8 Cowett et al: Tracking Changes to Urban Trees over 100 Years in Ithaca, NY, USA smaller growing trees such as Kwanzan cherry (Prunus serrulata) will never attain the size of larger growing trees such as a northern red oak (Quercus rubra)(Welch 1994). Nevertheless, urban forestry has borrowed from non-urban forestry the model in which a descending size class distribution from smaller to larger DBH size classes suggests a sustainable tree population in which there are sufficient young trees to compensate for tree mortality; conversely, a flat shaped distribution or a distribution with a hump in the mid- sized DBH classes suggests an aging and unsustain- able tree population (Richards 1979; McPherson and Rowntree 1989). For those inventories where DBH data were collected, street and park trees were aggre- gated into 8 DBH classes: 0 to 15.2 cm (0 to 6 in), 15.2 to 30.5 cm (6 to 12 in), 30.5 to 45.7 cm (12 to 18 in), 45.7 to 61.0 cm (18 to 24 in), 61.0 to 76.2 cm (24 to 30 in), 76.2 to 91.4 cm (30 to 36 in), 91.4 to 106.7 cm (36 to 42 in), and > 106.7 cm (> 42 in). Data for DBH were not collected in the 1902 inventory. The 1987 inventory contains DBH data aggregated at a finer scale than the 8 size classes above; these data were reworked to conflate with the 8 size classes. Relative size class distributions of prevalent tree species and genera were also generated, except for the 1928–1947 inventory, where distributions for street tree genera only could be generated. i-Tree (www.itreetools.org) is a suite of freely available software tools developed by the United States Forest Service to assess the benefits and values derived from trees and forests (Nowak et al. 2018). i-Tree Eco (2019) can be used to estimate the monetary benefits (i.e., annual US dollar values) provided by urban trees. It requires at a minimum species and DBH data for each tree. These data also enable the computation of importance values (i.e., the sum of the percentage of relative abundance and the percentage of leaf area) associated with a tree species in an urban tree popula- tion. Since larger-growing tree species can be expected to have more leaf surface area than smaller-growing tree species, and many benefits provided by urban trees correlate with leaf surface area, larger-growing tree species are typically associated with greater mon- etary benefits than smaller-growing tree species and with greater importance values (McPherson et al. 2007). For example, Sydnor and Subburayalu (2011) found in Brooklyn, Ohio that a larger-growing tree species (honeylocust, Gleditsia triacanthos) provided 7.5 times the estimated benefits that a smaller-growing ©2021 International Society of Arboriculture tree species (Lavalle hawthorn, Crataegus × lavallei) provided. i-Tree Eco version 6 was utilized to esti- mate in 2020 US dollars (i.e., as calculated in 2020, unadjusted for inflation) the gross monetary benefits provided by and the importance values associated with Ithaca street trees for the 1928–1947, 1996, 2006, 2013, and 2019 inventories and with Ithaca park trees for the 1996, 2006, 2013, and 2019 inven- tories. For the 1928–1947 inventory, in which data were collected mostly at the genus level, species was substituted for genus (e.g., Acer species for Acer genus). Monetary benefits and importance values could not be estimated for the 1902 and 1987 inven- tories because species, genus, and DBH data were not available for individual trees. Finally, to better understand the current status of and future prospects for city trees and the benefits they provide, the 2019 inventory was assessed in greater depth. Street tree stocking level and density statistics were calculated and then compared to statis- tics for New York State (Cowett and Bassuk 2014), California (McPherson et al. 2016), and the United States (Hauer and Peterson 2016). Street trees and park trees in the 0 to 15.2 cm (0 to 6 in) DBH class were analyzed for diversity and species and genus composition and then compared to the diversity and species and genus composition of all street and park trees. Finally, condition and maintenance ratings were associated with the species composition of all street and park trees. RESULTS Species and Genus Composition Street Trees Sugar maple (Acer saccharum) was the most preva- lent street tree species in the 1902 inventory (37.45%), but comprised just 4.80% of all street trees in the 2019 inventory (Table 3). American elm (Ulmus americana) was the second most prevalent street tree species in the 1902 inventory (28.91%), and elm (Ulmus spp.) the second most prevalent street tree genus in the 1928–1947 inventory (30.88%). However, following the onset of Dutch elm disease in the 1930s, only 5 elms, or 0.09% of all street trees, were found in the 1987 inventory; those numbers subsequently increased with the planting of DED-resistant elm spe- cies and cultivars, and 311 elms, or 3.80% of all street trees, were found in the 2019 inventory. Norway maple
January 2021
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