Arboriculture & Urban Forestry 47(1): January 2021 This study analyzes the structure of Ithaca’s street- and park-tree populations at multiple points in time based on the 7 tree inventories conducted between 1902 and 2019. Variables analyzed are conditioned by dif- ferences between the data sets and, in particular, between the inventories conducted prior to 1996 and those conducted in 1996 and thereafter. For example, plant- ing space data were not collected for the 1902, 1928– 1947, and 1987 inventories, and therefore stocking levels (i.e., the number of existing street trees divided by the number of available planting spaces) cannot be ascertained for those dates. Additional limitations in the data impacting analysis are described below. The relative abundance percentages of species and genera in relation to the population as a whole (i.e., species and genus composition) were calculated for each inventory where feasible. For example, percent- ages for street and park trees at both the species and genus level were calculated for the 2019 inventory, but street tree genera percentages only were calcu- lated for the 1928–1947 inventory, since species-level data and park-tree data were generally not collected for that inventory. Relative abundance percentages not only speak to the prevalence or scarcity of an individual tree species and genus, but they are also commonly used to evaluate the susceptibility of an urban tree population to pests and disease. For exam- ple, after street tree populations containing large numbers of American elm (Ulmus americana) were decimated by Dutch elm disease (DED, Ophiostoma spp.) beginning in the 1930s, Santamour (1990) hypothesized that the resilience of a tree population to pests and disease would be enhanced if no tree spe- cies exceeded 10%, no tree genus exceeded 20%, and no tree family exceeded 30% of a population. Santa- mour’s 10-20-30 rule has achieved wide acceptance, in part because it is easy to comprehend and calcu- late. However, it has also been criticized for many reasons, including the absence of evidence to validate its thresholds (Kendal et al. 2014), the threat posed by a polyphagous pest, such as the Asian longhorned beetle (ALB, Anoplophora glabripennis), that attacks more than one tree species or genus (Laćan and McBride 2008), and differences in the ability of tree species to cope with stressful urban conditions (Raupp et al. 2006). Notwithstanding these criti- cisms, Santamour’s 10-20-30 rule has been judged a reasoned approach to urban forest planning (Laćan and McBride 2008) and a useful measure of diversity for urban forest managers (Kendal et al. 2014). 7 In addition to relative abundance frequencies, diver- sity indices are often used to assess the potential resil- ience of an urban tree population because they consider additional factors such as the number of trees in the population and species and genus richness (i.e., the number of species and genera). Diversity statistics were calculated where feasible for each tree inven- tory at species and genus levels for street and park trees (Table 2). Simpson’s Diversity Index (SDI) (Simpson 1949) and the Shannon-Wiener Diversity Index (Shannon 1948) are two diversity indices often used in urban forest research. Simpson is sometimes preferred to Shannon-Wiener because it is more sen- sitive to population evenness (i.e., how evenly the members of a population are distributed between all the species and genera in the population) and gives less weight to rare species and genera; Shannon-Wiener is sometimes preferred to Simpson because it is more sensitive to species and genera richness and to sample size (Colwell 2009). A species diversity t-test (Hutcheson 1970) was utilized to assess the statistical signifi- cance of change ( p < 0.05) for Shannon-Wiener Index values between the inventories. Population evenness (Buzas and Gibson 1969) and statistics for the inverse of Simpson’s Diversity Index (1/SDI) were also cal- culated. Simpson’s Diversity Index measures domi- nance, meaning that the greater the SDI statistic, the greater the dominance level; with the Inverse SDI, the greater the Inverse SDI, the greater the diversity level (Sun 1992; Sreetheran et al. 2011). Finally, sta- tistics for effective diversity were calculated from the Shannon-Wiener Index statistics (Jost 2006). Because the Shannon-Wiener Index is logarithmic, the expo- nential of the Shannon-Wiener Index statistic, or eH where H is the Shannon-Wiener statistic, produces diversity statistics which are not logarithmic and are therefore more directly comparable. Diversity and evenness statistics and the diversity t-test were calcu- lated with PAST Paleontological Statistics software Version 4.2 (Hammer et al. 2001). The size structure of street and park trees was assessed by DBH, which is trunk diameter measured at breast height (1.37 m or 4.5 ft). The population dynam- ics of urban forests differ from those of non-urban forests (Halpin and Lorimer 2017). Differences are particularly acute for urban trees intentionally planted in park and streetscape settings (Roman et al. 2014; Smith et al. 2019). Additionally, DBH is an imperfect surrogate indicator for tree age, since growth rates vary both within and between tree species, and ©2021 International Society of Arboriculture
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