Arboriculture & Urban Forestry 45(5): September 2019 uneven-aged trees, annual mortality was calculated using provided periodic survival rate pt (i.e., the pro- portion surviving over the time interval t) and the equation (Roman et al. 2016). For both cohort studies and repeated inventory studies of uneven-aged trees, (Roman et al. 2016) was used. We assumed a con- stant rate of mortality when calculating annual mor- tality rates over a specified time period from a given study (Roman and Scatena 2011). We also summa- rized how the studies we reviewed defined and calcu- lated mortality. To summarize the typical mortality rates provided in the literature, we calculated the quartiles, since these summary statistics are less influenced by outli- ers than the mean. We did not use weighted quartiles by sample size because we did not want to attribute more weight to studies of larger tree populations based solely on that metric. If studies gave a range of mortality rates or study period lengths (i.e., time since last inventory for repeated inventories of uneven- aged trees, or time since planting for multi-year plant- ing cohort studies), the minimum and maximum values were used in calculations, and we reported the corresponding lower and higher quartile values. For such studies that reported ranges, lower values repre- sent the lowest possible interpretation of annual mor- tality, and higher values represent the highest possible interpretation of annual mortality. We did not use the mid-point because we could not be certain that the mid-point was representative of the underlying range of data. If a study provided mortality rates for one or more sub-groupings, we retained these sub-group values to use in summary tables and quartile calcula- tions if (1) this was the only information reported; or (2) the sub-groupings were based on time since plant- ing or tree size. For planting cohort studies, we used the quartile summary statistics for establishment and post-establishment mortality rates to create survivor- ship curves for fairly typical annual mortality (50th percentile), worse-than-normal (75th percentile), and better-than-normal (25th percentile). For two plant- ing cohort studies that provided annual mortality information for different age classes (Lu et al. 2010; Roman et al. 2016), we created mortality curves by 171 graphing annual mortality against time since plant- ing. Likewise, for repeat inventories of uneven-aged trees, when annual mortality information was pro- vided for varying size classes (Nowak 1986; Nowak et al. 2004; Roman et al. 2014a), we created mortality curves by graphing annual mortality rates against size class (using diameter at breast height, dbh). Factors associated with mortality outcomes from quantitative and qualitative studies were categorized as either human-related or biophysical and into sub-categories within these two major categories, recognizing the potential for interactive and coupled effects and the multi-scalar patterns in which they operate. We then grouped these factors as predispos- ing, inciting, or contributing, following the dis- ease-decline model from Manion (1981). Predisposing factors represent the human and biophysical context at the time of planting. These conditions can then cre- ate vulnerabilities to inciting factors, which are short- term stressors that impact tree vigor. The inciting factors, in turn, create vulnerabilities to contributing factors, which are the direct mechanisms leading to tree mortality. In addition to short summary tables in which we presented mortality rate quartiles and factors associ- ated with mortality, we created three comprehensive tables: one outlining mortality rates in cohort studies, one outlining mortality rates in repeat inventory stud- ies, and a final table summarizing studies that pro- vided statistical analysis of factors associated with mortality. In order for a study to be included in the quartile calculations for annual mortality rates, it had to report annual mortality rates (or sufficient informa- tion to calculate annual rates) and a time interval. Studies that did not provide mortality information, time intervals, or whose methodologies were vastly different than the majority of papers, were excluded from these tables but were still considered in other summary results and discussion of the literature reviewed. RESULTS AND DISCUSSION Characteristics of the Literature Fifty-six studies were analyzed. Fifty-two of the stud- ies were published in peer-reviewed journals, three were internal reports or extension articles, and one was a master’s thesis. Eighteen studies were pub- lished in the Journal of Arboriculture/Arboriculture ©2019 International Society of Arboriculture
September 2019
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