Arboriculture & Urban Forestry 47(1): January 2021 multiple stressors and disturbances (Richards 1983; Steenberg et al. 2017). For example, with respect to pest vulnerability, for pests such as the emerald ash borer (EAB)(Agrilus planipennis) which are primar- ily host specific, greater tree diversity can typically be expected to be associated with reduced tree loss (Jac- tel and Brockerhoff 2007). However, for non-host specific, polyphagous pests, such as the Asian long- horned beetle, a less diverse tree population com- prised of tree species resistant to such pests could potentially be more sustainable than a more diverse population comprised at least in part of vulnerable tree species (Berland and Hopton 2016). Addition- ally, since differences in tree growth rates and size translate into significant differences in the benefits provided by trees (McPherson and Peper 2012), increasing diversity by replacing larger growing tree species, such as Norway maple, with smaller growing tree species, such as crabapple, rather than with other larger growing tree species, can reduce the structural potential of the benefits provided by municipal trees (Sydnor and Subburayalu 2011). Finally, whereas native tree species may support a greater abundance and density of birds than non-native trees (Shackleton 2016) and contribute to ecological integrity (Ordóñez and Duinker 2012), and non-native tree species which are non-invasive and resistant to pests, diseases, and droughts may be better suited to harsh growing urban conditions (Riley et al. 2018), an emphasis on native tree plantings may negatively impact the sustainabil- ity of urban tree populations, particularly in regions with extreme environmental conditions (Sjöman et al. 2016). Therefore, although diversity is rightfully seen as an important contributor to sustainable municipal tree management, such management is more complex than simply increasing the number of tree species and genera in a municipal tree population. It requires accurately estimating the new plantings needed to account for tree mortality and removals (Roman et al. 2014). The large number of new plantings in Ithaca since 1987 has been sufficient to substantially alter the DBH size class distribution of its municipal trees and has likely placed them on a more sustainable long-term footing. It requires regular periodic moni- toring to provide the information necessary for effec- tive management (Nowak 2017). Ithaca has conducted 7 inventories of its street trees since 1902 and 4 inven- tories of its park trees since 1996, and this paper has been able to make findings from those inventories, 19 despite substantial differences between them (Table 1); importantly, the 2006, 2013, and 2019 inventories, by utilizing the same data fields and metrics and sharing the same unique identifier for individual trees, have established a standardized footing for future longitu- dinal comparisons and analysis, both on a population level and for individual trees, that will contribute to effective management, particularly if inventories continue to be conducted on a regular, periodic basis (Roman et al. 2013). It requires coordinated strategies of action linking actors at many geographic scales, including at the neighborhood level (Mincey et al. 2013). A volunteer citizen pruner program has assisted city crews in maintaining municipal trees for many years and pruned 446 trees in 2019; Ithaca’s GIS and Forestry departments have collaborated to create an interactive Tree Tour Map for city residents accessi- ble via the Internet; and Ithaca tree managers have partnered with nurseries to secure tree stock that is more diverse and/or available as bare root plantings (Denig 2014). Finally, it requires funding sufficient for tree planting and maintenance, for developing a long-term management plan, and for hiring and train- ing the staff required to implement the plan by plant- ing and maintaining trees (Clark et al. 1997; Kenney et al. 2011). Municipal funding proposed by Ithaca’s mayor and approved by its Common Council, cou- pled with grants from New York State’s Urban and Community Forestry program, allowed the Forestry Section within the Public Works Department to spend $26.21 per capita and $70.11 per tree to manage the city’s street and park trees in 2019. By comparison, in 2018, for municipalities in the Tree City USA pro- gram, the average spent per capita was $7.77 in New York State, $10.66 in New Jersey, and $5.64 in Penn- sylvania, with a national average of $8.40 (Arbor Day Foundation 2019). Additionally, Hauer and Peterson (2016) reported that, in 2014, a municipality of Ithaca’s population size (i.e., 25,000 to 49,999) spent on aver- age $9.75 per capita and $37.35 per tree to manage municipal trees. CONCLUSION Analysis of Ithaca’s 7 tree inventories revealed sub- stantial change in the population structure of Ithaca’s municipally managed urban trees. This change has taken time because of the legacy effect of past prac- tices and preferences, but progress has been made, especially in reducing the dominance of prevalent tree species and genera and increasing diversity, that ©2021 International Society of Arboriculture
January 2021
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