10 Cowett et al: Tracking Changes to Urban Trees over 100 Years in Ithaca, NY, USA most prevalent street tree genera exceeded his 20% rule for genus. Whereas the 5 most prevalent street tree species comprised 88.76% of all street trees in 1902 and 75.47% of all street trees in 1987, they com- prised 35.22% of all street trees in 2019, which equates to a 60.32% reduction from 1902 and a 53.33% reduction from 1987 (Table 4). Similarly, whereas the 5 most prevalent street tree genera com- prised 94.84% of all street trees in 1902 and 84.67% of all street trees in 1987, they comprised 55.93% of all street trees in 2019, which equates to a 41.03% reduction from 1902 and a 33.94% reduction from 1987 (Table 4). Park Trees Crabapple (Malus spp.) was the most prevalent park tree species (12.21% of all park trees) in the 1996 inventory, the first inventory in which data were col- lected for park trees, and has continued to be the most prevalent park tree species in subsequent inventories, comprising 8.22% of all park trees in 2019 (Table 3). Maple (Acer spp.) was the most prevalent park tree genus in the 1996 inventory, comprising 14.17% of all park trees, and has remained the most prevalent park tree genus in subsequent inventories, compris- ing 14.18% of all park trees in 2019 (Table 3). The percentage of crabapple relative to all park tree spe- cies in the 1996 and 2006 inventories exceeded San- tamour’s 10% rule for species, but did not exceed the 10% rule in the 2013 or 2019 inventories. The per- centage of maple relative to all park tree genera did not exceed Santamour’s 20% rule for genus in the 1996 inventory nor in any subsequent inventory. Whereas the 5 most prevalent park tree species comprised 33.79% of all park trees in 1996, they comprised 27.40% of all park trees in 2019, which equates to an 18.91% reduction from 1996 (Table 4). Similarly, whereas the 5 most prevalent park tree genera com- prised 52.20% of all park trees in 1996, they com- prised 43.43% of all park trees in 2019, which equates to a 16.81% reduction from 1996 (Table 4). Diversity Indices Street Trees Statistics were generated at species and genus levels for Simpson’s Diversity Index, the inverse of the Simp- son’s Diversity Index (Inverse SDI), the Shannon- Wiener Diversity Index, distribution evenness, and Jost’s effective diversity (Table 5). The 1902 inventory was comprised of 5,621 street trees, 48 species, and 29 genera; the 2019 inventory was comprised of 8,194 street trees, 185 species, and 65 genera. Between the 1902 and 2019 inventories, the Inverse SDI increased from 4.09 to 27.15 for species and from 3.27 to 9.40 for genus; the Shannon-Wiener Diversity Index increased from 1.83 to 4.04 for species and from 1.51 to 2.97 for genus; distribution evenness increased from 0.13 to 0.31 for species and from 0.16 to 0.30 for genus; and effective diversity increased from 6.26 to 56.88 for species and from 4.54 to 19.57 for genus. A diver- sity index t-test (Hutcheson 1970) found statistically significant differences ( p < 0.05) in the Shannon-Wiener Diversity Index between an inventory and the succes- sive inventory for street tree species for all invento- ries and for street tree genera for the 1987 through 2019 inventories (Table 6). For all inventories con- ducted between 1902 and 2019, with the exception of the 1928–1947 inventory, for which no species-level data are available, species diversity was found to be Table 4. Cumulative frequency of the 5 most prevalent tree species and genera from 1902 to 2019. Street trees 1902 Sum top 5 species ∆ from prior inventory Sum top 5 genera ∆ from prior inventory Park trees Sum top 5 species ∆ from prior inventory Sum top 5 genera ∆ from prior inventory 88.76% NA 94.84% NA 1902 NA NA NA NA 1928−1947 NA NA 91.61% ˗3.41% 1928−1947 NA NA NA NA 1987 75.47% ˗14.97% 84.67% ˗7.57% 1987 NA NA NA NA 1996 47.44% ˗37.14% 65.85% ˗22.23% 1996 33.79% NA 52.20% NA 2006 43.89% ˗7.48% 62.83% ˗4.58% 2006 33.01% ˗2.29% 51.52% ˗1.31% 2013 36.72% ˗16.35% 57.46% ˗8.55% 2013 28.16% ˗14.71% 44.75% ˗13.13% 2019 35.22% ˗4.08% 55.93% ˗2.65% 2019 27.40% ˗2.69% 43.43% ˗2.96% ©2021 International Society of Arboriculture
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