Arboriculture & Urban Forestry 45(5): September 2019 suggestions such as data censoring (Roman et al. 2014b; Roman et al. 2016; van Doorn and McPher- son 2018). More research needs to be conducted to determine whether constant rates of mortality are realistic and applicable to various urban tree data. For planting cohort mortality studies, we did break-down the annual mortality data into establishment (under five years post-planting) and post-establishment (over five years), however, this is a somewhat arbitrary cut- off. Further research could indicate when the key inflection points are for urban tree survivorship and mortality curves towards better explaining the estab- lishment phase in terms of reduction in annual sur- vival rates (Roman et al. 2014a; Sherman et al. 2016). Fifth, the studies we reviewed did not examine biophysical factors like soil characteristics and pests, which have been shown to influence tree mortality. Soil characteristics like structure, bulk density, and organic matter content have been shown to influence tree growth and health (Day and Bassuk, 1994; Xiao and McPherson 2011; Grabosky and Bassuk 2016; Scharenbroch et al. 2017). However, most of the urban tree studies that account for these soil differ- ences are experimental plantings, which we did not include in our review. There is also an abundance of urban forestry literature on pests and diseases (e.g., Cannon and Worley 1979; Aukema et al. 2011; Van- natta et al. 2012), but such studies do not generally address rates of mortality or predictive factors, there- fore they were not included in our analysis. Finally, and related to the last point, our results and conclusions are limited to trees planted and managed in situ, i.e., in actual urban areas and real-world con- ditions, as we intentionally excluded experimental planting studies. Though we did find a number in our searches (e.g., Insley 1980; Buckstrup and Bassuk 2000; Gilman 2004; Gerhold 2007; Gerhold 2008; Grabosky and Bassuk 2008; Etemadi et al. 2013; Oldfield et al. 2015; Grabosky and Bassuk 2016), we chose to leave out controlled experimental plantings in order to stay focused on straightforward compari- son of mortality rates and factors in real-world condi- tions. Nonetheless, experimental planting trials can pinpoint both biophysical (e.g., species, cultivar, nursery stock, soil characteristics) and human (e.g., stewardship or maintenance regimes, neighborhood sociodemographic characteristics) factors that obser- vational studies may miss due to confounding vari- ables. For example, studies by Gerhold (1994; 2008) document the performance of different species and 185 cultivars in urban settings. More recently, McPherson et al. (2018) outlined a method for selecting and eval- uating the performance of “climate ready trees” in California. This experimental study and others (e.g., Roloff et al. 2009) provide critical mortality informa- tion about new and underutilized urban species within the context of a changing climate. A separate review of mortality rates and factors in experimental plantings could be conducted to illuminate gaps in the literature where future experimental and in situ studies could complement each other. IMPLICATIONS FOR RESEARCH AND PRACTICE Despite the limitations described above, this review has some clear implications for research and practice. First and most importantly, for both research and practice, researchers, arborists, and urban forest prac- titioners should explicitly define mortality, survival, and the procedures used to measure and calculate each. Second, not only should definitions be clear, procedures should be standardized. The standardiza- tion issue has been recently discussed in a primer on urban tree mortality by Roman et al. (2016), an essay on the importance of standardizing at-planting data by Vogt et al. (2015b), and a report on software and data standards for urban tree monitoring by Boyer et al. (2016). Third, methods for calculating and analyz- ing empirical survival and mortality data from fields like ecology (Woodall et al. 2005; Das et al. 2007; Siccama et al. 2007; van Doorn et al. 2011; Fahey et al. 2013; Cleavitt et al. 2014; Levine et al. 2016) should be applied to urban forest mortality studies to gain a better understanding of population dynamics. Such demographic analytical techniques—like age- based life tables, survivorship and mortality curves, and lifespan metrics—can be applied to urban tree mortality data (Roman et al. 2016). Fourth, urban forestry programs can benefit greatly by conducting well-designed, long-term monitoring programs that address specific research questions (Lindenmayer and Likens 2010). Variables collected should relate directly to those questions to avoid being “snowed by a blizzard of ecological details” resulting from a “laundry list” of items being moni- tored (Lindenmayer and Likens 2010). Finally, a word of caution is warranted about use of the mortality rate ranges presented in this paper. It would be desirable for both researchers and practi- tioners if the mortality rates summarized in this ©2019 International Society of Arboriculture
September 2019
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