18 Cowett et al: Tracking Changes to Urban Trees over 100 Years in Ithaca, NY, USA Park trees were not inventoried until 1996. Accord- ingly, there is no way of assessing legacy effects dat- ing back to 1902, such as knowing whether, prior to 1996, the park tree population was dominated by a few species and genera and, if so, the extent of domi- nation. However, results from the 1996 inventory reveal that park trees were not dominated by a few species and genera to the same extent as street trees. For example, the 1996 inventory found that crabap- ple, the most prevalent park tree species, comprised 12.21% of all park tree species, whereas Norway maple, the most prevalent street tree species, com- prised 22.54% of all street tree species (Table 3). The 1996 inventory also found that maple, the most prev- alent park tree genus, comprised 14.17% of all park tree genera, whereas maple, the most prevalent street tree genus, comprised 44.76% of all street tree genera (Table 3). Additionally, in 1996, the sums of the 5 most prevalent park tree species and genera were less than the sums of the 5 most prevalent street tree spe- cies and genera (Table 4). Moreover, there were sub- stantial differences in the composition of the park tree population, with eastern hemlock, eastern white pine, and Norway spruce found to be prevalent park tree species, unlike in the street tree population. These findings for Ithaca’s park trees in 1996 par- allel findings made by Welch (1994) in Boston that the composition of Boston’s park tree population was very different than that of Boston’s street tree popula- tion, and that Boston’s park tree population was more diverse than its street tree population. Comparisons with the 2006, 2013, and 2019 inventories provide further insight. From 1996 to 2019, effective diver- sity (eH evenness for Ithaca’s street trees since 1996 has been substantial and has contributed to reducing the gap in diversity with Ithaca’s park trees, but, in 2019, Itha- ca’s park tree population continued to be more diverse than the population of its street trees. )(Jost 2006) increased for street tree species and genera and for park tree species and genera; while the number of street and park tree species and genera increased, the increase in effective diversity was cor- related more with an increase in the evenness of the street and park tree populations than with the increase in the number of species and genera (Table 5). Even- ness is important because even if the number of spe- cies or genera increases, if a species or genus is represented by only a few trees, then the most preva- lent species or genera remain comparatively domi- nant. For example, even though there was a 66.7% increase in the number of street tree genera between the 1928–1947 and 1987 inventories, there was no statistically significant increase in diversity ( p < 0.05) due to the dominance of the most prevalent street tree genera (Tables 5 and 6). The increase in population ©2021 International Society of Arboriculture While greater population evenness has contributed importantly to increased diversity for Ithaca’s street and park trees, the number of species and genera has also increased, and its impact should not be dis- counted. The increase at the species level is espe- cially pronounced with a 35.0% increase in the number of street tree species and a 24.6% increase in the number of park tree species between 1996 and 2019 (Table 5). This increase can be attributed in part to the increase in average minimum winter tempera- tures revealed in the 1990 and 2012 plant hardiness zones (USARS 1990; USDA 2012). Ithaca was reas- signed from Zone 5b (˗26 to ˗23 °C,˗10 to ˗5 °F) in 1990 to Zone 6a (˗23 to ˗20 °C, ˗10 to ˗5 °F) in 2012. Warmer average minimum winter temperatures have permitted cold hardy cultivars of species such as southern magnolia (Magnolia grandiflora) to survive now in Ithaca, whereas in the past they would not have been hardy. More significantly, however, the increase in the number of street and park tree species reflects an emphasis by municipal tree managers to diversify the municipal tree population. In particular, there has been an effort to seek out and plant tree spe- cies not typically available at nurseries nor planted in urban environments. Such plantings have included trees that are difficult to transplant successfully when harvested from the field, including pawpaw (Asimina triloba), pecan (Carya illinoiensis), shellbark hickory (Carya laciniosa), overcup oak (Quercus lyrata), swamp chestnut oak (Quercus michauxii), and chinkapin oak (Quercus muehlenbergii). Many tree species are not good candidates for urban plantings, especially as street trees, due to the many environ- mental stressors impacting tree health such as drought, compacted soil, de-icing salt, and air pollu- tion (Kargar et al. 2017), and not every introduction of new tree species has been successful. For example, European alder (Alnus glutinosa) did not do well because it is not drought tolerant, and water stress is the primary abiotic constraint for trees in urban land- scapes (Sjöman et al. 2018). Increased diversity is not a cure-all for the many challenges facing urban trees, nor does it automati- cally result in a stable and sustainable municipal tree population, given the urban forest’s vulnerability to
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