Arboriculture & Urban Forestry 47(5): September 2021 (Dirr 1998). Accordingly, Zones 3 and 4 were aggre- gated, and this is reflected in the weighting of data. Between the 75 inventories in this statewide assess- ment, 3 inventories are located in Zones 3 and 4, 25 inventories are located in Zone 5, 36 inventories are located in Zone 6, and 11 inventories are located in Zone 7 (Figure 2). In assessing the 32 municipalities with multiple inventories, data were not stratified and weighted with auxiliary information to correct for selection bias. Nevertheless, the distribution of these municipalities with respect to the 2012 Plant Hardiness Zones is worth noting. Between the 32 municipalities with newer and older inventories, 2 sets of inventories are located in Zones 3 and 4, 10 sets of inventories are located in Zone 5, 14 sets of inventories are located in Zone 6, and 6 sets of inventories are located in Zone 7 (Figure 2). Relative abundance percentages of street tree spe- cies and genera in relation to the street tree population as a whole (i.e., species and genus composition) were calculated for each municipal tree inventory. Such percentages are commonly utilized as a metric for diversity. Santamour (1990) proposed that no tree species should exceed 10%, no tree genus should exceed 20%, and no tree family should exceed 30% of a municipal tree population in order to guard against large-scale devastation by a pest or disease. Santamour’s 10-20-30 rule is not without critics, par- ticularly since it offers less protection against a polyphagous pest, such as the ALB, that attacks a host range wider than a single tree species (Watson 2017), but it has achieved wide acceptance by urban forest managers for providing benchmarks for diver- sity (Kendal et al. 2014). For this statewide assess- ment, relative abundance percentages for each municipality were allocated to the municipality’s 2012 Plant Hardiness Zone. Group means for tree species and genera percentages were calculated for each zone. These means were then weighted by the measures derived from United States Census Bureau TIGER- Line All Roads files to create weighted statewide per- centages according to the formula: where m1, m2, m3, and m4 denote the group means (i.e., means for species and genus relative abundance 201 percentages in the 2012 USDA Plant Hardiness Zones) and w1, w2, w3, and w4 denote the different weights for each group (i.e., the relative percentages of statewide street length contained within those zones). For the 75 inventories in this statewide assess- ment, comparisons were then made between these weighted statewide relative abundance percentages and those reported in the 2014 and 2017 statewide assessments. Comparisons were also made for the 32 municipalities with multiple inventories to ascertain any trends in relative abundance percentages for prevalent street tree species and genera. As stated above, data for this assessment were not stratified and weighted with auxiliary information to correct for selection bias. In addition to relative abundance percentages, diversity index statistics were calculated for each municipal street tree population. These indices, which are frequently used to make comparisons between biological populations, typically consider more than relative abundance and include additional factors, such as population size and the number of species and genera in that population, in their calculation. Simp- son’s Diversity Index (SDI)(Simpson 1949) and the Shannon-Wiener Diversity Index (Shannon 1948) are two diversity indices often utilized in urban forest research. The SDI is sometimes preferred because it is more sensitive to population evenness (i.e., how evenly the members of a population are distributed between all the species and genera in that population) and gives less weight to rare species and genera; the Shannon-Wiener Diversity Index is more sensitive to species and genera richness (i.e., the number of spe- cies and genera in a population) and to sample size (Colwell 2009). Because the SDI measures domi- nance (i.e., the greater the SDI statistic, the greater the dominance level), the Inverse SDI (1/SDI) is some- times preferred to the SDI as a measure of diversity (i.e., the greater the Inverse SDI statistic, the greater the diversity level)(Sun 1992). Additionally, because the Shannon-Wiener Diversity Index is logarithmic, effec- tive diversity, which is the exponential of the Shannon- Wiener statistic, or eH where H is the Shannon-Wiener statistic, has also been utilized in urban forest research, since it produces statistics that are not logarithmic and are therefore more directly comparable (Jost 2006). Statistics for the Inverse SDI, the Shannon-Wiener Diversity Index, and effective diversity were calcu- lated as measures of tree species and genera diversity. ©2021 International Society of Arboriculture
September 2021
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