Arboriculture & Urban Forestry 45(5): September 2019 either based on thorough evaluation of the tree or per- ceived problems (Koeser et al. 2015; Conway 2016; Koeser and Smiley 2017; Klein et al. 2019). Alterna- tively, removals can be entirely unrelated to health and risk, such as trees removed during construction activities or due to aesthetic preferences (Kirkpatrick et al. 2013; Steenberg et al. 2017). We therefore pro- pose three types of mortality outcomes for urban trees: death-in-place, preemptive removal for tree health and safety reasons, and removal unrelated to health or safety (Figure 6). Biophysical contributing factors, such as hurricanes, lethal pests, and accumulating site stressors, can result in death-in-place or preemptive removals. Human contributing factors can result in any of the three mortality outcome types. For exam- ple, mature Fraxinus spp. street trees threatened by EAB experience death-in-place if they are not treated with insecticide, and some municipalities are remov- ing untreated Fraxinus spp. trees preemptively before the disease hits when they have chosen not to treat (Hauer and Peterson 2017). In another example, site stressors and lack of maintenance can lead to death- in-place for recently planted trees (Roman et al. 2014b; Koeser et al. 2014; Vogt et al. 2015a), but if those mechanisms stress a tree without killing it, pre- emptive removals could occur because the tree was deemed unhealthy or undesirable. Humans may also make tree removal decisions independent of any health or safety considerations, such as removals due to construction, renovation, and aesthetic preferences (Kirkpatrick et al. 2013; Conway 2016; Steenberg et al. 2017). Notably, while we emphasized earlier that 183 urban tree mortality studies have typically defined mortality as a combination of trees observed standing dead and removed, the studies we reviewed here were generally not able to differentiate between the three mortality outcome types. During monitoring field work, it is not usually feasible to be certain whether a removed tree was healthy, unhealthy, or dead at the time of removal. Surveys of residents have yielded important information in this regard (Kirkpatrick et al. 2013; Conway 2016), and surveys of municipal arborists might likewise provide insights into the health status of trees at the time of removal. While this framework builds directly from our review, in that most of the factors listed were statisti- cally significant (bolded in Figure 6) or qualitatively important (italicized in Figure 6) in the studies we reviewed (Table 4; see also Appendix Table 3), there are a few factors listed which were not prominent in our review. For instance, native biome, and the asso- ciated precipitation and temperature patterns, is a log- ical predisposing factor that could relate to species suitability and therefore likelihood of stress and later mortality. This issue was lightly touched upon in a yard tree mortality study in Sacramento (Roman et al. 2014b), where lack of irrigation combined with the seasonal drought in a Mediterranean climate, and species drought tolerance, appeared related to young tree losses, but most studies did not explicitly link biome to mortality since trees within each study were most typically within a single biome. The papers we reviewed also did not raise the issue of landscaping norms and behaviors (an inciting factor in Figure 6), Figure 7. Tree mortality spirals depicting an example tree in a natural forest (adapted from Franklin et al. 1987) and an example planted urban tree. ©2019 International Society of Arboriculture
September 2019
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