186 review could be used to model urban tree population change over time, for example, in order to project the benefits resulting from planting trees in a particular location (as done in Widney et al. 2016), or predict and plan for urban forest management needs and costs in the future (as suggested by Vogt et al. 2015b; Vogt et al. 2015c). In addition, the survivorship curves (Figure 5) and life tables (Appendix Table 4) provide an example of how population demography approaches can be applied to urban forestry settings in order to maximize management strategies. However, defining and even standardizing calculation methods for mor- tality rates is insufficient for creating accurate tree population models or scenario-building tools. Instead, we need much more data on urban tree mortality rates in various circumstances—particularly for the under-studied human and biophysical facets of the urban environment: different land use types, socio- economic factors, municipal management strategies, stewardship regimes, types of planting locations, geographies and climates, and pests and diseases (Figure 6). More long-term mortality studies should be con- ducted to test for the significance of factors such as soil and microclimate, community and institutional structures (e.g., tree stewardship programs), sociode- mographic characteristics, and resident behaviors. Such future studies should focus on both existing and newly planted trees in many different planting sites and include regular monitoring. Municipal tree removal records like those used by Polanin (1991) and Morgenroth and Armstrong (2012) are an underused yet potentially helpful data source for mor- tality studies. Mixed methods research incorporating both rigorous statistical analysis of predictive factors (both biophysical and human) along with qualitative assessments of communities and institutions would provide a holistic understanding of mortality pro- cesses in the anthropogenically constructed urban forest. Further, studies should be conducted to inves- tigate the survival and mortality of large, old trees, perhaps exploring relationships between site and soil characteristics, as well as the role that removal prac- tices and tree protection policies play in creating a population of large, unusually healthy or protected trees. Eventually, with enough data, models could be built to aid in the management of forests in many dif- ferent cities and scenarios. Given the pace of urban tree mortality research over the past decade, the next ©2019 International Society of Arboriculture Hilbert et al: Urban Tree Mortality: A Literature Review decade will likely be very promising to further advance our understanding of the urban tree mortality process and capacity to build empirically-grounded population projection models. CONCLUSION This review yielded a handful of important take-away points: • Urban tree mortality studies span a range of quantitative and qualitative study designs, with a dramatic increase in the number of published studies over the past ten years. • For planting cohort studies, annual mortality tended to be higher during the first five years after planting, aligning with the establishment phase concept. • Based on mortality rates reported in planting cohort studies, the population half-life for planted urban trees (i.e., when survivorship is 50%) is around 7 to 11 years, 13 to 18 years, and 33 to 38 years for worse-than-normal, middle- of-the-road, and better-than-normal survivor- ship scenarios. • The 1st, 2nd, and 3rd quartiles of annual mortal- ity for repeated inventories of uneven-aged trees were more similar to cohort study annual mor- tality rates of the post-establishment phase (i.e., six or more years after planting) than those of the establishment phase (i.e., first five years after planting). • Characterizing the factors that influence mortal- ity into categories according to biophysical ver- sus human influences, and predisposing, inciting, and contributing factors (as outlined in Figure 6) is helpful to understanding the urban tree dis- ease-decline spiral. • Future research could examine topics that are understudied in the current literature, such as micro- climate, soil characteristics, institutional structures related to stewardship regimes, parcel-level sociodemographic factors, and resident behaviors. LITERATURE CITED Aukema, J.E., B. Leung, K. Kovacs, C. Chivers, K.O. Britton, J. Englin, S.J. Frankel, R.G. Haight, T.P. Holmes, A.M. Lieb- hold, D.G. McCullough, and B. Von Holle. 2011. Economic impacts of non-native forest insects in the continental United States. PLoS One 6, e24587. doi:10.1371/journal.pone.0024587.
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