarios (Roman et al. 2016). Recent research integrates concepts from demography into urban forest population studies, drawing attention to limitations like this and offering
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